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ELECTROSTATICS Electrostatics (ie. static electricity) is the study of electrical forces between stationary charges or charges that are hardly moving. Electric charge is a concept that is defined in terms of the effect it produces. It has no mass, color, length, or width. It is quantified based on multiples of a fundamental charge e - . Examples of electric charge are the nervous system of the human body and lightning.

ELECTROSTATICS Electrostatics (ie. static electricity) …hrsbstaff.ednet.ns.ca/tskelhor/Physics 12/Notes... · ELECTROSTATICS Electrostatics (ie. static electricity) is the study

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Page 1: ELECTROSTATICS Electrostatics (ie. static electricity) …hrsbstaff.ednet.ns.ca/tskelhor/Physics 12/Notes... · ELECTROSTATICS Electrostatics (ie. static electricity) is the study

ELECTROSTATICS

Electrostatics (ie. static electricity) is the study of

electrical forces between stationary charges or

charges that are hardly moving.

Electric charge is a concept that is defined in terms of

the effect it produces. It has no mass, color, length,

or width. It is quantified based on multiples of a

fundamental charge e-.

Examples of electric charge are the nervous system of

the human body and lightning.

Page 2: ELECTROSTATICS Electrostatics (ie. static electricity) …hrsbstaff.ednet.ns.ca/tskelhor/Physics 12/Notes... · ELECTROSTATICS Electrostatics (ie. static electricity) is the study

Atoms are electrically neutral. They consist of

protons, electrons, and neutrons.

Electrons in an atom occupy different orbits. Those in

orbits near the nucleus are tightly bound to it by their

electrical attraction. Those farthest from the nucleus

are loosely bound.

Since the outer electrons are loosely bound they may

be transferred to other atoms (chemically or by other

means).

If an atom has lost an electron, it becomes overall

positively charged and is called a cation.

If an atom has gained an electron, it becomes overall

negatively charged and is called an anion.

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Transfer of Electric Charge

An atom holds on to its electrons by the force of

electrical attraction to its oppositely charged nucleus.

The electrostatic series table lists many of the

substances that can be charged by friction.

The substance that is higher in the table becomes

negatively charged, while the other substance becomes

positively charged.

Electrostatic Series

- holds electrons tightly

sulphur

brass

copper

ebonite

paraffin wax

silk

lead

fur

wool

glass

+ holds electrons loosely

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Separation of Charge

Induced charge separation is caused by the presence

of a negative or positive distribution of charge on an

object.

Negative distribution of charge:

pith ball metal leaf electroscope

There is a repulsion between the object and the similar

charge on the far side. However, the strength of the

electric forces between similar and opposite charges

depends on the distance between the charges. As the

distance increases, the magnitude of the force of

attraction or repulsion decreases.

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Charge by Contact

When a negatively charged object (ebonite rod) is

touched to a neutral object, some of the excess

electrons on the rod, repelled by their neighbouring

electrons, move over to the neutral object. Both

objects have some of the excess electrons and both

become negatively charged.

If the object is positively charged (glass rod), some of

the electrons on the neutral object are attracted onto

the glass rod to reduce its deficit of electrons. Both

objects share the deficit of electrons and both

become positively charged.

An object charged by contact has the same sign as the

charging rod.

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Charge by Induction

An object charged by induction has a charge opposite

to that of the charging rod. (grounding)

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Charge Conservation

• if a glass rod is rubbed with silk, a positive charge

appears on the rod. Measurement shows that a

negative charge of equal magnitude appears on the

silk.

• this suggests that rubbing does not create charge

but only transfers it from one body to another.

• the hypothesis of charge conservation was put

forward by Benjamin Franklin and no exception has

been found either at the macroscopic or subatomic

level.

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The existence of electric charge and of charge

transfer can be demonstrated in the lab or classroom.

These activities suggest the opposite charges attract

and like charges repel.

This is the basic law of electrostatics.

The SI unit of charge is called the Coulomb.

1 C = the charge on 6.25 x 1018 protons

therefore, the charge on one proton is:

e = 1.60 x 10-19 C

Coulomb used a torsion balance similar to that used by

Cavendish in his study of gravitational forces. Coulomb

would bring small charged spheres towards a charged

sphere on the torsion balance and it would rotate.

The magnitude of the electrical force that one charge

exerts on another charge was determined in 1788:

221

r

qkqF e =

This force is also known as the Coulomb force.

k = 9.0 x 109 Nm2/C2

q = charge in Coulombs

r = distance in m

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Ex. An object A has a positive charge of 6.0 x 10-6 C.

Object B, carrying a positive charge of 3.0 x 10-6 C is

0.030 m away.

a) calculate the force on A.

b) what would be the force if the charge on B were

negative?

Ex. An object A with +6.0 x 10-6 C charge, has two

other charges nearby.

Object B, -3.0 x 10-6 C is 0.040 m to the right. Object

C, +1.5 x 10-6 C, is 0.030 m below. What is the net

force on A?

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Electric Fields (p.643)

When you bring an ebonite rod close to a neutral pith

ball, the pith ball is attracted to the rod - charge

separation.

This is an example of an action - at - a - distance force.

SO, how does a charge become aware that there is

another charge near it?

