Electrostatics involves electric charges, the forces between … · 2013-03-25 · 32 Electrostatics Electrostatics, or electricity at rest, involves electric charges, the forces

32 Electrostatics

Electrostatics involves

electric charges, the forces

between them, and their

behavior in materials.

32 Electrostatics

Electrostatics, or electricity at rest, involves electric charges, the forces between them, and their behavior in materials. An understanding of electricity requires a step-by-step approach, for one concept is the building block for the next.

32 Electrostatics

The fundamental rule at the base of all electrical

phenomena is that like charges repel and opposite

charges attract.

32.1 Electrical Forces and Charges

32 Electrostatics

Consider a force acting on you that is billions upon

billions of times stronger than gravity.

Suppose that in addition to this enormous force

there is a repelling force, also billions upon billions

of times stronger than gravity.

The two forces acting on you would balance each

other and have no noticeable effect at all.

A pair of such forces acts on you all the time—

electrical forces.


32 Electrostatics

The enormous attractive and

repulsive electrical forces

between the charges in Earth

and the charges in your body

balance out, leaving the relatively

weaker force of gravity, which

only attracts.


32 Electrostatics

The Atom

Electrical forces arise from particles in atoms.

The protons in the nucleus attract the electrons and hold them

in orbit. Electrons are attracted to protons, but electrons repel

other electrons.


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The fundamental electrical property to which the

mutual attractions or repulsions between electrons

or protons is attributed is called charge.

By convention, electrons are negatively charged

and protons positively charged.

Neutrons have no charge, and are neither attracted

nor repelled by charged particles.


32 Electrostatics

The helium nucleus is composed of two protons and

two neutrons. The positively charged protons attract

two negative electrons.


32 Electrostatics

Here are some important facts about atoms:

• Every atom has a positively charged nucleus

surrounded by negatively charged electrons.

• All electrons are identical.

• The nucleus is composed of protons and neutrons. All

protons are identical; similarly, all neutrons are identical.

• Atoms usually have as many electrons as protons, so

the atom has zero net charge.

A proton has nearly 2000 times the mass of an electron, but

its positive charge is equal in magnitude to the negative

charge of the electron.


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Attraction and Repulsion


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The fundamental rule of all

electrical phenomena is that

like charges repel and

opposite charges attract.


32 Electrostatics

What is the fundamental rule at the base of all

electrical phenomena?


32 Electrostatics

An object that has unequal numbers of electrons

and protons is electrically charged.

32.2 Conservation of Charge

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Electrons and protons have electric charge.

In a neutral atom, there are as many electrons as protons, so

there is no net charge.


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If an electron is removed from an atom, the atom is

no longer neutral. It has one more positive charge

than negative charge.

A charged atom is called an ion.

• A positive ion has a net positive charge; it has

lost one or more electrons.

• A negative ion has a net negative charge; it has

gained one or more extra electrons.


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Electrically Charged Objects

Matter is made of atoms, and atoms are made of

electrons and protons.

An object that has equal numbers of electrons

and protons has no net electric charge.

But if there is an imbalance in the numbers, the

object is then electrically charged.

An imbalance comes about by adding or

removing electrons.


32 Electrostatics

The innermost electrons in an atom are bound very tightly to

the oppositely charged atomic nucleus.

The outermost electrons of many atoms are bound very

loosely and can be easily dislodged.

How much energy is required to tear an electron away from

an atom varies for different substances.


32 Electrostatics

When electrons are transferred

from the fur to the rod, the rod

becomes negatively charged.


32 Electrostatics

Principle of Conservation of Charge

Electrons are neither created nor

destroyed but are simply

transferred from one material to

another. This principle is known as

conservation of charge.

In every event, whether large-scale

or at the atomic and nuclear level,

the principle of conservation of

charge applies.


32 Electrostatics

Any object that is electrically charged has an excess or

deficiency of some whole number of electrons—electrons

cannot be divided into fractions of electrons.

This means that the charge of the object is a whole-number

multiple of the charge of an electron.


32 Electrostatics

think!

If you scuff electrons onto your shoes while walking across

a rug, are you negatively or positively charged?


32 Electrostatics

think!

If you scuff electrons onto your shoes while walking across

a rug, are you negatively or positively charged?

Answer:

When your rubber- or plastic-soled shoes drag across the

rug, they pick up electrons from the rug in the same way

you charge a rubber or plastic rod by rubbing it with a cloth.

You have more electrons after you scuff your shoes, so you

are negatively charged (and the rug is positively charged).


32 Electrostatics

What causes an object to become

electrically charged?


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Coulomb’s law states that for charged particles or

objects that are small compared with the distance

between them, the force between the charges varies

directly as the product of the charges and inversely

as the square of the distance between them.

32.3 Coulomb’s Law

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Recall from Newton’s law of gravitation that the gravitational

force between two objects of mass m1 and mass m2 is

proportional to the product of the masses and inversely

proportional to the square of the distance d between them:


32 Electrostatics

Force, Charges, and Distance

The electrical force between any two objects obeys a similar

inverse-square relationship with distance.

The relationship among electrical force, charges, and

distance—Coulomb’s law—was discovered by the French

physicist Charles Coulomb in the eighteenth century.


32 Electrostatics

For charged objects, the force between the charges varies

directly as the product of the charges and inversely as the

square of the distance between them.

Where:

d is the distance between the charged particles.

q1 represents the quantity of charge of one particle.

q2 is the quantity of charge of the other particle.

k is the proportionality constant.


32 Electrostatics

The SI unit of charge is the coulomb, abbreviated C.

A charge of 1 C is the charge of 6.24 × 1018 electrons.

A coulomb represents the amount of charge that passes

through a common 100-W light bulb in about one second.


32 Electrostatics

The Electrical Proportionality Constant

The proportionality constant k in Coulomb’s law is similar to G

in Newton’s law of gravitation.

k = 9,000,000,000 N·m2/C2 or 9.0 × 109 N·m2/C2

If a pair of charges of 1 C each were 1 m apart, the force of

repulsion between the two charges would be

9 billion newtons.

That would be more than 10 times the weight of a battleship!


32 Electrostatics

Newton’s law of gravitation for masses is similar to Coulomb’s law for

electric charges.

Whereas the gravitational force of attraction between a pair of one-

kilogram masses is extremely small, the electrical force between a pair of

one-coulomb charges is extremely large.

The greatest difference between gravitation and electrical forces is that

gravity only attracts but electrical forces may attract or repel.


32 Electrostatics

Electrical Forces in Atoms

Because most objects have almost exactly equal numbers of

electrons and protons, electrical forces usually balance out.

Between Earth and the moon, for example, there is no

measurable electrical force.

In general, the weak gravitational force, which only attracts, is

the predominant force between astronomical bodies.


32 Electrostatics

Although electrical forces balance out for

astronomical and everyday objects, at the atomic

level this is not always true.

Often two or more atoms, when close together,

share electrons.

Bonding results when the attractive force between

the electrons of one atom and the positive nucleus

of another atom is greater than the repulsive force

between the electrons of both atoms. Bonding leads

to the formation of molecules.


32 Electrostatics

think!

What is the chief significance of the fact that G in Newton’s

law of gravitation is a small number and k in Coulomb’s law

is a large number when both are expressed in SI units?


32 Electrostatics

think!

What is the chief significance of the fact that G in Newton’s

law of gravitation is a small number and k in Coulomb’s law

is a large number when both are expressed in SI units?

Answer:

The small value of G indicates that gravity is a weak force;

the large value of k indicates that the electrical force is

enormous in comparison.


32 Electrostatics

think!

a. If an electron at a certain distance from a charged particle

is attracted with a certain force, how will the force compare

at twice this distance?


32 Electrostatics

think!




Answer:

a. In accord with the inverse-square law, at twice the

distance the force will be one fourth as much.


32 Electrostatics

think!




b. Is the charged particle in this case positive or negative?

Answer:




32 Electrostatics

think!




b. Is the charged particle in this case positive or negative?

Answer:



b. Since there is a force of attraction, the charges must be

opposite in sign, so the charged particle is positive.


32 Electrostatics

What does Coulomb’s law state?


32 Electrostatics

Electrons move easily in good conductors and

poorly in good insulators.

32.4 Conductors and Insulators

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Outer electrons of the atoms in a metal are not

anchored to the nuclei of particular atoms, but are

free to roam in the material.

Materials through which electric charge can flow are

called conductors.

Metals are good conductors for the motion of

electric charges because their electrons are “loose.”


32 Electrostatics

Electrons in other materials—rubber and glass, for

example—are tightly bound and remain with

particular atoms.

They are not free to wander about to other atoms in

the material.

These materials, known as insulators, are poor

conductors of electricity.


32 Electrostatics

A substance is classified as a

conductor or an insulator based on

how tightly the atoms of the

substance hold their electrons.

The conductivity of a metal can be

more than a million trillion times

greater than the conductivity of an

insulator such as glass.

In power lines, charge flows much

more easily through hundreds of

kilometers of metal wire than through

the few centimeters of insulating

material that separates the wire from

the supporting tower.


32 Electrostatics

Semiconductors are materials that can be made to behave

sometimes as insulators and sometimes as conductors.

Atoms in a semiconductor hold their electrons until given

small energy boosts.

This occurs in photovoltaic cells that convert solar energy

into electrical energy.

Thin layers of semiconducting materials sandwiched

together make up transistors.


32 Electrostatics

What is the difference between a good

conductor and a good insulator?


32 Electrostatics

Two ways electric charge can be transferred are by

friction and by contact.

32.5 Charging by Friction and Contact

32 Electrostatics

We can stroke a cat’s fur and hear the crackle of sparks that

are produced.

We can comb our hair in front of a mirror in a dark room and

see as well as hear the sparks of electricity.

We can scuff our shoes across a rug and feel the tingle as

we reach for the doorknob.

Electrons are being transferred by friction when one

material rubs against another.


32 Electrostatics

If you slide across a seat in an automobile, you are in

danger of being charged by friction.


32 Electrostatics

Electrons can also be transferred from one material to another

by simply touching.

When a charged rod is placed in contact with a neutral object,

some charge will transfer to the neutral object.

This method of charging is called charging by contact.

If the object is a good conductor, the charge will spread to all

parts of its surface because the like charges repel each other.


32 Electrostatics

What are two ways electric charge can

be transferred?


32 Electrostatics

If a charged object is brought near a conducting

surface, even without physical contact, electrons

will move in the conducting surface.

32.6 Charging by Induction

32 Electrostatics

Charging by induction can be illustrated using two insulated

metal spheres.

Uncharged insulated metal spheres touching each other, in

effect, form a single noncharged conductor.


32 Electrostatics

• When a negatively charged rod is held near one sphere, electrons in the

metal are repelled by the rod.

• Excess negative charge has moved to the other sphere, leaving the first

sphere with an excess positive charge.

• The charge on the spheres has been redistributed, or induced.


32 Electrostatics

• When the spheres are separated and the rod removed, the spheres are

charged equally and oppositely.

• They have been charged by induction, which is the charging of an

object without direct contact.


32 Electrostatics

Charge induction by grounding can be illustrated using a metal sphere hanging from

a nonconducting string.


32 Electrostatics



• A charge redistribution is induced by the presence of the charged rod. The

net charge on the sphere is still zero.


32 Electrostatics





• Touching the sphere removes electrons by contact and the sphere is left

positively charged.


32 Electrostatics






positively charged.

• The positively charged sphere is attracted to a negative rod.


32 Electrostatics






positively charged.

• The positively charged sphere is attracted to a negative rod.

• When electrons move onto the sphere from the rod, it becomes negatively

charged by contact.


32 Electrostatics

When we touch the metal surface with a finger, charges that

repel each other have a conducting path to a practically infinite

reservoir for electric charge—the ground.

When we allow charges to move off (or onto) a conductor by

touching it, we are grounding it.


32 Electrostatics

Charging by induction occurs during

thunderstorms.

The negatively charged bottoms of

clouds induce a positive charge on the

surface of Earth below.

Most lightning is an electrical

discharge between oppositely charged

parts of clouds.

The kind of lightning we are most

familiar with is the electrical discharge

between clouds and oppositely

charged ground below.


32 Electrostatics

If a rod is placed above a building and connected to the

ground, the point of the rod collects electrons from the air.

This prevents a buildup of positive charge by induction.

The primary purpose of the lightning rod is to prevent a

lightning discharge from occurring.

If lightning does strike, it may be attracted to the rod and short-

circuited to the ground, sparing the building.


32 Electrostatics

think!

Why does the negative rod in the two-sphere example have

the same charge before and after the spheres are charged, but

not when charging takes place in the single-sphere example?


32 Electrostatics

think!

Why does the negative rod in the two-sphere example have

the same charge before and after the spheres are charged, but

not when charging takes place in the single-sphere example?

Answer:

In the first charging process, no contact was made between

the negative rod and either of the spheres. In the second

charging process, however, the rod touched the sphere when

it was positively charged. A transfer of charge by contact

reduced the negative charge on the rod.


32 Electrostatics

What happens when a charged object is

placed near a conducting surface?


32 Electrostatics

Charge polarization can occur in insulators that are

near a charged object.

32.7 Charge Polarization

32 Electrostatics

Charging by induction is not restricted to conductors.

Charge polarization can occur in insulators that are

near a charged object.

When a charged rod is brought near an insulator, there

are no free electrons to migrate throughout the

insulating material.

Instead, there is a rearrangement of the positions of

charges within the atoms and molecules themselves.


32 Electrostatics

One side of the atom or molecule is induced to be slightly

more positive (or negative) than the opposite side.

The atom or molecule is said to be electrically polarized.


32 Electrostatics

a. When an external negative charge is brought closer

from the left, the charges within a neutral atom or

molecule rearrange.


32 Electrostatics

a. When an external negative charge is brought closer

from the left, the charges within a neutral atom or

molecule rearrange.

b. All the atoms or molecules near the surface of the

insulator become electrically polarized.


32 Electrostatics

Examples of Charge Polarization

Polarization explains why electrically neutral bits of paper are

attracted to a charged object, such as a charged comb.

Molecules are polarized in the paper, with the oppositely

charged sides of molecules closest to the charged object.


32 Electrostatics

The bits of paper experience a net attraction.

Sometimes they will cling to the charged object and

suddenly fly off.

Charging by contact has occurred; the paper bits have

acquired the same sign of charge as the charged object

and are then repelled.


