Electricity

All of us agree the importance of electricity in our daily lives.

But what is electricity?

Electric Charge

• Electric Charge and Electrical Forces:

• Electrons have a negative electrical charge.

• Protons have a positive electrical charge.

• These charges interact to create an electrical force.

– Like charges produce repulsive forces – so they repel each other (e.g. electron and electron or proton and proton repel each other).

– Unlike charges produce attractive forces – so they attract each other (e.g. electron and proton attract each other).

A very highly simplified model of an atom has most of the mass in a small, dense center called the nucleus. The nucleus has positively charged protons and neutral neutrons. Negatively charged electrons move around the nucleus at much greater distance. Ordinary atoms are neutral because there is a balance between the number of positively charged protons and negatively charged electrons.

– Electrostatic Charge:

• Electrons move from atom to atom to create ions.

–positively charge ions result from the loss of electrons and are called cations.

–Negatively charge ions result from the gain of electrons and are called anions.

(A) A neutral atom has no net charge because the numbers of electrons and protons are balanced. (B) Removing an electron produces a net positive charge; the charged atom is called a positive ion (cation). (C) The addition of an electron produces a net negative charge and a negative ion (anion).

Arbitrary numbers of protons (+) and electrons (-) on a comb and in hair (A) before and (B) after combing. Combing transfers electrons from the hair to the comb by friction, resulting in a negative charge on the comb and a positive charge on the hair.

• The charge on an ion is called an electrostatic charge.

• An object becomes electrostatically charged by–Friction,which transfers electrons between

two objects in contact,

–Contact with a charged body which results in the transfer of electrons,

–Induction which produces a charge redistribution of electrons in a material.

Charging by induction: The comb has become charged by friction, acquiring an excess of electrons. The paper (A) normally has a random distribution of (+) and (-) charges. (B) When the charged comb is held close to the paper, there is a reorientation of charges because of the repulsion of the charges. This leaves a net positive charge on the side close to the comb, and since unlike charges attract, the paper is attracted to the comb.

– Electrical Conductors and Insulators:

• Electrical conductors are materials that can move electrons easily.

– Good conductors include metals. Copper is the best electrical conductor.

• Electrical nonconductors (insulators) are materials that do not move electrons easily.

– Examples are wood, rubber etc.

• Semiconductors are materials that sometimes behave as conductors and sometimes behave as insulators.

Examples are silicon, arsenic, germanium.

• Measuring Electrical Charges:

– The fundamental charge is the electrical charge on an electron and has a magnitude of 1.6021892 X 10-19 C (Note that the electrical charge is measured in coulombs).

– A coulomb is the charge resulting from the transfer of 6.24 x 1018 of the charge carried by an electron.

– The magnitude of an electrical charge (q) is dependent upon how many electrons (n) have been moved to it or away from it.Mathematically,

q = n ewhere e is the fundamental charge.

• Coulomb’s law: Electrical force is proportional to the product of the

electrical charge and inversely proportional to the square of the distance. This is known as Coulomb’s law.Mathematically,

where,

• F is the force,

• k is a constant and has the value of 9.00 x 109 Newtonmeters2/coulomb2 (9.00 x 10 9 Nm2/C2),

• q1 represents the electrical charge of object 1 and q2 represents the electrical charge of object 2, and

• d is the distance between the two objects.

2

21

d

qqkF

• Force Fields:

– The condition of space around an object is changed by the presence of an electrical charge.

– The electrical charge produces a force field, that is called an electrical field since it is produced by electrical charge.

– A map of the electrical field can be made by bringing a positive test charge into an electrical field.

• When brought near a negative charge the test charge is attracted to the unlike charge and when brought near a positive charge the test charge is repelled.

• You can draw vector arrows to indicate the direction of the electrical field.

• This is represented by drawing lines of force or electrical field lines,

– These lines are closer together when the field is stronger and farther apart when it is weaker.

A positive test charge is used by convention to identify the properties of an electric field. The vector arrow points in the direction of the force that the test charge would experience.

Lines of force diagrams for (A) a negative charge and (B) a positive charge when the charges have the same magnitude as the test charge.

• Electrical Potential:– An electrical charge has an electrical field that surrounds

it.– In order to move a second charge through this field work

must be done.– Bringing a like charge particle into this field will require

work since like charges repel each other and bringing an opposite charged particle into the field will require work to keep the charges separated.

• In both of these cases the electrical potential is changed.

