electricity lecture

7/29/2019 Electricity Lecture

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Electricity


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All of us agree the importance of electricity in our daily

lives.

But what is electricity?


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Electric Charge


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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 forcesso they

repel each other (e.g. electron and electron or proton

and proton repel each other).

Unlike charges produce attractive forcesso they

attract each other (e.g. electron and proton attract

each other).


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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.


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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.


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(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 anelectron produces a net

negative charge and a

negative ion (anion).


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Arbitrary numbers

of protons (+) and

electrons (-) on acomb and in hair

(A) before and

(B) after combing.

Combing transferselectrons from the

hair to the comb by

friction, resulting

in a negative

charge on the comb

and a positive

charge on the hair.


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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.


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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.


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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.


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Measuring Electrical Charges:

The fundamental charge is the electrical charge on an

electron and has a magnitude of1.6021892 X 10-19 C(Note that the electrical charge is measured incoulombs).

A coulomb is the charge resulting from the transfer of6.24 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 oraway from it.Mathematically,

q = n e

where e is the fundamental charge.


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Coulombs law:

Electrical force is proportional to the product of theelectrical charge and inversely proportional to the

square of the distance. This is known as Coulombs law.Mathematically,

where,

F is the force,

kis a constant and has the value of9.00 x 109

Newtonmeters

2

/coulomb2

(9.00 x 109

Nm

2

/C2

), q1 represents the electrical charge of object 1 and q2represents

the electrical charge of object 2, and

d is the distance between the two objects.

2

21

d

qqkF


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Force Fields:

The condition of space around an object is changed bythe 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.


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

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

You can draw vector arrows to indicate the directionof the electrical field.

This is represented by drawing lines of force orelectrical field lines,

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


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A positive test

charge is used byconvention to

identify the

properties of an

electric field. The

vector arrow points

in the direction of

the force that thetest charge would

experience.


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Lines of force diagrams for

(A) a negative charge and

(B) a positive charge whenthe charges have the same

magnitude as the test

charge.


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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 anopposite charged particle into the field will require work

to keep the charges separated.

In both of these cases the electrical potential is

changed.


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The potential difference (PD) that is created by doing1.00 joule of work in moving 1.00 coulomb of charge isdefined as 1.00 volt.

A volt is a measure of the potential differencebetween two points,

electric potential = work done,

chargeOr, 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 chargecan do because of the position of the field.


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The falling water

can do work inturning the water

wheel only as

long as the pump

maintains the

potential

difference

between the upperand lower

reservoirs.


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Electric Current


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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.


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The Electric Circuit:

An electrical circuit contains some device that acts

as a source of energy as it gives charges a higherpotential 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.


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A simple electric circuit has a voltage source (such as agenerator 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.


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Voltageis 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).


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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.


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The Nature of Current:

Conventional current describes current as positive

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

It is actually the electron current that moves charges.


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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 (+).


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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.


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It is the electron field, and not the electrons, which doesthe 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, atnearly the speed of light, an electric fieldthroughout a circuit.

The field causes a net motion that constitutes a flowof charge.

The average velocity of the electrons moving as acurrent is very slow, even thought he electric fieldthat moves them travels with a speed close to thespeed of light.


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What is the nature of the electric current carried by theseconducting 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


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(A) A metal conductorwithout a current has

immovable positive ions

surrounded by a swarmof randomly moving

electrons.

(B) An electric fieldcauses the electrons to

shift positions, creating a

separation charge as the

electrons move with azigzag motion from

collisions with stationary

positive ions and other

electrons.


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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 ().


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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.


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The four factors that influence the resistance of anelectrical 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.


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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 sourceis equal to the work

done by the electrical field in an electrical device,Work = Power x Time.

The electrical potential is measured injoules/coulomb and a

quantity ofcharge is measured in coulombs, so the electrical

work is measure in joules. Ajoule/second is a unit of power called the watt.

Power = current x potential

Or, P = I V

Whatdoyousuppose it


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What do you suppose itwould cost to run each ofthese appliances for one

hour?(A) This light bulb isdesigned to operate on a

potential difference of 120

volts and will do work atthe 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.


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This metermeasures the amount of electric work done

in the circuits, usually over a time period of a month. The

work is measured in kWhr.


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Magnetism


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All of us are familiar with magnets. In a magnet we

have magnetic polesthe 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 polesattract.


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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.


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Magnetic Fields:

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

A magnetic field can be represented by field lines.

The strength of the magnetic field isgreaterwhere the

lines are closer togetherandweakerwhere they are

farther apart.


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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.


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The Source of Magnetic Fields:

Permanent Magnets:

Moving electrons produce magnetic fields.

In most materials these magnetic fields cancel oneanother and neutralize the overall magnetic effect.

In other materials such as iron, cobalt, and nickel, theatoms behave as tiny magnets because of certain

orientations of the electrons inside the atom.

These atoms are grouped in a tiny region called themagnetic domain.


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OurEarth is a big magnet.

The Earths magnetic field is thought to originate withmoving charges.

The core is probably composed of iron and nickel,

which flows as the Earth rotates, creating electricalcurrents that result in the Earths magnetic field.


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The earth's magnetic field.Note that the magnetic north

pole and the geographicNorth Pole are not in the

same place.

Note also that the magnetic

north pole acts as if the southpole of a huge bar magnet

were inside the earth. You

know that it must be a

magnetic south pole since thenorth end of a magnetic

compass is attracted to it and

opposite poles attract.


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A bar magnet cut into halves always makes new, completemagnets with both a north and a south pole. The poles

always come in pairs. You can not separate a pair into

single poles.


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Electric Currents

andMagnetism


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Oersted discovered that a

compass needle below a wire

(A) pointed north when

there was not a current,

(B) moved at right angleswhen a current flowed one

way, and

(C) moved at right angles in

the opposite direction when

the current was reversed.


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(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.


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A magneticcompass

shows the

presence and

direction of themagnetic field

around a

straight length

of current-carrying wire.


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Use (A) a right-hand rule of thumb to determine thedirection 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.


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When a current is run

through a cylindrical coil

of wire, a solenoid, it

produces a magneticfield like the magnetic

field of a bar magnet.

The solenoid is known as

electromagnet.

Applications of Electromagnets:


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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.


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A galvanometermeasures 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:


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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.


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Electromagnetic Induction

Ind ced C rrent:


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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:


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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).

electricity lecture

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