32 Electrostatics

A charged comb attracts an uncharged piece of paper

because the force of attraction for the closer charge is

greater than the force of repulsion for the farther charge.


32 Electrostatics

Rub an inflated balloon on your hair and it becomes charged.

Place the balloon against the wall and it sticks.

The charge on the balloon induces an opposite surface charge

on the wall. The charge on the balloon is slightly closer to the

opposite induced charge than to the charge of the same sign.


32 Electrostatics

Electric Dipoles

Many molecules—H2O, for

example—are electrically

polarized in their normal states.

The distribution of electric

charge is not perfectly even.

There is a little more negative

charge on one side of the

molecule than on the other.

Such molecules are said to be

electric dipoles.


32 Electrostatics


32 Electrostatics

In summary, objects are electrically charged in three ways.

• By friction, when electrons are transferred by friction from

one object to another.

• By contact, when electrons are transferred from one

object to another by direct contact without rubbing.

• By induction, when electrons are caused to gather or

disperse by the presence of nearby charge without

physical contact.


32 Electrostatics

If the object is an insulator, on the other hand, then a

realignment of charge rather than a migration of

charge occurs.

This is charge polarization, in which the surface near

the charged object becomes oppositely charged.


32 Electrostatics

What happens when an insulator is in the

presence of a charged object?


32 Electrostatics

1. If a neutral atom has 22 protons in its nucleus, the number of

surrounding electrons is

a. less than 22.

b. 22.

c. more than 22.

d. unknown.

Assessment Questions

32 Electrostatics

1. If a neutral atom has 22 protons in its nucleus, the number of

surrounding electrons is

a. less than 22.

b. 22.

c. more than 22.

d. unknown.

Answer: B


32 Electrostatics

2. When we say charge is conserved, we mean that charge can

a. be saved, like money in a bank.

b. only be transferred from one place to another.

c. take equivalent forms.

d. be created or destroyed, as in nuclear reactions.


32 Electrostatics

2. When we say charge is conserved, we mean that charge can

a. be saved, like money in a bank.

b. only be transferred from one place to another.

c. take equivalent forms.

d. be created or destroyed, as in nuclear reactions.

Answer: B


32 Electrostatics

3. A difference between Newton’s law of gravity and Coulomb’s law is

that only one of these

a. is a fundamental physical law.

b. uses a proportionality constant.

c. invokes the inverse-square law.

d. involves repulsive as well as attractive forces.


32 Electrostatics

3. A difference between Newton’s law of gravity and Coulomb’s law is

that only one of these

a. is a fundamental physical law.

b. uses a proportionality constant.

c. invokes the inverse-square law.

d. involves repulsive as well as attractive forces.

Answer: D


32 Electrostatics

4. Which is the predominant carrier of charge in copper wire?

a. protons

b. electrons

c. ions

d. neutrons


32 Electrostatics

4. Which is the predominant carrier of charge in copper wire?

a. protons

b. electrons

c. ions

d. neutrons

Answer: B


32 Electrostatics

5. When you scuff electrons off a rug with your shoes, your shoes

are then

a. negatively charged.

b. positively charged.

c. ionic.

d. electrically neutral.


32 Electrostatics

5. When you scuff electrons off a rug with your shoes, your shoes

are then

a. negatively charged.

b. positively charged.

c. ionic.

d. electrically neutral.

Answer: A


32 Electrostatics

6. When a cloud that is negatively charged on its bottom and

positively charged on its top moves over the ground below, the

ground acquires

a. a negative charge.

b. a positive charge.

c. no charge since the cloud is electrically neutral.

d. an electrically grounded state.


32 Electrostatics

6. When a cloud that is negatively charged on its bottom and

positively charged on its top moves over the ground below, the

ground acquires

a. a negative charge.

b. a positive charge.

c. no charge since the cloud is electrically neutral.

d. an electrically grounded state.

Answer: B


32 Electrostatics

7. When a negatively charged balloon is placed against a non-

conducting wall, positive charges in the wall are

a. attracted to the balloon.

b. repelled from the balloon.

c. too bound to negative charges in the wall to have any effect.

d. neutralized.


32 Electrostatics

7. When a negatively charged balloon is placed against a non-

conducting wall, positive charges in the wall are

a. attracted to the balloon.

b. repelled from the balloon.

c. too bound to negative charges in the wall to have any effect.

d. neutralized.

Answer: A


33 Electric Fields and Potential

An electric field is a

storehouse of energy.


The space around a concentration of electric charge is different from how it would be if the charge were not there. If you walk by the charged dome of an electrostatic machine—a Van de Graaff generator, for example—you can sense the charge. Hair on your body stands out—just a tiny bit if you’re more than a meter away, and more if you’re closer. The space is said to contain a force field.


The magnitude (strength) of an electric field can be

measured by its effect on charges located in the

field. The direction of an electric field at any point,

by convention, is the direction of the electrical force

on a small positive test charge placed at that point.

33.1 Electric Fields


If you throw a ball upward, it follows a curved path

due to interaction between the centers of gravity of

the ball and Earth.

The centers of gravity are far apart, so this is “action

at a distance.”

The concept of a force field explains how Earth can

exert a force on things without touching them.

The ball is in contact with the field all the time.



You can sense the force field that surrounds a

charged Van de Graaff generator.



An electric field is a force field that surrounds an electric charge or group

of charges.




of charges.

A gravitational force holds a satellite in orbit about a planet, and an electrical force holds an electron in orbit about a proton.




of charges.

A gravitational force holds a satellite in orbit about a planet, and an electrical force holds an electron in orbit about a proton.

The force that one electric charge exerts on another is the interaction

between one charge and the electric field of the other.



An electric field has both magnitude and direction. The

magnitude can be measured by its effect on charges

located in the field.

Imagine a small positive “test charge” placed in an

electric field.

• Where the force is greatest on the test charge, the

field is strongest.

• Where the force on the test charge is weak, the

field is small.



The direction of an electric field at any point, by

convention, is the direction of the electrical force on a

small positive test charge.

• If the charge that sets up the field is positive, the

field points away from that charge.

• If the charge that sets up the field is negative, the

field points toward that charge.



How are the magnitude and direction of an

electric field determined?



You can use electric field lines (also called lines of

force) to represent an electric field. Where the lines

are farther apart, the field is weaker.

33.2 Electric Field Lines


Since an electric field has both magnitude and direction, it is a

vector quantity and can be represented by vectors.

• A negatively charged particle is surrounded by vectors

that point toward the particle.

• For a positively charged particle, the vectors point away.

• Magnitude of the field is indicated by the vector length.

The electric field is greater where the vectors are longer.



You can use electric field lines to represent an electric field.

• Where the lines are farther apart, the field is weaker.

• For an isolated charge, the lines extend to infinity.

• For two or more opposite charges, the lines emanate

from a positive charge and terminate on a

negative charge.



a. In a vector representation of

an electric field, the length of

the vectors indicates the

magnitude of the field.



a. In a vector representation of

an electric field, the length of

the vectors indicates the

magnitude of the field.

b. In a lines-of-force

representation, the distance

between field lines indicates

magnitudes.



a. The field lines around a single positive charge extend to infinity.




b. For a pair of equal but opposite charges, the field lines emanate

from the positive charge and terminate on the negative charge.




b. For a pair of equal but opposite charges, the field lines emanate

from the positive charge and terminate on the negative charge.

c. Field lines are evenly spaced between two oppositely charged

capacitor plates.



You can demonstrate electric field patterns

by suspending fine thread in an oil bath with

charged conductors. The photos show

patterns for

a. equal and opposite charges;






patterns for


b. equal like charges;






patterns for



c. oppositely charged plates;






patterns for



c. oppositely charged plates;

d. oppositely charged cylinder and plate.



Bits of thread suspended in an oil bath surrounding charged

conductors line up end-to-end with the field lines.

Oppositely charged parallel plates produce nearly parallel field

lines between the plates. Except near the ends, the field

between the plates has a constant strength.

There is no electric field inside a charged cylinder. The

conductor shields the space from the field outside.



think! A beam of electrons is produced at one end of a glass tube and lights up a

phosphor screen at the other end. If the beam passes through the electric

field of a pair of oppositely charged plates, it is deflected upward as shown.

If the charges on the plates are reversed, in what direction will the beam

deflect?



think! A beam of electrons is produced at one end of a glass tube and lights up a

phosphor screen at the other end. If the beam passes through the electric

field of a pair of oppositely charged plates, it is deflected upward as shown.

If the charges on the plates are reversed, in what direction will the beam

deflect?

Answer:

When the charge on the plates is reversed, the electric field will be in the

opposite direction, so the electron beam will be deflected upward.



How can you represent an electric field?



If the charge on a conductor is not moving, the

electric field inside the conductor is exactly zero.

33.3 Electric Shielding


When a car is struck by

lightning, the occupant inside

the car is completely safe.

The electrons that shower

down upon the car are

mutually repelled and spread

over the outer metal surface.

It discharges when additional

sparks jump to the ground.

The electric fields inside the

car practically cancel to zero.



Charged Conductors

The absence of electric field within a conductor holding static

charge is not an inability of an electric field to penetrate

metals.

Free electrons within the conductor can “settle down” and stop

moving only when the electric field is zero.

The charges arrange to ensure a zero field with the material.



Consider a charged metal sphere.

Because of repulsion, electrons

spread as far apart as possible,

uniformly over the surface.

A positive test charge located exactly

in the middle of the sphere would feel

no force. The net force on a test

charge would be zero.

The electric field is also zero.

Complete cancellation will occur

anywhere inside the sphere.



If the conductor is not spherical, the charge distribution will not

be uniform but the electric field inside the conductor is zero.

If there were an electric field inside a conductor, then free

electrons inside the conductor would be set in motion.

They would move to establish equilibrium, that is, all the

electrons produce a zero field inside the conductor.



How to Shield an Electric Field

There is no way to shield gravity, because gravity

only attracts.

Shielding electric fields, however, is quite simple.

• Surround yourself or whatever you wish to

shield with a conducting surface.

• Put this surface in an electric field of whatever

field strength.

• The free charges in the conducting surface will

arrange on the surface of the conductor so that

fields inside cancel.



The metal-lined cover

shields the internal

electrical components

from external electric

fields. A metal cover

shields the cable.



think!

It is said that a gravitational field, unlike an electric field,

cannot be shielded. But the gravitational field at the center of

Earth cancels to zero. Isn’t this evidence that a gravitational

field can be shielded?



think!

It is said that a gravitational field, unlike an electric field,

cannot be shielded. But the gravitational field at the center of

Earth cancels to zero. Isn’t this evidence that a gravitational

field can be shielded?

Answer:

No. Gravity can be canceled inside a planet or between

planets, but it cannot be shielded. Shielding requires a

combination of repelling and attracting forces, and gravity

only attracts.



How can you describe the electric field within a

conductor holding static charge?



The electrical potential energy of a charged particle

is increased when work is done to push it against

the electric field of something else that is charged.

33.4 Electrical Potential Energy


Work is done when a force moves something in the

direction of the force.

An object has potential energy by virtue of its location,

say in a force field.

For example, doing work by lifting an object increases

its gravitational potential energy.



a. In an elevated position, the ram has gravitational

potential energy. When released, this energy is

transferred to the pile below.



a. In an elevated position, the ram has gravitational

potential energy. When released, this energy is

transferred to the pile below.

b. Similar energy transfer occurs for electric charges.



A charged object can have potential energy by virtue of

its location in an electric field.

Work is required to push a charged particle against the

electric field of a charged body.



To push a positive test charge closer to a

positively charged sphere, we will expend

energy to overcome electrical repulsion.

Work is done in pushing the charge against

the electric field.

This work is equal to the energy gained by

the charge.

The energy a charge has due to its location

in an electric field is called

electrical potential energy.

If the charge is released, it will accelerate

away from the sphere and electrical potential

energy transforms into kinetic energy.



How can you increase the electrical potential

energy of a charged particle?



Electric potential is not the same as electrical

potential energy. Electric potential is electrical

potential energy per charge.

33.5 Electric Potential


If we push a single charge against an electric field, we

do a certain amount of work. If we push two charges

against the same field, we do twice as much work.

Two charges in the same location in an electric field will

have twice the electrical potential energy as one; ten

charges will have ten times the potential energy.

It is convenient when working with electricity to consider

the electrical potential energy per charge.



The electrical potential energy per charge is the total electrical

potential energy divided by the amount of charge.

At any location the potential energy per charge—whatever the

amount of charge—will be the same.

The concept of electrical potential energy per charge has the

name, electric potential.



An object of greater charge has more electrical potential

energy in the field of the charged dome than an object of less

charge, but the electric potential of any charge at the same

location is the same.



The SI unit of measurement for electric potential is the

volt, named after the Italian physicist Allesandro Volta.

The symbol for volt is V.

Potential energy is measured in joules and charge is

measured in coulombs,



A potential of 1 volt equals 1 joule of energy per coulomb of charge.

A potential of 1000 V means that 1000 joules of energy per coulomb is

needed to bring a small charge from very far away and add it to the charge

on the conductor.

The small charge would be much less than one coulomb, so the energy

required would be much less than 1000 joules.

To add one proton to the conductor would take only 1.6 × 10–16 J.



Since electric potential is measured in volts, it is

commonly called voltage.

Once the location of zero voltage has been specified, a

definite value for it can be assigned to a location

whether or not a charge exists at that location.

We can speak about the voltages at different locations

in an electric field whether or not any charges occupy

those locations.



Rub a balloon on your hair and the

balloon becomes negatively charged,

perhaps to several thousand volts!

The charge on a balloon rubbed on

hair is typically much less than a

millionth of a coulomb.

Therefore, the energy is very small—

about a thousandth of a joule.

A high voltage requires great energy

only if a great amount of charge is

involved.



think! If there were twice as much charge on one of the

objects, would the electrical potential energy be

the same or would it be twice as great? Would

the electric potential be the same or would it be

twice as great?



think! If there were twice as much charge on one of the

objects, would the electrical potential energy be

the same or would it be twice as great? Would

the electric potential be the same or would it be

twice as great?

Answer:

Twice as much charge would cause the object to

have twice as much electrical potential energy,

because it would have taken twice as much work

to bring the object to that location. The electric

potential would be the same, because the electric

potential is total electrical potential energy divided

by total charge.