– The potential difference (PD) that is created by doing 1.00 joule of work in moving 1.00 coulomb of charge is defined as 1.00 volt.• A volt is a measure of the potential difference

between two points,• electric potential = work done,

charge Or, PD=W Q• The voltage of an electrical charge is the energy

transfer per coulomb.– The energy transfer can be measured by the work that is

done to move the charge or by the work that the charge can do because of the position of the field.

The falling water can do work in turning the water wheel only as long as the pump maintains the potential difference between the upper and lower reservoirs.

Electric Current

• Introduction:

– Electric current means a flow of charge in the same way that a water current flows.

– It is the charge that flows, and the current is defined as the flow of the charge.

• The Electric Circuit:An electrical circuit contains some device that acts as a source of energy as it gives charges a higher potential against an electrical field.

• The charges do work as they flow through the circuit to a lower potential.

• The charges flow through connecting wires to make a continuous path.

• A switch is a means of interrupting or completing the circuit.

– The source of the electrical potential is the voltage source.

A simple electric circuit has a voltage source (such as a generator or battery) that maintains the electrical potential, some device (such as a lamp or motor ) where work is done by the potential, and continuous pathways for the current to follow.

– Voltage is a measure of the potential difference between two places in a circuit.• Voltage is measured in joules/coloumb.

– The rate at which an electrical current (I) flows is the charge (q) that moves through a cross section of a conductor in a give unit of time (t),

I = q/t.

• the units of current are coulombs/second.• A coulomb/second is an ampere (amp).

A simple electric circuit carrying a current of 1.00 coulomb per second through a cross section of a conductor has a current of 1.00 amp.

• The Nature of Current:– Conventional current describes current as positive

charges that flow from the positive to the negative terminal of a battery.

– The electron current description is the opposite of the conventional current.

• The electron current describes current as a drift of negative charges that flow from the negative to the positive terminal of a battery.

• It is actually the electron current that moves charges.

A conventional current describes positive charges moving from the positive terminal (+) to the negative terminal (-). An electron current describes negative charges (-) moving from the negative terminal (-) to the positive terminal (+).

– The current that occurs when there is a voltage depends on:

• The number of electrons that are moved through the unit volume of the conducting material.

• The fundamental charge on each electron.

• The drift velocity which depends on the properties of the conducting material and the temperature.

• The cross-sectional area of the conducting wire.

– It is the electron field, and not the electrons, which does the work.• It is the electric field that accelerates electrons that

are already in the conducting material.

– It is important to understand that:• An electric potential difference establishes, at nearly

the speed of light, an electric field throughout a circuit.

• The field causes a net motion that constitutes a flow of charge.

• The average velocity of the electrons moving as a current is very slow, even thought he electric field that moves them travels with a speed close to the speed of light.

What is the nature of the electric current carried by these conducting lines? It is an electric field that moves at near the speed of light. The field causes a net motion of electrons that constitutes a flow of charge, a current.

(A) A metal conductor without a current has immovable positive ions surrounded by a swarm of randomly moving electrons.

(B) An electric field causes the electrons to shift positions, creating a separation charge as the electrons move with a zigzag motion from collisions with stationary positive ions and other electrons.

• Electrical Resistance:– Electrical resistance is the resistance to movement of

electrons being accelerated with an energy loss.• Materials have the property of reducing a current and that

is electrical resistance (R).

– Resistance is a ratio between the potential difference (V) between two points and the resulting current (I).

R = V/I

• The ratio of volts/amp is called an ohm ().

– The relationship between voltage, current, and resistance is:

V =I R

This is known as Ohms Law.

– The magnitude of the electrical resistance of a conductor depends on four variables:

• The length of the conductor.• The cross-sectional area of the conductor.• The material the conductor is made of.• The temperature of the conductor.

The four factors that influence the resistance of an electrical conductor are the length of the conductor, the cross-sectional area of the conductor, the material the conductor is made of, and the temperature of the conductor.

• Electrical Power and Electrical Work:– All electrical circuits have three parts in common.

• A voltage source.

• An electrical device

• Conducting wires.

– The work done (W) by a voltage source is equal to the work done by the electrical field in an electrical device,

Work = Power x Time.

• The electrical potential is measured in joules/coulomb and a quantity of charge is measured in coulombs, so the electrical work is measure in joules.

• A joule/second is a unit of power called the watt.

