Electricity in Your Life

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M

IRVING ^M»V.

80 illustrations in three colors

by Ru th Adler

EIYL $ 3.95

ELECTRICITY IN

YOUR

LIFE

By IRVING ADLER Author of Time in Your Life, 80 illustrations

etc.

in three colors

by Ruth Adler Electricity

is

responsible for

many

fasci-

nating things in nature, in homes, and in industry.

A

Hash of lightning is a great electrical spark. The ignition system of an automobile engine makes smaller sparks of the same kind. Calculating machines that work electrically are used to solye complicated problems. The human brain also works electrically, and is more complicated and more wonderful than

any calculating machine made by man.

On

a

summer

night,

we

see the

abdomens

glow by phosphorescence, caused by electricity. The same kind of glow makes the pictures on our television screens. of HreHies

The antennas of radio and teleyision pick up radio w ayes produced by electrical yibrations at broadcasting stations.

telescopes pick

Giant radio

up wav es produced by

elec-

trical yibrations in space.

many eyents \yorld is made of

Electricity plays a part in so

because eyerything in the electricity. By learning about electricity \ye get a better understanding of nature. We also learn hovy to in

make

electricity \york for us

such things as lamps, motors, telephones,

and

radios. 10213 Jacket design by

The Etheredges

THE JOHN DAY COMPANY New

York

STATE 0 FREE PU3LIC REGIONAL

V.!t; «

,

i\’

r

LIb.-v'AAv

ST. jlQHNSBURy', VtR.VJONf

Digitized by the Internet Archive in

2017 with funding from

Kahle/Austin Foundation

https://archive.org/details/electricityinyouOOadle

Electricity in Electricity

is

responsible for

Your Life

many

fascinating things in

nature, in homes, and in industry.

A

flash of

lightning

is

The

a great electrical spark.

igni-

makes smaller sparks the same kind. Calculating machines that work elec-

tion system of an automobile engine of

trically are

man

used to solve complicated problems.

The

hu-

more complicated and more wonderful than any calculating machine made brain also works electrically, and

is

by man.

On

a

summer

night,

we

see the

abdomens

of fireflies

glow by phosphorescence, caused by electricity. The same kind of glow makes the pictures on our television screens. The antennas of radio and television pick up radio waves produced by electrical vibrations at broadcasting stations. Giant radio telescopes pick up waves produced by electrical vibrations in space.

Electricity plays a part in so

thing in the world electricity

made

events because every-

of electricity.

By learning about

get a better understanding of nature.

how

make

work for us lamps, motors, telephones, and radios.

also learn

things as

we

is

many

to

electricity

We

in such

Books by Irving Adler

COLOR

IN

YOUR

LIFE

DUST THE ELEMENTARY MATHEMATICS OF THE ATOM

YOUR LIFE HOT AND COLD HOW LIFE BEGAN INSIDE THE NUCLEUS FIRE IN

LOGIC FOR BEGINNERS MAGIC HOUSE OF NUMBERS

MAN-MADE MOONS

MONKEY A

BUSINESS: Hoaxes

in the

Name

of Science

NEW LOOK AT ARITHMETIC THE NEW MATHEMATICS

PROBABILITY AND STATISTICS FOR EVERY THE SECRET OF LIGHT SEEING THE EARTH FROM SPACE

THE STARS: Stepping Stones into Space THE SUN AND ITS FAMILY THINKING MACHINES riME IN YOl’R LIFE

TOOLS

IN

YOUR

LIFE

THE FOOLS OF SCIENCE WEATHER IN YOUR LIFE WHAT WE WANE OF OUR SCHOOLS THE Reason Why BOOKS (with Ruth Adler)

MAN

IRVING ADLER

Electricity in

Your

Life Illustrated by

Ruth Adler

The John Day Company Nexu York

©

1965

BY IRVING AND

All rights reserved. This

RUTH ADLER

book or parts thereof, must not be

re-

produced in any form without permission. Published by The John

Day Company,

Inc.,

62 West 45th Street,

simultaneously in Canada by

New York

IN

and

Longmans Canada Limited, Toronto.