What is the difference between electric potential

and electrical potential energy?



The energy stored in a capacitor comes from the

work done to charge it.

33.6 Electrical Energy Storage


Electrical energy can be stored in a device called a capacitor.

• Computer memories use very tiny capacitors to store the

1’s and 0’s of the binary code.

• Capacitors in photoflash units store larger amounts of

energy slowly and release it rapidly during the flash.

• Enormous amounts of energy are stored in banks of

capacitors that power giant lasers in national

laboratories.



The simplest capacitor is a pair of

conducting plates separated by a small

distance, but not touching each other.

• Charge is transferred from one plate

to the other.

• The capacitor plates then have

equal and opposite charges.

• The charging process is complete

when the potential difference

between the plates equals the

potential difference between the

battery terminals—the battery

voltage.

• The greater the battery voltage and

the larger and closer the plates, the

greater the charge that is stored.



In practice, the plates may be thin metallic foils separated by a

thin sheet of paper.

This “paper sandwich” is then rolled up to save space and may

be inserted into a cylinder.



A charged capacitor is discharged when a conducing path is

provided between the plates.

Discharging a capacitor can be a shocking experience if you

happen to be the conducting path.

The energy transfer can be fatal where voltages are high, such

as the power supply in a TV set—even if the set has been

turned off.



The energy stored in a capacitor comes from the

work done to charge it.

The energy is in the form of the electric field

between its plates.

Electric fields are storehouses of energy.



Where does the energy stored in a capacitor

come from?



The voltage of a Van de Graaff generator can be

increased by increasing the radius of the sphere or

by placing the entire system in a container filled

with high-pressure gas.

33.7 The Van de Graaff Generator


A common laboratory device for building up high voltages is

the Van de Graaff generator.

This is the lightning machine often used by “evil scientists” in

old science fiction movies.



In a Van de Graaff generator, a moving rubber belt carries

electrons from the voltage source to a conducting sphere.



A large hollow metal sphere is supported by a cylindrical

insulating stand.

A rubber belt inside the support stand moves past metal

needles that are maintained at a high electric potential.

A continuous supply of electrons is deposited on the belt

through electric discharge by the points of the needles.

The electrons are carried up into the hollow metal sphere.



The electrons leak onto metal points attached to the inner

surface of the sphere.

Because of mutual repulsion, the electrons move to the outer

surface of the conducting sphere.

This leaves the inside surface uncharged and able to receive

more electrons.

The process is continuous, and the charge builds up to a very

high electric potential—on the order of millions of volts.



The physics enthusiast and the dome of the Van de Graaff

generator are charged to a high voltage.



A sphere with a radius of 1 m can be raised to a potential of 3

million volts before electric discharge occurs through the air.

The voltage of a Van de Graaff generator can be increased by

increasing the radius of the sphere or by placing the entire

system in a container filled with highpressure gas.

Van de Graaff generators in pressurized gas can produce

voltages as high as 20 million volts. These devices accelerate

charged particles used as projectiles for penetrating the nuclei

of atoms.



How can the voltage of a Van de Graaff generator

be increased?



1. An electric field has

a. no direction.

b. only magnitude.

c. both magnitude and direction.

d. a uniformed strength throughout.



1. An electric field has

a. no direction.

b. only magnitude.

c. both magnitude and direction.

d. a uniformed strength throughout.

Answer: C



2. In the electric field surrounding a group of charged particles, field

strength is greater where field lines are

a. thickest.

b. longest.

c. farthest apart.

d. closest.



2. In the electric field surrounding a group of charged particles, field

strength is greater where field lines are

a. thickest.

b. longest.

c. farthest apart.

d. closest.

Answer: D



3. Electrons on the surface of a conductor will arrange themselves such

that the electric field

a. inside cancels to zero.

b. follows the inverse-square law.

c. tends toward a state of minimum energy.

d. is shielded from external charges.



3. Electrons on the surface of a conductor will arrange themselves such

that the electric field

a. inside cancels to zero.

b. follows the inverse-square law.

c. tends toward a state of minimum energy.

d. is shielded from external charges.

Answer: A



4. The potential energy of a compressed spring and the potential

energy of a charged object both depend

a. only on the work done on them.

b. only on their locations in their respective fields.

c. on their locations in their respective fields and on the work

done on them.

d. on their kinetic energies exceeding their potential energies.



4. The potential energy of a compressed spring and the potential

energy of a charged object both depend

a. only on the work done on them.

b. only on their locations in their respective fields.

c. on their locations in their respective fields and on the work

done on them.

d. on their kinetic energies exceeding their potential energies.

Answer: C



5. Electric potential is related to electrical potential energy as

a. the two terms are different names for the same concept.

b. electric potential is the ratio of electrical potential energy per

charge.

c. both are measured using the units of coulomb.

d. both are measured using only the units of joules.



5. Electric potential is related to electrical potential energy as

a. the two terms are different names for the same concept.

b. electric potential is the ratio of electrical potential energy per

charge.

c. both are measured using the units of coulomb.

d. both are measured using only the units of joules.

Answer: B



6. A capacitor

a. cannot store charge.

b. cannot store energy.

c. can only store energy.

d. can store energy and charge.



6. A capacitor

a. cannot store charge.

b. cannot store energy.

c. can only store energy.

d. can store energy and charge.

Answer: D



7. What happens to the electric field inside the conducting sphere of a

Van de Graaff generator as it charges?

a. The field increases in magnitude as the amount of

charge increases.

b. The field decreases in magnitude as the amount of

charge increases.

c. The field will have a net force of one.

d. Nothing; the field is always zero.



7. What happens to the electric field inside the conducting sphere of a

Van de Graaff generator as it charges?

a. The field increases in magnitude as the amount of

charge increases.

b. The field decreases in magnitude as the amount of

charge increases.

c. The field will have a net force of one.

d. Nothing; the field is always zero.

Answer: D


34 Electric Current

Electric current is related to

the voltage that produces it,

and the resistance that

opposes it.

34 Electric Current

Voltage produces a flow of charge, or current, within a conductor. The flow is restrained by the resistance it encounters. The rate at which energy is transferred by electric current is power.

34 Electric Current

When the ends of an electric conductor are at

different electric potentials, charge flows from one

end to the other.

34.1 Flow of Charge

34 Electric Current

Heat flows through a conductor when a temperature

difference exists. Heat flows from higher temperature to lower

temperature.

When temperature is at equilibrium, the flow of heat ceases.

34.1 Flow of Charge

34 Electric Current

Charge flows in a similar way. Charge flows

when there is a potential difference, or

difference in potential (voltage), between the

ends of a conductor. The flow continues until

both ends reach the same potential.

When there is no potential difference, there is

no longer a flow of charge through the

conductor.

To attain a sustained flow of charge in a

conductor, one end must remain at a higher

potential than the other.

The situation is analogous to the flow of water.

34.1 Flow of Charge

34 Electric Current

a. Water flows from higher pressure to lower pressure. The

flow will cease when the difference in pressure ceases.

34.1 Flow of Charge

34 Electric Current

a. Water flows from higher pressure to lower pressure. The

flow will cease when the difference in pressure ceases.

b. Water continues to flow because a difference in pressure

is maintained with the pump. The same is true of electric

current.

34.1 Flow of Charge

34 Electric Current

What happens when the ends of a conductor are

at different electrical potentials?

34.1 Flow of Charge

34 Electric Current

A current-carrying wire has a

net electric charge of zero.

34.2 Electric Current

34 Electric Current

Electric current is the flow of electric charge.

In solid conductors, electrons carry the charge through the

circuit because they are free to move throughout the atomic

network.

These electrons are called conduction electrons.

Protons are bound inside atomic nuclei, locked in fixed

positions.

In fluids, such as the electrolyte in a car battery, positive and

negative ions as well as electrons may flow.


34 Electric Current

Measuring Current

Electric current is measured in

amperes, symbol A.

An ampere is the flow of

1 coulomb of charge per second.

When the flow of charge past any

cross section is 1 coulomb (6.24

billion billion electrons) per

second, the current is 1 ampere.


34 Electric Current

Net Charge of a Wire

While the current is flowing, negative electrons swarm

through the atomic network of positively charged

atomic nuclei.

Under ordinary conditions, the number of electrons in

the wire is equal to the number of positive protons in

the atomic nuclei.

As electrons flow, the number entering is the same as

the number leaving, so the net charge is normally zero

at every moment.


34 Electric Current

What is the net flow of electric charge in a

current-carrying wire?


34 Electric Current

Voltage sources such as batteries and generators

supply energy that allows charges to move steadily.

34.3 Voltage Sources

34 Electric Current

Charges do not flow unless there is a potential difference.

Something that provides a potential difference is known as a

voltage source.

Batteries and generators are capable of maintaining a

continuous flow of electrons.


34 Electric Current

Steady Voltage Sources

In a battery, a chemical reaction releases electrical energy.

Generators—such as the alternators in automobiles—convert

mechanical energy to electrical energy.

The electrical potential energy produced is available at the

terminals of the battery or generator.


34 Electric Current

The potential energy per

coulomb of charge available to

electrons moving between

terminals is the voltage.

The voltage provides the

“electric pressure” to move

electrons between the terminals

in a circuit.


34 Electric Current

Power utilities use electric generators to provide the 120 volts

delivered to home outlets.

The alternating potential difference between the two holes in

the outlet averages 120 volts.

When the prongs of a plug are inserted into the outlet, an

average electric “pressure” of 120 volts is placed across the

circuit.

This means that 120 joules of energy is supplied to each

coulomb of charge that is made to flow in the circuit.


34 Electric Current

Distinguishing Between Current and Voltage

There is often some confusion between charge flowing

through a circuit and voltage being impressed across a circuit.


34 Electric Current

Consider a long pipe filled with water.

• Water will flow through the pipe if there is a difference in

pressure across the pipe or between its ends.

• Water flows from high pressure to low pressure.

Similarly, charges flow through a circuit because of an applied

voltage across the circuit.

• You don’t say that voltage flows through a circuit.

• Voltage doesn’t go anywhere, for it is the charges that

move.

• Voltage causes current.


34 Electric Current

What are two voltage sources used to provide

the energy that allows charges to move steadily?


34 Electric Current

The resistance of a wire depends on the

conductivity of the material used in the wire (that

is, how well it conducts) and also on the thickness

and length of the wire.

34.4 Electric Resistance

34 Electric Current

The amount of charge that flows in a circuit depends on the

voltage provided by the voltage source.

The current also depends on the resistance that the conductor

offers to the flow of charge—the electric resistance.

This is similar to the rate of water flow in a pipe, which

depends on the pressure difference and on the resistance of

the pipe.


34 Electric Current

For a given pressure, more

water passes through a

large pipe than a small

one. Similarly, for a given

voltage, more electric

current passes through a

large-diameter wire than a

small-diameter one.


34 Electric Current

A simple hydraulic circuit is analogous to an electric circuit.


34 Electric Current

The resistance of a wire depends on the conductivity of the

material in the wire and on the thickness and length of the

wire.

• Thick wires have less resistance than thin wires.

• Longer wires have more resistance than short wires.

• Electric resistance also depends on temperature. For

most conductors, increased temperature means

increased resistance.


34 Electric Current

The resistance of some materials becomes zero at very

low temperatures, a phenomenon known as

superconductivity.

Certain metals acquire superconductivity (zero

resistance to the flow of charge) at temperatures near

absolute zero.

Superconductivity at “high” temperatures (above 100 K)

has been found in a variety of nonmetallic compounds.

In a superconductor, the electrons flow indefinitely.


34 Electric Current

What factors affect the resistance of a wire?


34 Electric Current

Ohm’s law states that the current in a circuit is

directly proportional to the voltage impressed

across the circuit, and is inversely proportional to

the resistance of the circuit.

34.5 Ohm’s Law

34 Electric Current

Electric resistance is measured in units called ohms.

Georg Simon Ohm, a German physicist, tested wires in circuits

to see what effect the resistance of the wire had on the current.

The relationship among voltage, current, and resistance is

called Ohm’s law.

34.5 Ohm’s Law

34 Electric Current

For a given circuit of constant resistance, current and

voltage are proportional.

Twice the current flows through a circuit for twice the

voltage across the circuit. The greater the voltage, the

greater the current.

If the resistance is doubled for a circuit, the current will

be half what it would be otherwise.

34.5 Ohm’s Law

34 Electric Current

The relationship among the units of measurement is:

A potential difference of 1 volt impressed across a circuit that

has a resistance of 1 ohm will produce a current of 1 ampere.

If a voltage of 12 volts is impressed across the same circuit,

the current will be 12 amperes.

34.5 Ohm’s Law

34 Electric Current

The resistance of a typical lamp cord is much less than 1 ohm,

while a typical light bulb has a resistance of about 100 ohms.

An iron or electric toaster has a resistance of 15 to 20 ohms.

The low resistance permits a large current, which produces

considerable heat.

34.5 Ohm’s Law

34 Electric Current

Current inside electric devices is regulated by circuit elements

called resistors.

The stripes on these resistors are color coded to indicate the

resistance in ohms.

34.5 Ohm’s Law

34 Electric Current

think!

How much current is drawn by a lamp that has a resistance of

100 ohms when a voltage of 50 volts is impressed across it?

34.5 Ohm’s Law

34 Electric Current

think!

How much current is drawn by a lamp that has a resistance of

100 ohms when a voltage of 50 volts is impressed across it?

Answer:

34.5 Ohm’s Law

34 Electric Current

What does Ohm’s law state?

34.5 Ohm’s Law

34 Electric Current

The damaging effects of electric shock are the

result of current passing through the body.

34.6 Ohm’s Law and Electric Shock

34 Electric Current

From Ohm’s law, we

can see that current

depends on the voltage

applied, and also on the

electric resistance of

the human body.


34 Electric Current

The Body’s Resistance

Your body’s resistance ranges from about 100 ohms if soaked with salt

water to about 500,000 ohms if your skin is very dry.

Touch the electrodes of a battery with dry fingers and your resistance to

the flow of charge would be about 100,000 ohms.

You would not feel 12 volts, and 24 volts would just barely tingle.

With moist skin, however, 24 volts could be quite uncomfortable.


34 Electric Current


34 Electric Current

Many people are killed each year by current from common

120-volt electric circuits.

Touch a faulty 120-volt light fixture while standing on the

ground and there is a 120-volt “pressure” between you and

the ground.

