•Electricity

The importance of electrical power seems obvious in a modern industrial society. What is not so obvious is the role of electricity in magnetism, light, chemical change,

and as the very basis for the structure of matter. All matter, in fact, is electrical in nature, as you will see.

• Electric Charge

• Electric Theory of 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.– Unlike charges produce attractive forces.

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

neurons. Negatively charged electrons move around the nucleus a much greater distance than is suggested by this

simplified model. 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 charges ions result from the loss of electrons and are called cations

– Negatively charge ions result from the gain of electros 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. (C) The addition of an electron produces a net negative charge and a negative

ion.

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.

• Electrical nonconductors are materials that do not move electrons easily

– These are also known as insulators• Semiconductors are materials that vary in their

conduction and nonconduction, sometimes conducting sometimes not conducting.

• Measuring Electrical Charges.– The magnitude of an electrical charge is dependent upon

how many electrons have been moved to it or away from it.

– 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 fundamental charge is the electrical charge on an electron and has a magnitude of 1.6021892 X 10-19 C

– To determine the quantity of an electrical charge you simply multiple the number of electrons by the fundamental charge on an electron or:

• q=ne

• Where q is the magnitude of the charge, n is the number of electrons, and e is the fundamental charge.

Coulomb constructed a torsion balance to test the relationships between a quantity of charge, the distance between the

charges, and the electrical force produced.

He found the inverse square law held

accurately for various charges and distances

• Measuring Electrical Forces.– Force is proportional to the product of the electrical

charge and inversely proportional to the square of the distance.

– Coulomb’s Law

• 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

• 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– All electrical charges are surrounded by an electrical

field just like all masses are surrounded by gravitational fields.

– 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 more 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.

Electric potential results from moving a

positive coulomb if charge into the electric

field of a second positive coulomb of change. When 1.00

joule of work is done in moving 1.00

coulomb of charge, 1.00 volt of potential

results. A volt is a joule/coulomb.

– 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 to create .

potential charge moved• 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.– In order to have an electric current there must be a

separation of the charge maintaining the electrical field (a potential difference).

• This potential difference can push a charge through a conductor.

– An electrical current is maintained by pumping charges to a higher electrical potential and the then do work as they flow back to a lower potential

– 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.

– The device where the charges do work is the voltage drop.

– 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 quantity (q) that moves through a cross section of a conductor in a give unit of time (t)

– I=q/t• he units of current are coulombs/second.• A coulomb/second is an ampere (amp)

– In an electrical circuit the rate of current is directly proportional to the difference in electrical potential between two parts of the circuit IPD.

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.

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 electronic

structure of the conducting material and the temperature.

• The cross-sectional area of eh 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 chaotically 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.

Electrons move very slowly in a direct

current circuit. With a drift velocity of 0.01 cm/s, more than 5 hr would be required for an electron to travel 200 cm from a car battery to the brake

light. It is the electric field, not the

electrons, that moves at near the speed of light in an electric

circuit.

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

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

electrical resistance (R).

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

• R=PD/I

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

– The relationship between voltage, current, an resistance is• V=IR• 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 by a voltage source is equal to the work done by the electrical field in an electrical device.• W=PD• 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

– P=IV

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.

• Magnetic Poles.– 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.

• Since electrons are charges in motion, they 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 electrons are oriented in such a ways as to impart magnetic properties to the atomic structure.

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

– Earth's Magnetic Field.• 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 is must be a magnetic south pole since

the north end of a magnetic compass is attracted to it and opposite poles attract

This magnetic declination map shows the approximate number of degrees east or west of the true geographic north that a magnetic compass will point in various

locations

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, and the separation of a pair into

single poles, called monopoles, has never been accomplished.

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 an unmagnetized piece of iron, the magnetic domains have random arrangement that cancels any

overall magnetic effect. (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 is magnetized

• Electric Currents and Magnetism.

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

• Current Loops.– A current-carrying wire that is formed into a loop has

perpendicular, circular field lines that pass through the inside of the loop in the same direction.

• This has the effect of concentrating the field lines, which increases the magnetic field intensity.

• Since the field lines pass through the loop in the same direction, the loop has a north and south pole.

– Many loops of wire formed into a cylindrical coil are called a solenoid.

• When a current is in the solenoid a magnetic field around the solenoid is created that acts like a magnetic field and is called an electromagnet

(A) Forming a wire into a loop causes the magnetic field to pass through the loop in the same direction. (B) This

gives one side of the loop a north pole and the other side a south pole.

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

• 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.

– Electromagnetic Switches.• A relay is an electromagnetic switch device that makes possible

the use of low voltage control current to switch a larger, high voltage circuit on and off

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

You can use the materials shown here to create and detect

an electric current.

A schematic of a relay circuit. The mercury vial turns as changes in the temperature expand or contract the coil, moving the mercury and making or breaking contact with the relay circuit. When the mercury moves to close to the relay circuit, a small current activates the electromagnet, which closes the contacts on the large-current circuit

(A) Sound waves are converted into a changing electrical current in a telephone. (B) Changing electrical current can be changed to sound waves in a speaker by the action of an electromagnet pushing and pulling on a permanent magnet. The permanent magnet is attached to a stiff paper cone or some other material that makes sound waves as it moves in and out

– 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.

A schematic of a simple electric motor

• Electromagnetic Induction.

• Introduction– 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 interaction is called electromagnetic induction.

– Electromagnetic induction occurs when the loop of wire cuts across magnetic field lines or when magnetic field lines cut across the loop.

– 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.

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

• Generators.– A generator is basically an axle with many wire loops

that rotates in a magnetic field.• The axle is turned by some form of mechanical

energy, such as a water turbine or a steam engine.

(A) Schematic of a simple alternator

(ac generator) with one output loop. (B) Output of the

single loop turning in a constant

magnetic field, which alternates the

induced current each half cycle

(A) Schematic of a simple dc generator with one

output loop. (B) Output of the single

loop turning in a constant magnetic field. The split

ring (commutator) reverses the sign of the output when the voltage starts to reverse, so the

induced current has half-cycle voltages of a

constant sign, which is the definition of direct

current.

• Transformers.– A transformer has two basic parts.

• A primary coil, which is connected to a source of alternating current

• A secondary coil, which is close by.– A growing and collapsing magnetic field in the primary

coil induces a voltage in the secondary coil.

– A step up or step down transformer steps up or steps down the voltage of an alternating current according to the ratio of wire loops in the primary and secondary coils.• The power input on the primary coil equals the power

output on the secondary coil.• Energy losses in transmission are reduced by stepping

up the voltage.

(A) This step-down transformer has 10 turns on the primary for each turn on the secondary

and reduces the voltage from 120 V to 12 V.

(B) This step-up transformer increases the

voltage from 120 V to 1,200 V, since there are

10 turns on the secondary to each turn

on the primary

Energy losses in transmission are reduced by increasing the voltage, so the voltage of generated power is stepped up at the power plant. (A) These transformers, for example, might step up the voltage from tens to hundreds of thousands of volts. After a step-down transformer reduces the voltage at a substation, still another transformer (B) reduces the voltage to 120 for transmission to three or four houses