Library of Corigress Catalogue Card

MANUFACTURED

36, N.Y.,

Number: 65-12275

THE UNITED STATES OF AMERICA

Contents I.

The Wonders

II.

Electrical Particles

13

III.

Rubbing

18

IV.

Falling Electrons and E.M.F.

V.

Insulators, Conductors,

VI.

Resistance and

VII.

Direct and Alternating Current

44

VIII.

A Current

51

IX.

Electric

X.

Measures of Electrical Power and Energy

62

XI.

Magnets

66

XII.

Electromagnets

71

XIII.

An

77

XIV.

Electric

Motors

81

x\^

Electric

Power Generator

86

XVI.

Transformers

90

XVII.

The Telephone

94

XVIII.

Radio

98

XIX.

How a

XX.

Inside a Radio

XXI. XXII.

Television

109

Radar

114

XXIII.

Electricity in Space

119

Index

125

of Electricity

off Electrons

and Current

Ohm’s Law

Makes Heat

Lamps

9

23 32 38

56

Electric Bell

Radio Wave Carries Sound

Tube

85)1 ‘^‘^1

102 105

,

Part of an electronic calculator

The

calculator

The human

and the brain both work

brain

electrically.

In industry and in scientific laboratories, calculating machines that

work

complicated problems.

electrically are used to solve

The human

brain also works

more complicated and more wonderful than any calculating machine made by man. On a summer night, we see spots of light in the air, when the abdomens of fireflies glow like tiny lamps.

electrically,

and

is

This glow, called phosphoresceyice is caused by electricity. The same kind of glow makes the pictures we see on our television screens. •J

•*r

makes

a firefly glow. It makes the glow of a television screen, too. This glow is called Electricity

phosphorescence.

A TV antenna and waves

The

a radio telescope pick produced by electrical vibrations.

up radio

and television sets pick up are produced by electrical vibrations

antenyias of radio

radio waves that

broadcasting stations. Giant, bowl-shaped radio telescopes pick up radio waves produced by electrical vibraat

tions far out in space.

from exploding

stars.

scattered in the space

Some of these radio waves come Some come from hydrogen atoms between the

stars.

Radio waves come from the Crab Nebula formbd from a star that exploded 900 years ago.

Electricity plays a part in so

everything in the world

We

also

made

we get learn how

ing about electricity nature.

is

for us in such things as

many

events because

of electricity.

By

learn-

a better understanding of

make

work lamps, motors, telephones, and to

electricity

radios.

f

l

'

1

;

.

i

\ i

^ i

'iHr

\ \ Vj

II

Electrical Particles

A

ll matter

is

molecules.

made up

There

of small particles called

are hundreds of thousands

of different kinds of molecules.

made up

The

molecules are

of smaller particles called atoms.

There are

only about a hundred different kinds of atoms.

...join

to

make one molecule

of water.

The

Two protons repel each other.

Two

electrons

repel each other, too.

But a proton and an electron attract each other.

atoms, in turn, are

made up

of three kinds of particles,

and neutrons. Electrons and protons are electrical particles. Each particle carries an electrical charge. These charges cause them to push or pull each other. Such a push or pull by electrical charges is called electrostatic force. Neutrons do not called electrons, protons

carry any charge.

The charge on The charge on an ticles

a

proton

electron

is is

called a positive charge.

called negative.

Two

par-

with the same kind of charge repel each other, or

tend to push each other apart. Thus, an electron repels

an electron.

A

proton repels a proton. Particles with

opposite charges attract each other, or tend to pull gether.

A

to-

proton and an electron attract each other.

In each atom, the protons

and neutrons are crowded

together in a cluster called the nucleus. Special forces 14

The nucleus of a hydrogen atom has one proton. So it can hold only one electron

The smallest atoms are those

in

place.

of hydrogen.

within the nucleus keep the protons from flying apart.