The soles of your shoes normally provide a very large

resistance, so the current would probably not be enough to do

serious harm.


34 Electric Current

If you are standing barefoot in a wet bathtub, the resistance

between you and the ground is very small.

Your overall resistance is so low that the 120-volt potential

difference may produce a harmful current through your body.

Drops of water that collect around the on/off switches of

devices such as a hair dryer can conduct current to the user.


34 Electric Current

Although distilled water is a good insulator, the ions in

ordinary water greatly reduce the electric resistance.

There is also usually a layer of salt on your skin, which

when wet lowers your skin resistance to a few hundred

ohms or less.

Handling electric devices while taking a bath is

extremely dangerous.


34 Electric Current

Handling a wet hair dryer can be like sticking your

fingers into a live socket.


34 Electric Current

High-Voltage Wires

You probably have seen birds

perched on high-voltage wires.

Every part of the bird’s body is at the

same high potential as the wire, and

it feels no ill effects.

For the bird to receive a shock, there

must be a difference in potential

between one part of its body and

another part.

Most of the current will then pass

along the path of least electric

resistance connecting these two

points.


34 Electric Current

Suppose you fall from a bridge and manage to grab onto

a high-voltage power line, halting your fall.

If you touch nothing else of different potential, you will

receive no shock, even if the wire is thousands of volts

above ground potential.

No charge will flow from one hand to the other because

there is no appreciable difference in electric potential

between your hands.


34 Electric Current

Ground Wires

Mild shocks occur when the surfaces of appliances are at an

electric potential different from other nearby devices.

If you touch surfaces of different potentials, you become a

pathway for current.

To prevent this, electric appliances are connected to a

ground wire, through the round third prong of a three-wire

electric plug.


34 Electric Current

All ground wires in all plugs are connected together through

the wiring system of the house.

The two flat prongs are for the current-carrying double wire.

If the live wire accidentally comes in contact with the metal

surface of an appliance, the current will be directed to ground

rather than shocking you if you handle it.


34 Electric Current

Health Effects

One effect of electric shock is to overheat tissues in the body

or to disrupt normal nerve functions.

It can upset the nerve center that controls breathing.


34 Electric Current

think!

If the resistance of your body were 100,000 ohms, what would

be the current in your body when you touched the terminals of

a 12-volt battery?


34 Electric Current

think!

If the resistance of your body were 100,000 ohms, what would

be the current in your body when you touched the terminals of

a 12-volt battery?

Answer:


34 Electric Current

think!

If your skin were very moist, so that your resistance was only

1000 ohms, and you touched the terminals of a 24-volt battery,

how much current would you draw?


34 Electric Current

think!

If your skin were very moist, so that your resistance was only

1000 ohms, and you touched the terminals of a 24-volt battery,

how much current would you draw?

Answer:

You would draw

or 0.024 A, a dangerous amount of current!


34 Electric Current

What causes the damaging effects

of electric shock?


34 Electric Current

Electric current may be DC or AC.

34.7 Direct Current and Alternating Current

34 Electric Current

By DC, we mean direct current, which refers to a flow of charge that

always flows in one direction.

• A battery produces direct current in a circuit because the terminals

of the battery always have the same sign of charge.

• Electrons always move through the circuit from the negative

terminal toward the positive terminal.

• Even if the current moves in unsteady pulses, so long as it moves

in one direction only, it is DC.


34 Electric Current

Alternating current (AC), as the name implies, is electric current that

repeatedly reverses direction.

• Electrons in the circuit move first in one direction and then in the

opposite direction.

• They alternate back and forth about relatively fixed positions.

• This is accomplished by alternating the polarity of voltage at the

generator or other voltage source.


34 Electric Current

Voltage Standards

Voltage of AC in North America is normally 120 volts.

In the early days of electricity, higher voltages burned out the

filaments of electric light bulbs.

Power plants in the United States prior to 1900 adopted 110

volts (or 115 or 120 volts) as standard.


34 Electric Current

By the time electricity became popular in Europe, light

bulbs were available that would not burn out so fast at

higher voltages.

Power transmission is more efficient at higher voltages,

so Europe adopted 220 volts as their standard.

The United States stayed with 110 volts (today, officially

120 volts) because of the installed base of 110-volt

equipment.


34 Electric Current

Three-Wire Service

Although lamps in an American home operate on 110–120

volts, electric stoves and other appliances operate on 220–

240 volts.

Most electric service in the United States is three-wire:

• one wire at 120 volts positive

• one wire at zero volts (neutral)

• one wire at a negative 120 volts


34 Electric Current

In AC, the positive and negative alternate at 60 hertz. A

wire that is positive at one instant is negative 1/120 of a

second later.

Most home appliances are connected between the

neutral wire and either of the other two wires, producing

120 volts.

When the plus-120 is connected to the minus-120, it

produces a 240-volt difference—just right for electric

stoves, air conditioners, and clothes dryers.


34 Electric Current

The popularity of AC arises from the fact that electrical energy

in the form of AC can be transmitted great distances.

Easy voltage step-ups result in lower heat losses in the wires.

The primary use of electric current, whether DC or AC, is to

transfer energy from one place to another.


34 Electric Current

What are the two types of electric current?


34 Electric Current

With an AC-DC converter, you can operate a

battery-run device on AC instead of batteries.

34.8 Converting AC to DC

34 Electric Current

The current in your home is AC. The current in a battery-

operated device, such as a laptop computer or cell phone,

is DC.

With an AC-DC converter, you can operate a battery-run

device on AC instead of batteries.


34 Electric Current

A converter uses a transformer to

lower the voltage and a diode, an

electronic device that allows

electron flow in only one direction.

Since alternating current vibrates in

two directions, only half of each

cycle will pass through a diode.

The output is a rough DC, off half

the time.

To maintain continuous current

while smoothing the bumps, a

capacitor is used.


34 Electric Current

Recall that a capacitor acts as a storage reservoir for charge.

Just as it takes time to raise or lower the water level in a

reservoir, it takes time to add or remove electrons from the

capacitor.

A capacitor therefore produces a retarding effect on changes

in current flow and smoothes the pulsed output.


34 Electric Current

a. When input to a diode is AC,


34 Electric Current


b. output is pulsating DC.


34 Electric Current



c. Charging and discharging of a capacitor provides

continuous and smoother current.


34 Electric Current



c. Charging and discharging of a capacitor provides

continuous and smoother current.

d. In practice, a pair of diodes is used so there are no gaps

in current output.


34 Electric Current

How can you operate a battery-run

device on AC?


34 Electric Current

In a current-carrying wire, collisions interrupt the

motion of the electrons so that their actual drift

speed, or net speed through the wire due to the

field, is extremely low.

34.9 The Speed of Electrons in a Circuit

34 Electric Current

When you flip on the light switch on your wall and the circuit is

completed, the light bulb appears to glow immediately.

Energy is transported through the connecting wires at nearly

the speed of light.

The electrons that make up the current, however, do not move

at this high speed.


34 Electric Current

The electrons inside a metal wire have an average speed of a

few million kilometers per hour due to their thermal motion.

This does not produce a current because the motion is

random. There is no net flow in any one direction.

When a battery or generator is connected, an electric field is

established inside the wire.


34 Electric Current

A pulsating electric field can travel through a circuit at nearly

the speed of light.

The electrons continue their random motions in all directions

while simultaneously being nudged along the wire by the

electric field.

The conducting wire acts as a “pipe” for electric field lines.

Inside the wire, the electric field is directed along the wire.


34 Electric Current

The electric field lines between the terminals of a battery are

directed through a conductor, which joins the terminals.


34 Electric Current

Conduction electrons are accelerated by the field.

Before the electrons gain appreciable speed, they “bump into”

metallic ions and transfer some of their kinetic energy.

• Collisions interrupt the motion of the electrons. Their

actual drift speed, or net speed through the wire, is

extremely low.

• In the electric system of an automobile, electrons have a

net average drift speed of about 0.01 cm/s.


34 Electric Current

The solid lines depict a random path of an electron bouncing

off atoms in a conductor. The dashed lines show an

exaggerated view of how this path changes when an electric

field is applied. The electron drifts toward the right with an

average speed less than a snail’s pace.


34 Electric Current

In an AC circuit, the conduction electrons don’t

make any net progress in any direction.

• In a single cycle they drift a tiny fraction of a

centimeter in one direction, and then the same

distance in the opposite direction.

• They oscillate rhythmically about relatively

fixed positions.

• On a conventional telephone, it is the pattern

of oscillating motion that is carried at nearly

the speed of light.

• The electrons in the wires vibrate to the

rhythm of the traveling pattern.


34 Electric Current

Why is the drift speed of electrons in a

current-carrying wire extremely low?


34 Electric Current

The source of electrons in a circuit is the

conducting circuit material itself.

34.10 The Source of Electrons in a Circuit

34 Electric Current

You can buy a water hose that is empty of water, but

you can’t buy a piece of wire, an “electron pipe,” that is

empty of electrons.

The source of electrons in a circuit is the conducting

circuit material itself.

Electrons do not travel appreciable distances through a

wire in an AC circuit. They vibrate to and fro about

relatively fixed positions.


34 Electric Current

When you plug a lamp into an AC outlet,

energy flows from the outlet into the lamp,

not electrons.

Energy is carried by the electric field and

causes a vibratory motion of the electrons

that already exist in the lamp filament.

Most of this electrical energy appears as

heat, while some of it takes the form of

light.

Power utilities do not sell electrons. They

sell energy. You supply the electrons.


34 Electric Current

When you are jolted by an AC electric shock, the

electrons making up the current in your body originate

in your body.

Electrons do not come out of the wire and through your

body and into the ground; energy does.

The energy simply causes free electrons in your body to

vibrate in unison.

Small vibrations tingle; large vibrations can be fatal.


34 Electric Current

What is the source of electrons in a circuit?


34 Electric Current

Electric power is equal to the product of

current and voltage.

34.11 Electric Power

34 Electric Current

Unless it is in a superconductor, a charge moving in a circuit

expends energy.

This may result in heating the circuit or in turning a motor.

Electric power is the rate at which electrical energy is

converted into another form such as mechanical energy,

heat, or light.


34 Electric Current

Electric power is equal to the product of current and voltage.

electric power = current × voltage

If the voltage is expressed in volts and the current in

amperes, then the power is expressed in watts.

1 watt = (1 ampere) × (1 volt)


34 Electric Current

The power and voltage on the light bulb read “60 W 120 V.”

The current that would flow through the bulb is:

I = P/V = (60 W)/(120 V) = 0.5 A.


34 Electric Current

A lamp rated at 120 watts operated

on a 120-volt line will draw a current

of 1 ampere:

120 watts = (1 ampere) × (120 volts).

A 60-watt lamp draws 0.5 ampere on

a 120-volt line.


34 Electric Current

A kilowatt is 1000 watts, and a kilowatt-hour represents the

amount of energy consumed in 1 hour at the rate

of 1 kilowatt.

Where electrical energy costs 10 cents per kilowatt-hour, a

100-watt light bulb burns for 10 hours for 10 cents.

A toaster or iron, which draws more current and therefore

more power, costs several times as much to operate for the

same time.


34 Electric Current

think!

How much power is used by a calculator that operates on 8

volts and 0.1 ampere? If it is used for one hour, how much

energy does it use?


34 Electric Current

think!

How much power is used by a calculator that operates on 8

volts and 0.1 ampere? If it is used for one hour, how much

energy does it use?

Answer:

Power = current × voltage = (0.1 A) × (8 V) = 0.8 W.

Energy = power × time = (0.8 W) × (1 h) = 0.8 watt-hour,

or 0.0008 kilowatt-hour.


34 Electric Current

think!

Will a 1200-watt hair dryer operate on a 120-volt line if the

current is limited to 15 amperes by a safety fuse? Can two hair

dryers operate on this line?


34 Electric Current

think!

Will a 1200-watt hair dryer operate on a 120-volt line if the

current is limited to 15 amperes by a safety fuse? Can two hair

dryers operate on this line?

Answer:

One 1200-W hair dryer can be operated because the circuit

can provide (15 A) × (120 V) = 1800 W. But there is

inadequate power to operate two hair dryers of combined

power 2400 W. In terms of current, (1200 W)/(120 V) = 10 A;

so the hair dryer will operate when connected to the circuit. But

two hair dryers will require 20 A and will blow the 15-A fuse.


34 Electric Current

How can you express electric power in terms of

current and voltage?


34 Electric Current

1. Electric charge will flow in an electric circuit when

a. electrical resistance is low enough.

b. a potential difference exists.

c. the circuit is grounded.

d. electrical devices in the circuit are not defective.


34 Electric Current

1. Electric charge will flow in an electric circuit when

a. electrical resistance is low enough.

b. a potential difference exists.

c. the circuit is grounded.

d. electrical devices in the circuit are not defective.

Answer: B


34 Electric Current

2. The electric current in a copper wire is normally composed of

a. electrons.

b. protons.

c. ions.

d. amperes.


34 Electric Current

2. The electric current in a copper wire is normally composed of

a. electrons.

b. protons.

c. ions.

d. amperes.

Answer: A


34 Electric Current

3. Which statement is correct?

a. Voltage flows in a circuit.

b. Charge flows in a circuit.

c. A battery is the source of electrons in a circuit.

d. A generator is the source of electrons in a circuit.


34 Electric Current

3. Which statement is correct?

a. Voltage flows in a circuit.

b. Charge flows in a circuit.

c. A battery is the source of electrons in a circuit.

d. A generator is the source of electrons in a circuit.

Answer: B


34 Electric Current

4. Which of the following type of copper wire would you expect to

have the least electric resistance?

a. a thick long wire

b. a thick short wire

c. a thin long wire

d. a thin short wire


34 Electric Current

4. Which of the following type of copper wire would you expect to

have the least electric resistance?

a. a thick long wire

b. a thick short wire

c. a thin long wire

d. a thin short wire

Answer: D


34 Electric Current

5. When you double the voltage in a simple electric circuit, you

double the

a. current.

b. resistance.

c. ohms.

d. resistors.


34 Electric Current

5. When you double the voltage in a simple electric circuit, you

double the

a. current.

b. resistance.

c. ohms.

d. resistors.

Answer: A


34 Electric Current

6. To receive an electric shock there must be

a. current in one direction.

b. moisture in an electrical device being used.

c. high voltage and low body resistance.

d. a difference in potential across part or all of your body.