Power = current x potential

Or, P = I V

What do you suppose it would cost to run each of these appliances for one hour? (A) This light bulb is designed to operate on a potential difference of 120 volts and will do work at the rate of 100 W. (B) The finishing sander does work at the rate of 1.6 amp x 120 volts or 192 W. (C) The garden shredder does work at the rate of 8 amps x 120 volts, or 960 W.

This meter measures the amount of electric work done in the circuits, usually over a time period of a month. The work is measured in kWhr.

Magnetism

All of us are familiar with magnets. In a magnet we have magnetic poles – the north and the south pole.

– A North seeking pole is called the North Pole.– A South seeking pole is called the South Pole.

Like magnetic poles repel and unlike magnetic poles attract.

Every magnet has ends, or poles, about which the magnetic properties seem to be concentrated. As this

photo shows, more iron filings are attracted to the poles, revealing their location.

• Magnetic Fields:

– A magnet that is moved in space near a second magnet experiences a magnetic field.

• A magnetic field can be represented by field lines.

– The strength of the magnetic field is greater where the lines are closer together and weaker where they are farther apart.

These lines are a map of the magnetic field around a bar magnet. The needle of a magnetic compass will follow the lines, with the north end showing the direction of the field.

• The Source of Magnetic Fields:

– Permanent Magnets:• Moving electrons produce magnetic fields.

• In most materials these magnetic fields cancel one another and neutralize the overall magnetic effect.

• In other materials such as iron, cobalt, and nickel, the atoms behave as tiny magnets because of certain orientations of the electrons inside the atom.

– These atoms are grouped in a tiny region called the magnetic domain.

Our Earth is a big magnet.

• The Earth’s magnetic field is thought to originate with moving charges.

• The core is probably composed of iron and nickel, which flows as the Earth rotates, creating electrical currents that result in the Earth’s magnetic field.

The earth's magnetic field. Note that the magnetic north pole and the geographic North Pole are not in the same place. Note also that the magnetic north pole acts as if the south pole of a huge bar magnet were inside the earth. You know that it must be a magnetic south pole since the north end of a magnetic compass is attracted to it and opposite poles attract.

A bar magnet cut into halves always makes new, complete magnets with both a north and a south pole. The poles always come in pairs. You can not separate a pair into single poles.

Electric Currents

and

Magnetism

Oersted discovered that a compass needle below a wire (A) pointed north when there was not a current, (B) moved at right angles when a current flowed one way, and (C) moved at right angles in the opposite direction when the current was reversed.

(A) In a piece of iron, the magnetic domains have random arrangement that cancels any overall magnetic effect (not magnetic). (B) When an external magnetic field is applied to the iron, the magnetic domains are realigned, and those parallel to the field grow in size at the expense of the other domains, and the iron becomes magnetized.

A magnetic compass shows the presence and direction of the magnetic field around a straight length of current-carrying wire.

Use (A) a right-hand rule of thumb to determine the direction of a magnetic field around a conventional current and (B) a left-hand rule of thumb to determine the direction of a magnetic field around an electron current.

When a current is run through a cylindrical coil of wire, a solenoid, it produces a magnetic field like the magnetic field of a bar magnet. The solenoid is known as electromagnet.

• Applications of Electromagnets:– Electric Meters:

• The strength of the magnetic field produced by an electromagnet is proportional to the electric current in the electromagnet.

• A galvanometer measures electrical current by measuring the magnetic field.

• A galvanometer can measure current, potential difference, and resistance.

A galvanometer measures the direction and relative strength of an electric current from the magnetic field it produces. A coil of wire wrapped around an iron core becomes an electromagnet that rotates in the field of a permanent magnet. The rotation moves pointer on a scale.

– Electric Motors:• An electrical motor is an electromagnetic device that

converts electrical energy into mechanical energy.

• A motor has two working parts - a stationary magnet called a field magnet and a cylindrical, movable electromagnet called an armature.

• The armature is on an axle and rotates in the magnetic field of the field magnet.

• The axle is used to do work.

Electromagnetic Induction

• Induced Current:– If a loop of wire is moved in a magnetic field a voltage

is induced in the wire.• The voltage is called an induced voltage and the resulting

current is called an induced current.

• The induction is called electromagnetic induction.

A current is induced in a

coil of wire moved

through a magnetic field.

The direction of the

current depends on the

direction of motion.

The magnitude of the induced voltage is proportional to:

• The number of wire loops cutting across the magnetic field lines.

• The strength of the magnetic field.

• The rate at which magnetic field lines are cut by the wire.

• Applications:– DC and AC Generators,– Transformers (step-up and step-down).