The

atom surround the nucleus and like planets revolving around the

electrons of the

revolve around sun. Because of

it,

its

protons, every nucleus has a positive

charge. This charge attracts the electrons that surround the nucleus

and keeps them near

one proton, the charge

is

it.

If a

nucleus has

strong enough to hold one

electron in place. If the nucleus has two protons, the

charge holds two electrons in place. In a complete

number of electrons that surround a nucleus same as the number of protons in the nucleus. As

atom, the is

the

a result, their opposite charges balance out,

atom

is

The

and the

electrically neutral.

smallest atoms are those of hydrogen.

Each hy-

drogen atom contains only one proton. The proton may be alone in the nucleus, or it may be accompanied 15

The nucleus of a helium atom has two protons. So it

can hold two electrons

in place.

The nucleus of a carbon atom has six protons. So it can hold six electrons in place.

A

hydrogen nucleus has one electron revolving around it. The largest atoms found in nature are those of uranium. Each uranium nucleus contains 92 protons, and is surrounded by 92 electrons. The 92 protons in the nucleus are usually accompanied by one or two neutrons.

by 146 neutrons, making a total of 238 particles in the nucleus. In this case the

atom

is

known

as

Uranium

hundred uranium atoms the 92 protons are accompanied by only 143 neutrons in the nucleus. This type of uranium, known 238. In about two out of every three

as

Uranium 16

235,

is

used in atomic energy reactors.

Individual atoms are too small to be seen, even with the most powerful microscope. small,

it

would take 250 million

A of

hydrogen atom

them placed

is

so

side by

one inch long. An electron or proton is still smaller. A hydrogen atom is about 100, 000 times as wide as either of them.

side to

make

a line

URANIUM RODS

GRAPHITE BLOCK

FISSIONABLE MATERIAL

MODERATOR

sustains chain reaction

slows Fiss ion

CONTROL RODS

down

absorb excess neutrons and

neutrons

control rate ol chain reaction

CONCRETE

NUCLEAR REACTOR

ENCASEMENT protects

workers from intense

radiation

produced

power

reactor

radioisotopes

in

fissionable material

SHIELD

An atomic energy

reactor. the fuels used in reactors.

PRODUCTION UNIT

Uranium 235

is

one 17

of

Ill

Rubbing Off Electrons

T

he electrons

surround

that

a

nucleus are

held in place by the pull of the nucleus. But

When

they can be torn away from the nucleus.

this

happens, the balance between the negative and positive charges in the atom

have it

lost electrons

is

has

destroyed.

A

body whose atoms

more protons than

electrons, so

has a positive charge. Electrons lost by one body are

A

body that has gained extra electrons besides those usually in its atoms has a

sometimes gained by another. negative charge.

This body

is electrically

because

has just as many

it

neutral

protons as electrons.

This body

is

because

has

it

positively

charged r\(p

lost electrons.

This body

because

it

^

v

negatively charged has gained electrons. is

I

Comb your hair with One way

a hard rubber

to tear electrons

out of atoms

comb. is

to

rub one

object against another. As the objects are rubbed, their

atoms bump into each other, and electrons are knocked out of place. Sometimes one of the objects picks up dislodged electrons, while the other one loses them. This happens, for example, when you comb your hair with a hard rubber comb. The comb knocks electrons out of your hair and carries them away with it. As a result, your hair becomes positively charged, and the comb becomes negatively charged. The presence of

Before you comb your hair, the positive and negative charges of your hair are balanced and the positive and negative charges of the comb are balanced.

When you comb your

hair,

the

comb knocks

trons out of your hair and carries it.

So your

is

negatively charged.

hair

these charges

is

is

positively

elec-

them away with

charged and the comb

shown by what happens

next.

When

each hair carries a positive charge, the hairs tend to pull

away from each

other.

They

stand

up on your

you bring your comb near the hair again, the negative charges on the comb attract the positive charges on the hair. Then head, like the quills of a porcupine.