34 Electric Current

6. To receive an electric shock there must be

a. current in one direction.

b. moisture in an electrical device being used.

c. high voltage and low body resistance.

d. a difference in potential across part or all of your body.

Answer: D


34 Electric Current

7. The difference between DC and AC in electrical circuits is that

in DC

a. charges flow steadily in one direction only.

b. charges flow in one direction.

c. charges steadily flow to and fro.

d. charges flow to and fro.


34 Electric Current

7. The difference between DC and AC in electrical circuits is that

in DC

a. charges flow steadily in one direction only.

b. charges flow in one direction.

c. charges steadily flow to and fro.

d. charges flow to and fro.

Answer: B


34 Electric Current

8. To convert AC to a fairly steady DC, which devices are used?

a. diodes and batteries

b. capacitors and diodes

c. capacitors and batteries

d. resistors and batteries


34 Electric Current

8. To convert AC to a fairly steady DC, which devices are used?

a. diodes and batteries

b. capacitors and diodes

c. capacitors and batteries

d. resistors and batteries

Answer: B


34 Electric Current

9. What is it that travels at about the speed of light in an electric circuit?

a. charges

b. current

c. electric field

d. voltage


34 Electric Current

9. What is it that travels at about the speed of light in an electric circuit?

a. charges

b. current

c. electric field

d. voltage

Answer: C


34 Electric Current

10. When you buy a water pipe in a hardware store, the water

isn’t included. When you buy copper wire, electrons

a. must be supplied by you, just as water must be supplied

for a water pipe.

b. are already in the wire.

c. may fall out, which is why wires are insulated.

d. enter it from the electric outlet.


34 Electric Current

10. When you buy a water pipe in a hardware store, the water

isn’t included. When you buy copper wire, electrons

a. must be supplied by you, just as water must be supplied

for a water pipe.

b. are already in the wire.

c. may fall out, which is why wires are insulated.

d. enter it from the electric outlet.

Answer: B


34 Electric Current

11. If you double both the current and the voltage in a circuit, the power

a. remains unchanged if resistance remains constant.

b. halves.

c. doubles.

d. quadruples.


34 Electric Current

11. If you double both the current and the voltage in a circuit, the power

a. remains unchanged if resistance remains constant.

b. halves.

c. doubles.

d. quadruples.

Answer: D


35 Electric Circuits

Any path along

which electrons can

flow is a circuit.


Mechanical things seem to be easier to figure out for most people than electrical things. Maybe this is because most people have had experience playing with blocks and mechanical toys. Hands-on laboratory experience aids your understanding of electric circuits. The experience can be a lot of fun, too!


In a flashlight, when the switch is turned on to

complete an electric circuit, the mobile conduction

electrons already in the wires and the filament begin

to drift through the circuit.

35.1 A Battery and a Bulb


A flashlight consists of a reflector cap, a light bulb, batteries,

and a barrel-shaped housing with a switch.



There are several ways to connect the battery and bulb from

a flashlight so that the bulb lights up.

The important thing is that there must be a complete path, or

circuit, that

• includes the bulb filament

• runs from the positive terminal at the top of the battery

• runs to the negative terminal at the bottom of the battery



Electrons flow

• from the negative part of the battery through the wire

• to the side (or bottom) of the bulb

• through the filament inside the bulb

• out the bottom (or side)

• through the wire to the positive part of the battery

The current then passes through the battery to complete

the circuit.



a. Unsuccessful ways to light a bulb.



a. Unsuccessful ways to light a bulb.

b. Successful ways to light a bulb.



The flow of charge in a circuit is very much like the flow of

water in a closed system of pipes.

In a flashlight, the battery is analogous to a pump, the wires

are analogous to the pipes, and the bulb is analogous to

any device that operates when the water is flowing.

When a valve in the line is opened and the pump is

operating, water already in the pipes starts to flow.



Neither the water nor the electrons

concentrate in certain places.

They flow continuously around a

loop, or circuit.

When the switch is turned on, the

mobile conduction electrons in the

wires and the filament begin to drift

through the circuit.



Electrons do not pile up inside

a bulb, but instead flow through

its filament.



What happens to the mobile conduction

electrons when you turn on a flashlight?



For a continuous flow of electrons, there must

be a complete circuit with no gaps.

35.2 Electric Circuits


Any path along which electrons can

flow is a circuit.

A gap is usually provided by an

electric switch that can be opened

or closed to either cut off or allow

electron flow.



The water analogy is useful but has some limitations.

• A break in a water pipe results in a leak, but a

break in an electric circuit results in a complete

stop in the flow.

• Opening a switch stops the flow of electricity. An

electric circuit must be closed for electricity to

flow. Opening a water faucet, on the other hand,

starts the flow of water.



Most circuits have more than one device that receives

electrical energy.

These devices are commonly connected in a circuit in

one of two ways, series or parallel.

• When connected in series, the devices in a

circuit form a single pathway for electron flow.

• When connected in parallel, the devices in a

circuit form branches, each of which is a

separate path for electron flow.



How can a circuit achieve a continuous

flow of electrons?



If one device fails in a series circuit, current in

the whole circuit ceases and none of the

devices will work.

35.3 Series Circuits


If three lamps are connected in series with a battery,

they form a series circuit. Charge flows through each

in turn.

When the switch is closed, a current exists almost

immediately in all three lamps.

The current does not “pile up” in any lamp but flows

through each lamp. Electrons in all parts of the circuit

begin to move at once.



Eventually the electrons move all the way around the circuit.

A break anywhere in the path results in an open circuit, and

the flow of electrons ceases.

Burning out of one of the lamp filaments or simply opening

the switch could cause such a break.



In this simple series circuit, a 9-volt battery provides 3 volts

across each lamp.



For series connections:

• Electric current has a single pathway through the circuit.

• The total resistance to current in the circuit is the sum of

the individual resistances along the circuit path.

• The current is equal to the voltage supplied by the source

divided by the total resistance of the circuit. This is Ohm’s

law.

• The voltage drop, or potential difference, across each

device depends directly on its resistance.

• The sum of the voltage drops across the individual

devices is equal to the total voltage supplied by the

source.



The main disadvantage of a

series circuit is that when one

device fails, the current in the

whole circuit stops.

Some cheap light strings are

connected in series. When one

lamp burns out, you have to

replace it or no lights work.



think!

What happens to the light intensity of each lamp in a series

circuit when more lamps are added to the circuit?



think!

What happens to the light intensity of each lamp in a series

circuit when more lamps are added to the circuit?

Answer:

The addition of more lamps results in a greater circuit

resistance. This decreases the current in the circuit (and in

each lamp), which causes dimming of the lamps.



think!

A series circuit has three bulbs. If the current through one of

the bulbs is 1 A, can you tell what the current is through each

of the other two bulbs? If the voltage across bulb 1 is 2 V, and

across bulb 2 is 4 V, what is the voltage across bulb 3?



think!

A series circuit has three bulbs. If the current through one of

the bulbs is 1 A, can you tell what the current is through each

of the other two bulbs? If the voltage across bulb 1 is 2 V, and

across bulb 2 is 4 V, what is the voltage across bulb 3?

Answer:

The same current, 1 A, passes through every part of a series

circuit. Each coulomb of charge has 9 J of electrical potential

energy (9 V = 9 J/C). If it spends 2 J in one bulb and 4 in

another, it must spend 3 J in the last bulb. 3 J/C = 3 V



What happens to current in other lamps if

one lamp in a series circuit burns out?



In a parallel circuit, each device operates

independent of the other devices. A break in any

one path does not interrupt the flow of charge in

the other paths.

35.4 Parallel Circuits


In a parallel circuit having three lamps, each electric

device has its own path from one terminal of the battery

to the other.

There are separate pathways for current, one through

each lamp.

In contrast to a series circuit, the parallel circuit is

completed whether all, two, or only one lamp is lit.

A break in any one path does not interrupt the flow of

charge in the other paths.



In this parallel circuit, a 9-volt battery provides 9 volts

across each activated lamp. (Note the open switch in the

lower branch.)



Major characteristics of parallel connections:

• Each device connects the same two points A and B of

the circuit. The voltage is therefore the same across

each device.

• The total current divides among the parallel branches.

• The amount of current in each branch is inversely

proportional to the resistance of the branch.

• The total current is the sum of the currents in its

branches.

• As the number of parallel branches is increased, the

total current through the battery increases.



From the battery’s perspective, the overall resistance of the

circuit is decreased.

This means the overall resistance of the circuit is less than the

resistance of any one of the branches.



think!

What happens to the light intensity of each lamp in a parallel

circuit when more lamps are added in parallel to the circuit?



think!

What happens to the light intensity of each lamp in a parallel

circuit when more lamps are added in parallel to the circuit?

Answer:

The light intensity for each lamp is unchanged as other lamps

are introduced (or removed). Although changes of resistance

and current occur for the circuit as a whole, no changes occur

in any individual branch in the circuit.



What happens if one device in a parallel

circuit fails?



In a schematic diagram, resistance is shown by a

zigzag line, and ideal resistance-free wires are shown

with solid straight lines. A battery is represented with

a set of short and long parallel lines.

35.5 Schematic Diagrams


Electric circuits are frequently

described by simple diagrams, called

schematic diagrams.

• Resistance is shown by a

zigzag line, and ideal

resistance-free wires are shown

with solid straight lines.

• A battery is shown by a set of

short and long parallel lines, the

positive terminal with a long line

and the negative terminal with a

short line.



These schematic diagrams represent

a. a circuit with three lamps in series, and



These schematic diagrams represent

a. a circuit with three lamps in series, and

b. a circuit with three lamps in parallel.



What symbols are used to represent resistance,

wires, and batteries in schematic diagrams?



The equivalent resistance of resistors connected in

series is the sum of their values. The equivalent

resistance for a pair of equal resistors in parallel is

half the value of either resistor.

35.6 Combining Resistors in a Compound Circuit


Sometimes it is useful to know the equivalent resistance

of a circuit that has several resistors in its network.

The equivalent resistance is the value of the single

resistor that would comprise the same load to the battery

or power source.

The equivalent resistance of resistors connected in

series is the sum of their values. For example, the

equivalent resistance for a pair of 1-ohm resistors in

series is simply 2 ohms.



The equivalent resistance for a pair of equal resistors in

parallel is half the value of either resistor.

The equivalent resistance for a pair of 1-ohm resistors in

parallel is 0.5 ohm.

The equivalent resistance is less because the current

has “twice the path width” when it takes the parallel path.



a. The equivalent resistance of two 8-ohm resistors in

series is 16 ohms.



a. The equivalent resistance of two 8-ohm resistors in

series is 16 ohms.

b. The equivalent resistance of two 8-ohm resistors in

parallel is 4 ohms.



For the combination of three 8-ohm resistors, the two

resistors in parallel are equivalent to a single 4-ohm resistor.

They are in series with an 8-ohm resistor, adding to produce

an equivalent resistance of 12 ohms.

If a 12-volt battery were connected to these resistors, the

current through the battery would be 1 ampere.

(In practice it would be less, for there is resistance inside the

battery as well, called the battery’s internal resistance.)



Schematic diagrams for an arrangement of various electric

devices. The equivalent resistance of the circuit is 10 ohms.



think!

In the circuit shown below, what is the current in amperes

through the pair of 10-ohm resistors? Through each of the 8-

ohm resistors?



think!

In the circuit shown below, what is the current in amperes

through the pair of 10-ohm resistors? Through each of the 8-

ohm resistors?

Answer:

The total resistance of the middle branch is 20 Ω. Since the

voltage is 60 V, the current = (voltage)/(resistance) =

(60V)/(2 Ω) = 3 A. The current through the pair of 8-Ω resistors

is 3 A, and the current through each is therefore 1.5 A.



What is the equivalent resistance of resistors in

series? Of equal resistors in parallel?



To prevent overloading in circuits, fuses or

circuit breakers are connected in series along

the supply line.

35.7 Parallel Circuits and Overloading


Electric current is fed into a home by two wires called lines.

About 110 to 120 volts are impressed on these lines at the

power utility.

These lines are very low in resistance and are connected to

wall outlets in each room.

The voltage is applied to appliances and other devices that

are connected in parallel by plugs to these lines.



As more devices are connected to the lines, more

pathways are provided for current.

The additional pathways lower the combined resistance

of the circuit. Therefore, a greater amount of current

occurs in the lines.

Lines that carry more than a safe amount of current are

said to be overloaded, and may heat sufficiently to melt

the insulation and start a fire.



Consider a line connected to a toaster that draws 8 amps, a heater that

draws 10 amps, and a lamp that draws 2 amps.

• If the toaster is operating, the total line current is 8 amperes.

• When the heater is also operating, the total line current increases to

18 amperes.

• If you turn on the lamp, the line current increases to 20 amperes.



To prevent overloading in circuits, fuses or circuit breakers are

connected in series along the supply line.

The entire line current must pass through the fuse.

If the fuse is rated at 20 amperes, it will pass up to 20 amperes.

A current above 20 amperes will melt the fuse ribbon, which

“blows out” and breaks the circuit.



Before a blown fuse is replaced, the cause of

overloading should be determined and remedied.

Insulation that separates the wires in a circuit can wear

away and allow the wires to touch.

This effectively shortens the path of the circuit, and is

called a short circuit.

A short circuit draws a dangerously large current

because it bypasses the normal circuit resistance.



Circuits may also be protected by circuit breakers, which use

magnets or bimetallic strips to open the switch.

Utility companies use circuit breakers to protect their lines all

the way back to the generators.

Circuit breakers are used in modern buildings because they

do not have to be replaced each time the circuit is opened.



How can you prevent overloading in circuits?



1. In a light bulb, the amount of current in the filament is

a. slightly less than the current in the connecting wires.

b. the same as the current in the connecting wires.

c. slightly greater than the current in the connecting wires.

d. twice as great as the current that is in the connecting wires.



1. In a light bulb, the amount of current in the filament is

a. slightly less than the current in the connecting wires.

b. the same as the current in the connecting wires.

c. slightly greater than the current in the connecting wires.

d. twice as great as the current that is in the connecting wires.

Answer: B



2. The flow of charge in an electric circuit is

a. much like the flow of water in a system of pipes.

b. very different from water flow in pipes.

c. like an electric valve.

d. like an electric pump.