Faraday (1792 - 1867) proposed the concept of a

FIELD.

A field is a property of space. An object influences

the space around it setting up an electric field. This

field in turn exerts a force on other objects located

within it.

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The direction of the electric field at any point in space

is:

the direction of the force on a positive test charge at

that point.

Page 12: ELECTROSTATICS Electrostatics (ie. static electricity) …hrsbstaff.ednet.ns.ca/tskelhor/Physics 12/Notes... · ELECTROSTATICS Electrostatics (ie. static electricity) is the study

Example.

-

P

+ P

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An electric field is defined in terms of the electric

force that acts on a charged particle at a point in

space.

+=q

ε: the electric field (N/C or V/m) F: the force (N)

q+: a small imaginary test charge (C)

The electric field is a property of the space that

surrounds the charge.

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Electrical Fields Inside and Surrounding a Conductor

Conductor: materials through which charged particles

move readily.

Charges on a conductor are spread apart as far as

possible until they come to rest in electrostatic

equilibrium. The result is that all the charges are on

the surface of a conductor and there is no net field on

the inside of the conductor.

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If the net electric field inside the conductor is zero,

there will be no net electric forces acting on the

charges along the surface.

The electric field will be perpendicular to the surface

at all points. If the shape of the surface is not

symmetrical, the charges will be concentrated near the

surfaces that are more pointed. The electric fields

are greater at these points. If the fields become very

large, the molecules of air near the sharp edge become

ionized and a spark occurs.

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Electrical Potential Energy

Two or more like charges have more electric potential

energy when pushed together than when they are apart

because work has been done to bring them together.

Oppositely charged particles have more electric

potential energy when pulled apart than when they are

close together.

Electric potential energy:

r

qkqE e

21=

units: Nm (J)

Electric potential difference, V, is the change in

electrical potential energy that occurs when a test

charge, q+ is in the field of any other charge.

r

kq

q

EV e == +

Units - J/C or V (scalar)

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1 volt is the change in electrical potential difference

that occurs when 1 Coulomb of charge experiences a

change in electrical potential energy of 1 Joule.

If A and B are two points in an electric field and a test

charge moves from A to B, then, regardless of the

actual path taken by q,

the change in electric = the work required to

potential energy of q, in move q from A to B,

moving from A to B against the electric field

d

V

FdW

qVqVE ABe

=

=−=∆

ε

This is the expression for the magnitude of the

electric field at any point in the space between two

large parallel plates. The electric field direction is

from the + plate to the - plate, in the direction of

decreasing potential.

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Volta (1800) developed the voltaic pile.

• A zinc disk and silver disk were used with a piece of cardboard soaked in salt water.

• When a wire is connected to the bottom zinc disk to the top silver disk, the pile produced repeated

sparks.

Before the pile, these sparks were created by friction

(this involved work).

The voltaic pile provided a continuous source of charge

flow.

Volta's pile is widely regarded as the first battery.

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Batteries generate a difference in electric potential

between two points.

The potential difference is the difference in voltage.

IE. A 12 V battery generates an electric field usually

via a chemical process. The potential difference

V+ � V-

Between the positive and negative terminals is 12 V.

In order to move a positive charge of 1 C from the -

to the + terminal of the battery, 12 J of work must be

done against the electric field.

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Conventional current �

Electron flow

Current flows from the positive terminal to the

negative terminal.

Electrons flow from the negative terminal to the

positive terminal.

e- : negative, therefore, it attracts to the positive

terminal.

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When Ben Franklin arbitrarily chose + and - charges, he

also determined the direction of current.

Current flows from regions of + charge to regions of -

charge. This is the convention followed to this day.

When JJ Thomson discovered the electron, more was

known about electric charge. Only e- flow in a current

carrying wire.

The directions of current and electron flow are

OPPOSITE!

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Current Electricity

When a conductor acquires an excess or deficit of

electrons, we say it has an electric charge. For this to

happen, electrons must be able to move.

When electric charges move from one place to another,

we say that they constitute an electric current.

In metals, electric current is defined as the amount of

charge that moves past a given point in a conductor per

second.

When a total charge, Q, flows through an area in a

time of t, the electric current, I, through the wire is:

t

qI = Units - Amperes (A)

1 A is the electric current when 1 C of charge moves

past a point in a conductor in 1 s.

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Electric Potential

Electrons move through a conductor and thus

constitute and electric current. What causes them to

move through the conductor? What pushes or pulls

them?

� Voltage pushes the current though the wire.

If there are two oppositely charged particles near one

another, work must be done on the particle to

overcome the electric force and pull it away from the

oppositely charged particle.

Charged particles moving in the presence of an electric

field and converting electric potential energy into some

other form of energy constitute an electric current.

The amount of work that is done on an electric charge

to move it through an electric field is equal to the

increase of electric potential energy, E, of the charge.

q

WV =

Electric potential difference, V, is often referred to

as "voltage".

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It is easier to measure current, I, through a potential

difference, V, for a time, t.

Therefore, electrical potential energy, E is

represented by:

VItE

Itq

qVE

===

To measure the potential difference, V, a voltmeter is

used.

As charges move from one point to another through a

conductor, they lose energy. This is the electric

potential difference.

The charges experience a decrease in electric potential

difference.