Then the

positively

from each other. 20

If

charged hairs tend to

pull

away

you bring your comb near your hair again, the negative charges on the comb attract the positive charges on the hair. Then your hair leans toward the comb. If

your hair leans over toward the comb. is

very close to the hair, the extra electrons

the

comb back

to the hair in

electric current called sparks.

ling a

When

sound that you can hear

comb

jump from

sudden swift surges of

The as

the

sparks

make

a crack-

you comb your hair on

dry day.

the comb is very close to your hair, the extra electrons jump from the comb back to your hair, causing electric sparks. Then your hair lies flat again because the positive and negative charges on your hair balance each other.

When

21

The same

happens when you shuffle across a thick carpet. As your feet rub against the carpet, an electrical charge builds up on your body. Then, if you touch your finger to a piece of metal, a spark jumps from your finger, making a flash of light and a crackling sound.

99

sort of thing

IV Falling Electrons and E. M.F.

T HELPS us to understand the behavior of elec-

I

trostatic force

The

if

we compare

it

with the force

on all bodies that are near it. Wdien we raise a ball above the ground we oppose the pull of gravity. Energy is used up when we raise the ball. This used-up energy is stored as hidden energy in the raised position of the ball. If the ball is dropped, the pull of gravity makes it fall back to the ground. Then the hidden energy stored in the raised position of the ball comes out of its hiding place and appears in the form of energy of motion of the of gravity.

gravity of the earth pulls

When we

above the ground, energy is used up because we oppose the pull of gravity. This used up energy raise a ball

stored in the raised position of the ball. is

23

If

the ball

is

dropped, the

pull of gravity

makes

it

back to the ground. The stored energy appears in the form of energy of fall

motion.

GROUND

falling ball. In the

same way, when electrons are pulled

away from a positive charge, they tend to fall back to it, and will fall if they are not held back. As they fall, hidden energy comes out of its hiding place and appears in the form of energy of motion of the electrons. The higher we raise a ball, the more hidden energy there will be in the ball in the hidden energy

is

its

raised position. Because

caused by the force of gravity,

it is

hidden gravitational energy. At a high level, the ball has a high amount of hidden gravitational energy. At a low level the ball has a low amount of hidden gravitational energy. So a high level is high in a called

double is

sense. It

also high in

son

is

hidden gravitational energy. For

we may think

may be

high in distance above the ground.

gravitational energy.

24

is,

this rea-

which a ball levels of hidden

of the possible levels to

raised as energy levels, that

It

#“

When an

electron is pulled away from a positive charge, electrical energy is used up. The electrical energy is hidden in the higher energy level of the electron.

A

j I

»

^

•"

In the same way, an electron, being pushed or pulled

by an

electrical force,

trical

energy

an electron

is

levels.

A

may be low

a place that

is

placed at different elec-

electrical

energy level for

near positive charges, away

from negative charges. A high electrical energy level for an electron is a place that is near negative charges, away from positive charges. An electron, acted on by electrical force, tends to fall from a high electrical energy level to a low electrical energy level.

•I

} I

•-

When

the electron is released, the pull of the positive charge makes the electron fall to a lower energy level. The hidden energy appears in the form of energy of motion of the electron. 25

Low

High electrical energy level

electrical

energy

level

An

electron acted on by an electrical force tends to fall from a high electrical energy level to a low electrical

energy

level.

Try this experiment. Poke three holes in the side of an empty tin can, making each hole at a different level. Fill

the can with water.

holes hole.

The

water will flow out of the

and the flow will be strongest from the lowest The flow from each hole will gradually become

weaker.

It will

stop

when

the level of the water in the

can reaches the level of the lowest hole.

The

water

pushed })y water pressure. The strength of the pressure depends on how high the level of the top of the water is above the level flows out of each hole because

it

is

of the hole.

Poke three holes can with water.

in

the side of a can and

fill

the

Level of top of water

The water will flow out of the holes, and the flow will be strongest from the lowest hole.