2. The flow of charge in an electric circuit is

a. much like the flow of water in a system of pipes.

b. very different from water flow in pipes.

c. like an electric valve.

d. like an electric pump.

Answer: A



3. In a series circuit, if the current in one lamp is 2 amperes, the current

in the battery is

a. half, 1 A.

b. 2 A.

c. not necessarily 2 A, depending on internal battery resistance.

d. more than 2 A.



3. In a series circuit, if the current in one lamp is 2 amperes, the current

in the battery is

a. half, 1 A.

b. 2 A.

c. not necessarily 2 A, depending on internal battery resistance.

d. more than 2 A.

Answer: B



4. In a circuit with two lamps in parallel, if the current in one lamp is

2 amperes, the current in the battery is

a. half, 1 A.

b. 2 A.

c. more than 2 A.

d. cannot be calculated from the information given



4. In a circuit with two lamps in parallel, if the current in one lamp is

2 amperes, the current in the battery is

a. half, 1 A.

b. 2 A.

c. more than 2 A.

d. cannot be calculated from the information given

Answer: C



5. In a circuit diagram there may be

a. no switches.

b. at most, one switch.

c. two switches.

d. any number of switches.



5. In a circuit diagram there may be

a. no switches.

b. at most, one switch.

c. two switches.

d. any number of switches.

Answer: D



6. Consider a compound circuit consisting of a pair of 6-ohm resistors in

parallel, which are in series with two 6-ohm resistors in series. The

equivalent resistance of the circuit is

a. 9 ohms.

b. 12 ohms.

c. 15 ohms.

d. 24 ohms.



6. Consider a compound circuit consisting of a pair of 6-ohm resistors in

parallel, which are in series with two 6-ohm resistors in series. The

equivalent resistance of the circuit is

a. 9 ohms.

b. 12 ohms.

c. 15 ohms.

d. 24 ohms.

Answer: C



7. One way to prevent overloading in your home circuit is to

a. operate fewer devices at the same time.

b. change the wiring from parallel to series for troublesome

devices.

c. find a way to bypass the fuse.

d. find a way to bypass the circuit breaker.



7. One way to prevent overloading in your home circuit is to

a. operate fewer devices at the same time.

b. change the wiring from parallel to series for troublesome

devices.

c. find a way to bypass the fuse.

d. find a way to bypass the circuit breaker.

Answer: A


36 Magnetism

A moving electric charge

is surrounded by a

magnetic field.

36 Magnetism

Electricity and magnetism were regarded as unrelated phenomena until it was noticed that an electric current caused the deflection of the compass needle. Then, magnets were found to exert forces on current-carrying wires. The stage was set for a whole new technology, which would eventually bring electric power, radio, and television.

36 Magnetism

Like poles repel; opposite poles attract.

36.1 Magnetic Poles

36 Magnetism

Magnets exert forces on one another.

They are similar to electric charges, for they can both attract

and repel without touching.

Like electric charges, the strength of their interaction depends

on the distance of separation of the two magnets.

Electric charges produce electrical forces and regions called

magnetic poles produce magnetic forces.

36.1 Magnetic Poles

36 Magnetism

Which interaction has the greater strength—the gravitational

attraction between the scrap iron and Earth, or the magnetic

attraction between the magnet and the scrap iron?

36.1 Magnetic Poles

36 Magnetism

If you suspend a bar magnet from its center by a

piece of string, it will act as a compass.

• The end that points northward is called the

north-seeking pole.

• The end that points southward is called the

south-seeking pole.

• More simply, these are called the north and

south poles.

• All magnets have both a north and a south

pole. For a simple bar magnet the poles are

located at the two ends.

36.1 Magnetic Poles

36 Magnetism

If the north pole of one magnet is brought near the north pole of another

magnet, they repel.

The same is true of a south pole near a south pole.

If opposite poles are brought together, however, attraction occurs.

36.1 Magnetic Poles

36 Magnetism

Magnetic poles behave similarly to electric charges in some

ways, but there is a very important difference.

• Electric charges can be isolated, but magnetic poles

cannot.

• A north magnetic pole never exists without the presence

of a south pole, and vice versa.

• The north and south poles of a magnet are like the head

and tail of the same coin.

36.1 Magnetic Poles

36 Magnetism

If you break a bar magnet in half, each half still behaves as a

complete magnet.

Break the pieces in half again, and you have four complete

magnets.

Even when your piece is one atom thick, there are two poles.

This suggests that atoms themselves are magnets.

36.1 Magnetic Poles

36 Magnetism

36.1 Magnetic Poles

36 Magnetism

think!

Does every magnet necessarily have a north

and a south pole?

36.1 Magnetic Poles

36 Magnetism

think!

Does every magnet necessarily have a north

and a south pole?

Answer:

Yes, just as every coin has two sides, a “head”

and a “tail.” (Some “trick” magnets have more

than two poles.)

36.1 Magnetic Poles

36 Magnetism

How do magnetic poles affect each other?

36.1 Magnetic Poles

36 Magnetism

The direction of the magnetic field outside a

magnet is from the north to the south pole.

36.2 Magnetic Fields

36 Magnetism

Iron filings sprinkled on a sheet of paper over a bar magnet will

tend to trace out a pattern of lines that surround the magnet.

The space around a magnet, in which a magnetic force is

exerted, is filled with a magnetic field.

The shape of the field is revealed by magnetic field lines.


36 Magnetism

Magnetic field lines spread out from one pole, curve around

the magnet, and return to the other pole.


36 Magnetism

Magnetic field patterns for a pair of magnets when

a. opposite poles are near each other


36 Magnetism

Magnetic field patterns for a pair of magnets when

a. opposite poles are near each other

b. like poles are near each other


36 Magnetism

The direction of the magnetic field

outside a magnet is from the north

to the south pole.

Where the lines are closer together,

the field strength is greater.

The magnetic field strength is

greater at the poles.

If we place another magnet or a

small compass anywhere in the

field, its poles will tend to line up

with the magnetic field.


36 Magnetism

What is the direction of the magnetic field

outside a magnet?


36 Magnetism

A magnetic field is produced by the motion of

electric charge.

36.3 The Nature of a Magnetic Field

36 Magnetism

Magnetism is very much related to electricity.

Just as an electric charge is surrounded by an electric

field, a moving electric charge is also surrounded by a

magnetic field.

Charges in motion have associated with them both an

electric and a magnetic field.


36 Magnetism

Electrons in Motion

Where is the motion of electric charges in a common

bar magnet?

The magnet as a whole may be stationary, but it is

composed of atoms whose electrons are in constant

motion about atomic nuclei.

This moving charge constitutes a tiny current and

produces a magnetic field.


36 Magnetism

More important, electrons can be thought of as spinning

about their own axes like tops.

A spinning electron creates another magnetic field.

In most materials, the field due to spinning

predominates over the field due to orbital motion.


36 Magnetism

Spin Magnetism

Every spinning electron is a tiny magnet.

• A pair of electrons spinning in the same direction

makes up a stronger magnet.

• Electrons spinning in opposite directions work

against one another.

• Their magnetic fields cancel.


36 Magnetism

Most substances are not magnets because the various

fields cancel one another due to electrons spinning in

opposite directions.

In materials such as iron, nickel, and cobalt, however,

the fields do not cancel one another entirely.

An iron atom has four electrons whose spin magnetism

is not canceled.

Each iron atom, then, is a tiny magnet. The same is true

to a lesser degree for the atoms of nickel and cobalt.


36 Magnetism

How is a magnetic field produced?


36 Magnetism

Permanent magnets are made by simply placing

pieces of iron or certain iron alloys in strong

magnetic fields.

36.4 Magnetic Domains

36 Magnetism

The magnetic fields of individual iron atoms are strong.

• Interactions among adjacent iron atoms cause

large clusters of them to line up with one another.

• These clusters of aligned atoms are called

magnetic domains.

• Each domain is perfectly magnetized, and is made

up of billions of aligned atoms.

• The domains are microscopic, and there are many

of them in a crystal of iron.


36 Magnetism

The difference between a piece of ordinary iron and an iron magnet is the

alignment of domains.

• In a common iron nail, the domains are randomly oriented.

• When a strong magnet is brought nearby, there is a growth in size of

domains oriented in the direction of the magnetic field.

• The domains also become aligned much as electric dipoles are aligned in

the presence of a charged rod.

• When you remove the nail from the magnet, thermal motion causes most of

the domains to return to a random arrangement.


36 Magnetism

Permanent magnets are made by simply placing pieces

of iron or certain iron alloys in strong magnetic fields.

Another way of making a permanent magnet is to stroke

a piece of iron with a magnet.

The stroking motion aligns the domains in the iron.

If a permanent magnet is dropped or heated, some of

the domains are jostled out of alignment and the

magnet becomes weaker.


36 Magnetism

The arrows represent

domains, where the

head is a north pole

and the tail a south

pole. Poles of

neighboring domains

neutralize one

another’s effects,

except at the ends.


36 Magnetism

think!

Iron filings sprinkled on paper that covers a magnet were not

initially magnetized. Why, then, do they line up with the

magnetic field of the magnet?


36 Magnetism

think!

Iron filings sprinkled on paper that covers a magnet were not

initially magnetized. Why, then, do they line up with the

magnetic field of the magnet?

Answer:

Domains align in the individual filings, causing them to act like

tiny compasses. The poles of each “compass” are pulled in

opposite directions, producing a torque that twists each filing

into alignment with the external magnetic field.


36 Magnetism

How can you make a permanent magnet?


36 Magnetism

An electric current produces a magnetic field.

36.5 Electric Currents and Magnetic Fields

36 Magnetism

A moving charge produces a magnetic field.

An electric current passing through a conductor

produces a magnetic field because it has many charges

in motion.


36 Magnetism

The magnetic field surrounding a current-carrying

conductor can be shown by arranging magnetic

compasses around the wire.

The compasses line up with the magnetic field

produced by the current, a pattern of concentric circles

about the wire.

When the current reverses direction, the compasses

turn around, showing that the direction of the magnetic

field changes also.


36 Magnetism

a. When there is no current in the wire, the compasses

align with Earth’s magnetic field.


36 Magnetism

a. When there is no current in the wire, the compasses

align with Earth’s magnetic field.

b. When there is a current in the wire, the compasses

align with the stronger magnetic field near the wire.


36 Magnetism

If the wire is bent into a loop, the magnetic field lines become bunched up

inside the loop.

If the wire is bent into another loop, the concentration of magnetic field lines

inside the double loop is twice that of the single loop.

The magnetic field intensity increases as the number of loops is increased.


36 Magnetism

A current-carrying coil of wire is an electromagnet.


36 Magnetism

Iron filings sprinkled on paper reveal the magnetic field

configurations about

a.a current-carrying wire


36 Magnetism




b.a current-carrying loop


36 Magnetism




b.a current-carrying loop

c. a coil of loops


36 Magnetism

Sometimes a piece of iron is placed inside the coil of

an electromagnet.

The magnetic domains in the iron are induced into

alignment, increasing the magnetic field intensity.

Beyond a certain limit, the magnetic field in iron

“saturates,” so iron is not used in the cores of the

strongest electromagnets.


36 Magnetism

A superconducting electromagnet can generate a powerful

magnetic field indefinitely without using any power.

At Fermilab near Chicago, superconducting electromagnets

guide high-energy particles around the four-mile-

circumference accelerator.

Superconducting magnets can also be found in magnetic

resonance imaging (MRI) devices in hospitals.


36 Magnetism

Why does a current-carrying wire deflect a

magnetic compass?


36 Magnetism

A moving charge is deflected when it crosses

magnetic field lines but not when it travels

parallel to the field lines.

36.6 Magnetic Forces on Moving Charged Particles

36 Magnetism

If the charged particle moves in a magnetic field, the charged particle experiences

a deflecting force.

• This force is greatest when the particle moves in a direction perpendicular

to the magnetic field lines.

• At other angles, the force is less.

• The force becomes zero when the particle moves parallel to the field lines.

• The direction of the force is always perpendicular to both the magnetic

field lines and the velocity of the charged particle.


36 Magnetism

The deflecting force is different from other forces,

such as the force of gravitation between masses, the

electrostatic force between charges, and the force

between magnetic poles.

The force that acts on a moving charged particle acts

perpendicular to both the magnetic field and the

electron velocity.


36 Magnetism

The deflection of charged

particles by magnetic fields

provides a TV picture.

Charged particles from outer

space are deflected by Earth’s

magnetic field, which reduces the

intensity of cosmic radiation.

A much greater reduction in

intensity results from the

absorption of cosmic rays in the

atmosphere.


36 Magnetism

What happens when a charged particle moves in

a magnetic field?


36 Magnetism

Since a charged particle moving through a magnetic

field experiences a deflecting force, a current of

charged particles moving through a magnetic field

also experiences a deflecting force.

36.7 Magnetic Forces on Current-Carrying Wires

36 Magnetism

If the particles are inside a wire, the wire will also move.

• If the direction of current in the wire is reversed, the deflecting force acts in

the opposite direction.

• The force is maximum when the current is perpendicular to the magnetic

field lines.

• The direction of force is along neither the magnetic field lines nor the

direction of current.

• The force is perpendicular to both field lines and current, and it is a

sideways force.


36 Magnetism

Just as a current-carrying wire will deflect a magnetic

compass, a magnet will deflect a current-carrying wire.

Both cases show different effects of the same phenomenon.

The discovery that a magnet exerts a force on a current-

carrying wire created much excitement.

People began harnessing this force for useful purposes—

electric meters and electric motors.


36 Magnetism

think!

What law of physics tells you that if a current-carrying wire

produces a force on a magnet, a magnet must produce a force

on a current-carrying wire?


36 Magnetism

think!

What law of physics tells you that if a current-carrying wire

produces a force on a magnet, a magnet must produce a force

on a current-carrying wire?

Answer:

Newton’s third law, which applies to all forces in nature.


36 Magnetism

How is current affected by a magnetic field?


36 Magnetism

The principal difference between a galvanometer

and an electric motor is that in an electric motor, the

current is made to change direction every time the

coil makes a half revolution.

36.8 Meters to Motors

36 Magnetism

The simplest meter to detect electric current consists of a

magnetic needle on a pivot at the center of loops of

insulated wire.

When an electric current passes through the coil, each

loop produces its own effect on the needle.

A very small current can be detected. A sensitive current-

indicating instrument is called a galvanometer.


36 Magnetism

Common Galvanometers

A more common design employs more loops of wire

and is therefore more sensitive.

The coil is mounted for movement and the magnet is

held stationary.

The coil turns against a spring, so the greater the

current in its loops, the greater its deflection.


36 Magnetism

a. A common galvanometer consists of a stationary

magnet and a movable coil of wire.


36 Magnetism

a. A common galvanometer consists of a stationary

magnet and a movable coil of wire.

b. A multimeter can function as both an ammeter and a

voltmeter.


36 Magnetism

A galvanometer may be calibrated to measure current

(amperes), in which case it is called an ammeter.

Or it may be calibrated to measure electric potential (volts),

in which case it is called a voltmeter.


36 Magnetism

Electric Motors

If the design of the galvanometer is slightly modified, you have

an electric motor.

The principal difference is that in an electric motor, the current

changes direction every time the coil makes a half revolution.

After it has been forced to rotate one half revolution, it

overshoots just in time for the current to reverse.

The coil is forced to continue another half revolution, and so

on in cyclic fashion to produce continuous rotation.


36 Magnetism

In a simple DC motor, a permanent magnet produces a magnetic field in a

region where a rectangular loop of wire is mounted.

• The loop can turn about an axis.

• When a current passes through the loop, it flows in opposite

directions in the upper and lower sides of the loop.

• The loop is forced to move as if it were a galvanometer.


36 Magnetism

• The current is reversed during each half revolution by

means of stationary contacts on the shaft.

• The parts of the wire that brush against these contacts

are called brushes.

• The current in the loop alternates so that the forces in

the upper and lower regions do not change directions as

the loop rotates.

• The rotation is continuous as long as current is supplied.


36 Magnetism

Larger motors, DC or AC, are made by replacing the

permanent magnet with an electromagnet, energized by

the power source.

Many loops of wire are wound about an iron cylinder,

called an armature, which then rotates when energized

with electric current.


36 Magnetism

think!

How is a galvanometer similar to a simple electric motor? How

do they fundamentally differ?


36 Magnetism

think!

How is a galvanometer similar to a simple electric motor? How

do they fundamentally differ?

Answer:

A galvanometer and a motor are similar in that they both employ coils

positioned in magnetic fields. When current passes through the coils, forces

on the wires rotate the coils. The fundamental difference is that the

maximum rotation of the coil in a galvanometer is one half turn, whereas in

a motor the coil (armature) rotates through many complete turns. In the

armature of a motor, the current is made to change direction with each half

turn of the armature.


36 Magnetism

What is the main difference between a

galvanometer and an electric motor?


36 Magnetism

A compass points northward because Earth

itself is a huge magnet.

36.9 Earth’s Magnetic Field

36 Magnetism

The compass aligns with the magnetic

field of Earth, but the magnetic poles of

Earth do not coincide with the

geographic poles.

The magnetic pole in the Northern

Hemisphere, for example, is located

some 800 kilometers from the

geographic North Pole.

This means that compasses do not

generally point to true north.

The discrepancy is known as the

magnetic declination.


36 Magnetism

Moving Changes Within Earth

The configuration of Earth’s magnetic field is like that of a

strong bar magnet placed near the center of Earth.

Earth is not a magnetized chunk of iron like a bar magnet. It is

simply too hot for individual atoms to remain aligned.


36 Magnetism

Currents in the molten part of Earth beneath

the crust provide a better explanation for

Earth’s magnetic field.

Most geologists think that moving charges

looping around within Earth create its

magnetic field. Because of Earth’s great size,

the speed of charges would have to be less

than one millimeter per second to account for

the field.

Another possible cause for Earth’s magnetic

field is convection currents from the rising

heat of Earth’s core. Perhaps such

convection currents combined with the

rotational effects of Earth produce Earth’s

magnetic field.


36 Magnetism

Magnetic Field Reversals

The magnetic field of Earth is not stable. Magnetic rock strata

show that it has flip-flopped throughout geologic time.

Iron atoms in a molten state align with Earth’s magnetic field.

When the iron solidifies, the direction of Earth’s field is

recorded by the orientation of the domains in the rock.


36 Magnetism

On the ocean floor at mid-ocean ridges, continuous

eruption of lava produces new seafloor.

This new rock is magnetized by the existing magnetic field.

Alternating magnetic stripes show that there have been

times when the Earth’s magnetic field has dropped to zero

and then reversed.


36 Magnetism

More than 20 reversals have taken place in the past 5 million

years. The most recent occurred 780,000 years ago.

We cannot predict when the next reversal will occur because

the reversal sequence is not regular.

Recent measurements show a decrease of over 5% of

Earth’s magnetic field strength in the last 100 years. If this

change is maintained, there may be another field reversal

within 2000 years.


36 Magnetism

Why does a magnetic compass

point northward?


36 Magnetism

1. For magnets, like poles repel each other and unlike poles

a. also repel each other.

b. attract each other.

c. can disappear into nothingness.

d. can carry a lot of energy.


36 Magnetism

1. For magnets, like poles repel each other and unlike poles

a. also repel each other.

b. attract each other.

c. can disappear into nothingness.

d. can carry a lot of energy.

Answer: B


36 Magnetism

2. The space surrounding a magnet is known as a(n)

a. electric field.

b. magnetic field.

c. magnetic pole.

d. electric pole.


36 Magnetism

2. The space surrounding a magnet is known as a(n)

a. electric field.

b. magnetic field.

c. magnetic pole.

d. electric pole.

Answer: B


36 Magnetism

3. Moving electric charges are surrounded by

a. only electric fields.

b. only magnetic fields.

c. both magnetic and electric fields.

d. nothing.


36 Magnetism

3. Moving electric charges are surrounded by

a. only electric fields.

b. only magnetic fields.

c. both magnetic and electric fields.

d. nothing.

Answer: C


36 Magnetism

4. The magnetic domains in a magnet produce a weaker magnet

when the

a. magnet is heated.

b. magnet is brought in contact with steel.

c. magnet is brought in contact with another strong magnet.

d. magnetic domains are all in alignment.


36 Magnetism

4. The magnetic domains in a magnet produce a weaker magnet

when the

a. magnet is heated.

b. magnet is brought in contact with steel.

c. magnet is brought in contact with another strong magnet.

d. magnetic domains are all in alignment.

Answer: A


36 Magnetism

5. The magnetic field lines about a current-carrying wire form

a. circles.

b. radial lines.

c. eddy currents.

d. spirals.


36 Magnetism

5. The magnetic field lines about a current-carrying wire form

a. circles.

b. radial lines.

c. eddy currents.

d. spirals.

Answer: A


36 Magnetism

6. A magnetic force cannot act on an electron when it moves

a. perpendicular to the magnetic field lines.

b. at an angle between 90° and 180° to the magnetic field lines.

c. at an angle between 45° and 90° to the magnetic field lines.

d. parallel to the magnetic field lines.


36 Magnetism

6. A magnetic force cannot act on an electron when it moves

a. perpendicular to the magnetic field lines.

b. at an angle between 90° and 180° to the magnetic field lines.

c. at an angle between 45° and 90° to the magnetic field lines.

d. parallel to the magnetic field lines.

Answer: D


36 Magnetism

7. A magnetic force acts most strongly on a current-carrying wire

when it is

a. parallel to the magnetic field.

b. perpendicular to the magnetic field.

c. at an angle to the magnetic field that is less than 90°.

d. at an angle to the magnetic field that is more than 90°.


36 Magnetism

7. A magnetic force acts most strongly on a current-carrying wire

when it is

a. parallel to the magnetic field.

b. perpendicular to the magnetic field.

c. at an angle to the magnetic field that is less than 90°.

d. at an angle to the magnetic field that is more than 90°.

Answer: B


36 Magnetism

8. Your teacher gives you two electrical machines and asks you to identify

which is a galvanometer and which is an electric motor. How can you tell the

difference between the two?

a. In a galvanometer, the current changes direction every time the


b. In an electric motor, the current changes direction every time the


c. In a galvanometer, the current changes direction every time the

coil makes a whole revolution.

d. In an electric motor, the current changes direction every time the coil

makes a whole revolution.


36 Magnetism

8. Your teacher gives you two electrical machines and asks you to identify

which is a galvanometer and which is an electric motor. How can you tell the

difference between the two?

a. In a galvanometer, the current changes direction every time the


b. In an electric motor, the current changes direction every time the


c. In a galvanometer, the current changes direction every time the

coil makes a whole revolution.

d. In an electric motor, the current changes direction every time the coil

makes a whole revolution.

Answer: B


36 Magnetism

9. The magnetic field surrounding Earth

a. is caused by magnetized chunks of iron in Earth’s crust.

b. is likely caused by magnetic declination.

c. never changes.

d. is likely caused by electric currents in its interior.


36 Magnetism

9. The magnetic field surrounding Earth

a. is caused by magnetized chunks of iron in Earth’s crust.

b. is likely caused by magnetic declination.

c. never changes.

d. is likely caused by electric currents in its interior.

Answer: D


37 Electromagnetic Induction

Magnetism can produce

electric current, and

electric current can

produce magnetism.


In 1831, two physicists, Michael Faraday in England and Joseph Henry in the United States, independently discovered that magnetism could produce an electric current in a wire. Their discovery was to change the world by making electricity so commonplace that it would power industries by day and light up cities by night.


Electric current can be produced in a wire by simply

moving a magnet into or out of a wire coil.

37.1 Electromagnetic Induction


No battery or other voltage

source was needed to produce a

current—only the motion of a

magnet in a coil or wire loop.

Voltage was induced by the

relative motion of a wire with

respect to a magnetic field.



The production of voltage depends only on the relative motion

of the conductor with respect to the magnetic field.

Voltage is induced whether the magnetic field moves past a

conductor, or the conductor moves through a magnetic field.

The results are the same for the same relative motion.



The amount of voltage induced depends on how quickly the

magnetic field lines are traversed by the wire.

• Very slow motion produces hardly any voltage at all.

• Quick motion induces a greater voltage.

Increasing the number of loops of wire that move in a

magnetic field increases the induced voltage and the current

in the wire.

Pushing a magnet into twice as many loops will induce twice

as much voltage.



Twice as many loops as another means twice as much

voltage is induced. For a coil with three times as many loops,

three times as much voltage is induced.



We don’t get something

(energy) for nothing by simply

increasing the number of loops

in a coil of wire.

Work is done because the

induced current in the loop

creates a magnetic field that

repels the approaching magnet.

If you try to push a magnet into

a coil with more loops, it

requires even more work.



Work must be done to move the magnet.

a.Current induced in the loop produces a magnetic field

(the imaginary yellow bar magnet), which repels the bar magnet.



Work must be done to move the magnet.

a.Current induced in the loop produces a magnetic field

(the imaginary yellow bar magnet), which repels the bar magnet.

b.When the bar magnet is pulled away, the induced

current is in the opposite direction and a magnetic field

attracts the bar magnet.



The law of energy conservation applies here.

The force that you exert on the magnet multiplied by the

distance that you move the magnet is your input work.

This work is equal to the energy expended (or possibly

stored) in the circuit to which the coil is connected.



If the coil is connected to a resistor, more induced voltage in

the coil means more current through the resistor.

That means more energy expenditure.

Inducing voltage by changing the magnetic field around a

conductor is electromagnetic induction.



How can you create a current using a

wire and a magnet?



Faraday’s law states that the induced voltage in

a coil is proportional to the product of the

number of loops, the cross-sectional area of

each loop, and the rate at which the magnetic

field changes within those loops.

37.2 Faraday’s Law


Faraday’s law describes the relationship between

induced voltage and rate of change of a magnetic field:

The induced voltage in a coil is proportional to the product

of the number of loops, the cross-sectional area of each

loop, and the rate at which the magnetic field changes

within those loops.



The current produced by electromagnetic induction

depends upon

• the induced voltage,

• the resistance of the coil, and the circuit to

which it is connected.

For example, you can plunge a magnet in and out of

a closed rubber loop and in and out of a closed loop

of copper.

The voltage induced in each is the same but the

current is quite different—a lot in the copper but

almost none in the rubber.



think!

If you push a magnet into a coil connected to a resistor you’ll

feel a resistance to your push. For the same pushing speed,

why is this resistance greater in a coil with more loops?



think!

If you push a magnet into a coil connected to a resistor you’ll

feel a resistance to your push. For the same pushing speed,

why is this resistance greater in a coil with more loops?

Answer:

More work is required because more voltage is induced, producing more

current in the resistor and more energy transfer. When the magnetic fields

of two magnets overlap, the two magnets are either forced together or

forced apart. When one of the fields is induced by motion of the other, the

polarity of the fields is always such as to force the magnets apart. Inducing

more current in more coils increases the induced magnetic field and the

resistive force.



What does Faraday’s law state?



Whereas a motor converts electrical energy into

mechanical energy, a generator converts

mechanical energy into electrical energy.

37.3 Generators and Alternating Current


A current can be generated by plunging a magnet into and out

of a coil of wire.

• As the magnet enters, the magnetic field strength inside

the coil increases and induced voltage in the coil is

directed one way.

• As the magnet leaves, the magnetic field strength

diminishes and voltage is induced in the

opposite direction.

• Greater frequency of field change induces

greater voltage.

• The frequency of the alternating voltage is the frequency

of the changing magnetic field within the loop.



It is more practical to move the coil instead of moving the

magnet, by rotating the coil in a stationary magnetic field.

A machine that produces electric current by rotating a coil

within a stationary magnetic field is called a generator.

A generator is essentially the opposite of a motor, converting

mechanical energy into electrical energy.



Simple Generators

Starting perpendicular to the field, the loop has the largest

number of lines inside.

As it rotates, the loop encircles fewer of the field lines until it

lies along the field lines, when it encloses none at all.

As rotation continues, it encloses more field lines, reaching a

maximum when it has made a half revolution.

The magnetic field inside the loop changes in cyclic fashion.















As the loop rotates, the magnitude and direction of the induced

voltage (and current) change. One complete rotation of the

loop produces one complete cycle in voltage (and current).



The voltage induced by the

generator alternates, and the current

produced is alternating current (AC).

The current changes magnitude and

direction periodically.

The standard AC in North America

changes magnitude and direction

during 60 complete cycles per

second—60 hertz.



Complex Generators

The generators used in power plants are much more

complex than the model discussed here.

Huge coils made up of many loops of wire are wrapped on

an iron core, to make an armature much like the armature of

a motor.

They rotate in the very strong magnetic fields of powerful

electromagnets.



The armature is connected externally to an assembly of paddle wheels

called a turbine.

While wind or falling water can be used to produce rotation of the turbine,

most commercial generators are driven by moving steam.

At the present time, a fossil fuel or nuclear fuel is used as the energy

source for the steam.



An energy source of some kind is required to operate

a generator.

Some fraction of energy from the source is converted to

mechanical energy to drive the turbine.

The generator converts most of this to electrical energy.

Some people think that electricity is a source of energy. It is

not. It is a form of energy that must have a source.



How is a generator different from a motor?



Moving charges experience a force that is

perpendicular to both their motion and the

magnetic field they traverse.

37.4 Motor and Generator Comparison


An electric current is deflected in a magnetic field,

which underlies the operation of the motor.

Electromagnetic induction underlies the operation of

a generator.

We will call the deflected wire the motor effect and

the law of induction the generator effect.



a. When a current moves to the right, there is a force on the

electrons, and the wire is tugged upward.



a. When a current moves to the right, there is a force on the

electrons, and the wire is tugged upward.

b. When a wire with no current is moved downward, the

electrons in the wire experience a force, creating current.



The motor effect occurs when a current moves through a

magnetic field.

• The magnetic field creates a perpendicular upward

force on the electrons.

• Because the electrons can’t leave the wire, the

entire wire is tugged along with the electrons.



In the generator effect, a wire with no current is moved

downward through a magnetic field.

• The electrons in this wire experience a force

perpendicular to their motion, which is along the wire.

• A current begins to flow.



A striking example of a device functioning as both motor and

generator is found in hybrid automobiles.

• When extra power for accelerating or hill climbing is

needed, this device draws current from a battery and

acts as a motor.

• Braking or rolling downhill causes the wheels to exert a

torque on the device so it acts as a generator and

recharges the battery.

• The electrical part of the hybrid engine is both a motor

and a generator.



How does a magnetic field affect a

moving charge?



A transformer works by inducing a changing

magnetic field in one coil, which induces an

alternating current in a nearby second coil.

37.5 Transformers


Consider a pair of coils, side by side, one connected to a

battery and the other connected to a galvanometer.

It is customary to refer to the coil connected to the power

source as the primary (input), and the other as the

secondary (output).

37.5 Transformers


As soon as the switch is closed in the primary and current

passes through its coil, a current occurs in the secondary.

When the primary switch is opened, a surge of current again

registers in the secondary but in the opposite direction.

Whenever the primary switch is opened or closed, voltage is

induced in the secondary circuit.

37.5 Transformers


The magnetic field that builds up around the primary

extends into the secondary coil.

Changes in the magnetic field of the primary are sensed

by the nearby secondary.

These changes of magnetic field intensity at the

secondary induce voltage in the secondary, in accord

with Faraday’s law.

37.5 Transformers


If we place an iron core inside

both coils, alignment of its

magnetic domains intensifies the

magnetic field within the primary.

The magnetic field is

concentrated in the core, which

extends into the secondary, so

the secondary intercepts more

field change.

The galvanometer will show

greater surges of current when

the switch of the primary is

opened or closed.

37.5 Transformers


Instead of opening and closing a switch to produce the

change of magnetic field, an alternating current can

power the primary.

Then the rate of magnetic field changes in the primary

(and in the secondary) is equal to the frequency of the

alternating current.

Now we have a transformer, a device for increasing or

decreasing voltage through electromagnetic induction.

37.5 Transformers


If the iron core forms a complete loop, guiding all

magnetic field lines through the secondary, the

transformer is more efficient.

All the magnetic field lines within the primary are

intercepted by the secondary.

37.5 Transformers


Voltage

Voltages may be stepped up or stepped down with a

transformer.

Suppose the primary consists of one loop connected to a

1-V alternating source.

• Consider the arrangement of a one-loop secondary

that intercepts all the changing magnetic field lines

of the primary.

• Then a voltage of 1 V is induced in the secondary.

37.5 Transformers


If another loop is wrapped around the core, the

induced voltage will be twice as much, in accord

with Faraday’s law.

If the secondary has a hundred times as many turns

as the primary, then a hundred times as much

voltage will be induced.

This arrangement of a greater number of turns on

the secondary than on the primary makes up a step-

up transformer.

Stepped-up voltage may light a neon sign or

operate the picture tube in a television receiver.

37.5 Transformers


a. 1 V induced in the secondary equals the voltage of the primary.

37.5 Transformers



b. 1 V is induced in the added secondary also because it intercepts the

same magnetic field change from the primary.

37.5 Transformers



b. 1 V is induced in the added secondary also because it intercepts the

same magnetic field change from the primary.

c. 2 V is induced in a single two-turn secondary.

37.5 Transformers


If the secondary has fewer turns than the primary, the

alternating voltage in the secondary will be lower than that in

the primary.

The voltage is said to be stepped down.

If the secondary has half as many turns as the primary, then

only half as much voltage is induced in the secondary.

37.5 Transformers


The relationship between primary and secondary voltages with

respect to the relative number of turns is

37.5 Transformers


A practical transformer uses many coils. The relative

numbers of turns in the coils determines how much the

voltage changes.

37.5 Transformers


Power

You don’t get something for nothing with a transformer that

steps up the voltage, for energy conservation is always in

control.

The transformer actually transfers energy from one coil to the

other. The rate at which energy is transferred is the power.

The power used in the secondary is supplied by the primary.

37.5 Transformers


The primary gives no more power than the secondary uses.

If the slight power losses due to heating of the core are

neglected, then the power going in equals the power

coming out.

Electric power is equal to the product of voltage and current:

(voltage × current)primary = (voltage × current)secondary

37.5 Transformers


If the secondary has more

voltage, it will have less current

than the primary.

If the secondary has less

voltage, it will have more

current than the primary.

37.5 Transformers


This transformer lowers 120 V to

6 V or 9 V. It also converts AC to

DC by means of a diode that acts

as a one-way valve.

37.5 Transformers


think! When the switch of the primary is opened or

closed, the galvanometer in the secondary

registers a current. But when the switch remains

closed, no current is registered on the

galvanometer of the secondary. Why?

37.5 Transformers


think! When the switch of the primary is opened or

closed, the galvanometer in the secondary

registers a current. But when the switch remains

closed, no current is registered on the

galvanometer of the secondary. Why?

Answer:

A current is only induced in a coil when there is a

change in the magnetic field passing through it.

When the switch remains in the closed position,

there is a steady current in the primary and a

steady magnetic field about the coil.

37.5 Transformers


think! If the voltage in a transformer is stepped up, then the current is

stepped down. Ohm’s law says that increased voltage will produce

increased current. Is there a contradiction here, or does Ohm’s Law

not apply to transformers?

37.5 Transformers


think! If the voltage in a transformer is stepped up, then the current is

stepped down. Ohm’s law says that increased voltage will produce

increased current. Is there a contradiction here, or does Ohm’s Law

not apply to transformers?

Answer:

Ohm’s law still holds, and there is no contradiction. The voltage

induced across the secondary circuit, divided by the load

(resistance) of the secondary circuit, equals the current in the

secondary circuit. The current is stepped down in comparison with

the larger current that is drawn in the primary circuit.

37.5 Transformers


How does a transformer work?

37.5 Transformers


Almost all electric energy sold today is in the

form of alternating current because of the ease

with which it can be transformed from one

voltage to another.

37.6 Power Transmission


Power is transmitted great

distances at high voltages and

correspondingly low currents.

This reduces energy losses

due to the heating of the wires.

Power may be carried from

power plants to cities at about

120,000 volts or more, stepped

down to about 2400 volts in the

city, and finally stepped down

again to 120 volts.



Power transmission uses transformers to increase voltage for

long-distance transmission and decrease it before it reaches

your home.



Energy, then, is transformed from

one system of conducting wires

to another by electromagnetic

induction.

The same principles account for

sending energy from a radio-

transmitter antenna to a radio

receiver many kilometers away.

The effects of electromagnetic

induction are very far-reaching.



Why is almost all electrical energy sold today in

the form of alternating current?



A magnetic field is created in any region of space in

which an electric field is changing with time.

37.7 Induction of Electric and Magnetic Fields


Electromagnetic induction has thus far been discussed in

terms of the production of voltages and currents.

The more fundamental way to look at it is in terms of the

induction of electric fields.

The electric fields, in turn, give rise to voltages and currents.



Induction takes place whether or not a conducting wire or

any material medium is present.

Faraday’s law states that an electric field is created in

any region of space in which a magnetic field is changing

with time.

The magnitude of the created electric field is proportional

to the rate at which the magnetic field changes.

The direction of the created electric field is at right angles

to the changing magnetic field.



If electric charge happens to be present where the

electric field is created, this charge will experience a

force.

• For a charge in a wire, the force could cause it to

flow as current, or to push the wire to one side.

• For a charge in the chamber of a particle

accelerator, the force can accelerate the charge to

high speeds.



There is a second effect, which is

the counterpart to Faraday’s law.

It is just like Faraday’s law, except

that the roles of electric and

magnetic fields are interchanged.



A magnetic field is created in any region of space in

which an electric field is changing with time.

• The magnitude of the magnetic field is proportional

to the rate at which the electric field changes.

• The direction of the created magnetic field is at

right angles to the changing electric field.



How can an electric field

create a magnetic field?



An electromagnetic wave is composed of

oscillating electric and magnetic fields that

regenerate each other.

37.8 Electromagnetic Waves


Shake the end of a stick back and forth in still water and you

will produce waves on the water surface.

Similarly shake a charged rod back and forth in empty space

and you will produce electromagnetic waves in space.



The shaking charge can be considered an electric current.

• A magnetic field surrounds an electric current.

• A changing magnetic field surrounds a changing

electric current.

• A changing magnetic field creates a changing electric

field.

• The changing electric field creates a changing

magnetic field.



An electromagnetic wave is composed of oscillating electric and magnetic fields that

regenerate each other.

No medium is required. The oscillating fields emanate from the vibrating charge.

At any point on the wave, the electric field is perpendicular to the magnetic field.

Both are perpendicular to the direction of motion of the wave.



Speed of Electromagnetic Waves

For electromagnetic radiation, there is only

one speed—the speed of light—no matter

what the frequency or wavelength or intensity

of the radiation.

The changing electric field induces a

magnetic field. The changing magnetic field

acts back to induce an electric field.

Only one speed could preserve this

harmonious balance of fields.



If the wave traveled at less than the speed of light, the fields

would rapidly die out.

The electric field would induce a weaker magnetic field, which

would induce a still weaker electric field.

If the wave traveled at more than the speed of light, the fields

would build up in a crescendo of ever greater magnitudes.

At some critical speed, however, mutual induction continues

indefinitely, with neither a loss nor a gain in energy.



From his equations of electromagnetic induction,

Maxwell calculated the value of this critical speed

and found it to be 300,000 kilometers per second.

He used only the constants in his equations

determined by simple laboratory experiments with

electric and magnetic fields.

He didn’t use the speed of light. He found the speed

of light!



Nature of Light

Maxwell quickly realized that he had discovered the

solution to one of the greatest mysteries of the

universe—the nature of light.

Maxwell realized that radiation of any frequency would

propagate at the same speed as light.



This radiation includes radio waves, which can be

generated and received by antennas.

• A rotating device in the sending antenna alternately

charges the upper and lower parts of the antenna

positively and negatively.

• The charges accelerating up and down the antenna

transmit electromagnetic waves.

• When the waves hit a receiving antenna, the electric

charges inside vibrate in rhythm with the variations of

the field.



What makes up an

electromagnetic wave?



1. A voltage will be induced in a wire loop when the magnetic field within

that loop

a. changes.

b. aligns with the electric field.

c. is at right angles to the electric field.

d. converts to magnetic energy.



1. A voltage will be induced in a wire loop when the magnetic field within

that loop

a. changes.

b. aligns with the electric field.

c. is at right angles to the electric field.

d. converts to magnetic energy.

Answer: A



2. If you change the magnetic field in a closed loop of wire, you induce in

the loop a

a. current.

b. voltage.

c. electric field.

d. all of these



2. If you change the magnetic field in a closed loop of wire, you induce in

the loop a

a. current.

b. voltage.

c. electric field.

d. all of these

Answer: D



3. The essential concept in an electric motor and a generator is

a. Coulomb’s law.

b. Ohm’s law.

c. Faraday’s law.

d. Newton’s second law.



3. The essential concept in an electric motor and a generator is

a. Coulomb’s law.

b. Ohm’s law.

c. Faraday’s law.

d. Newton’s second law.

Answer: C



4. A motor and a generator are

a. similar devices.

b. very different devices with different applications.

c. only found in hybrid cars.

d. energy sources.



4. A motor and a generator are

a. similar devices.

b. very different devices with different applications.

c. only found in hybrid cars.

d. energy sources.

Answer: A



5. A step-up transformer in an electrical circuit can

a. increase voltage.

b. decrease energy.

c. increase current.

d. increase energy.



5. A step-up transformer in an electrical circuit can

a. increase voltage.

b. decrease energy.

c. increase current.

d. increase energy.

Answer: A



6. To keep heat losses down when power is carried across the

countryside, it is best that current in the wires is

a. low.

b. high.

c. not too low and not too high.

d. replaced with voltage.



6. To keep heat losses down when power is carried across the

countryside, it is best that current in the wires is

a. low.

b. high.

c. not too low and not too high.

d. replaced with voltage.

Answer: A



7. According to James Clerk Maxwell, if the magnitude of the created magnetic

field increases, the change in the electric field will

a. stay the same.

b. increase.

c. decrease.

d. always disappear.



7. According to James Clerk Maxwell, if the magnitude of the created magnetic

field increases, the change in the electric field will

a. stay the same.

b. increase.

c. decrease.

d. always disappear.

Answer: B



8. Electricity and magnetism connect to form

a. mass.

b. energy.

c. ultra high-frequency sound.

d. light.



8. Electricity and magnetism connect to form

a. mass.

b. energy.

c. ultra high-frequency sound.

d. light.

Answer: D


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Electrostatics involves electric charges, the forces between … · 2013-03-25 · 32 Electrostatics Electrostatics, or electricity at rest, involves electric charges, the forces