Now

suppose two cans,

connected by pipe A, pipe

is

ment.

as

filled to different levels, are

shown

in the diagram.

like a hole in the can in the

The

water in each can

is

The

preceding experi-

being pushed toward

the pipe by water pressure. But the pressure

is

greater

from the can where the water reaches a higher level. The greater pressure from this can overcomes the smaller pressure from the other can. So the water flows from the can with the higher water level to the can with the lower water level. As a result, the higher of the two water levels begins to fall, and the lower of the

Water flows from the can with the higher water level to the can with the lower water level until the levels become the same.

can be kept different by pumping the water back through pipe B.

The water

two

level

levels rises.

When

the two levels are the same, the

water stops flowing through the pipe.

There

is

a

way

of keeping the water levels in the

cans from changing even

if

two

the levels are different.

Imagine that the cans are joined by another pipe, B, and that there is a pump in the pipe. Suppose that, as fast as the water flows through pipe A from the can with the higher water level to the can with the lower

water

level, the

pump

pushes water through pipe B

from the can with the lower water

Then change. One

the higher water level.

two cans

level to the can

with

the water levels in the

remain higher than the other, and the flow of water through pipe A will not

level will

will not stop.

The same trons. Just as

happens with a flow of elecwater tends to flow from a higher gravita-

sort of thing

tional energy level to a lower gravitationel energy level,

electrons tend to flow from a higher electrical energy level to a

lower electrical energy

level.

If

two places

that are at different electrical energy levels are joined

by a metal wire, the wire serves as a pipe through which 28

Lower energy

Higher energy

level

level

Electrical pressure called voltage or electromotive

force pushes the electrons through the wire. the electrons can flow. that tends to

There

is

an

electrical pressure

push the electrons through the wire from

the higher electrical energ)’ level to the lower electrical

energy

level.

This pressure

is

called voltage or electro-

?notive force, abbreviated as e.m.f. It

is

measured

in

units called volts.

many ways of producing an electrical presvoltage. One way, already mentioned, is to tear

There sure or

are

electrons out of an object by rubbing one object against

another.

A

typical instrument for

machine,

electrostatic

There metal

is

doing

this, called

shown on the next

an

page.

are two plates in the machine, each joined to a ball.

When

one plate

is

turned so that

against the other, electrons are pulled

it

rubs

away from one

and piled up on the other. The ball that loses the electrons becomes a low electrical energy level for an electron. The ball that gains the electrons becomes a ball

high electrical energy level for an electron.

The

balls

are like water cans that are filled to different levels.

If

the balls are joined by a wire, electrons flow from the

higher energy level to the lower energy

level.

I'he

electrons will keep flowing as long as the plates are

The pump in

turned.

turning of the plates

the

the example of the cans of water.

is

like the action of

29

An

electrostatic

Another way of producing chemical action. In a dry

machine

electrical pressure uses

cell, a special

chemical mix-

ture interacts with the zinc wall of the cell

carbon rod held

center of the

in the

action creates an e.m.f. of wall and the carbon rod.

tached to the to tlie

the

I

cell,

carbon rod.

I/2

If

tlie

as long as tlie

I

i/o

wo

a

d he chemical between the zinc

cell,

volts

metal terminals are

to the zinc wall,

at-

and the other

the terminals are joined by a wire,

volts of e.m.f.

wire Irom

30

one

1

and with

make

How through the The How continues

electrons

zinc to the carbon.

chemical action

in the

dry

cell

continues.

The

chemical action keeps the electrical energy levels

at the terminals different, just as the

pump

kept the

water levels different in the two cans described on

page 28.

The is

terminal attached to the zinc wall of a dry

called the negative terminal.

to the

carbon rod

is

The

cell

terminal attached

called the positive terminal.

a wire joins the terminals, electrons flow

When

through the

wire from the negative terminal to the positive terminal.

produced with the help of motion or light. These methods are described on pages 86-88 and 111. The e.m.f. supplied by the electric power company to your home is usually 110 volts. \^oltages can also be

Positive

(

+

)

terminal

Negative

(

—

)

terminal

Carbon rod

Chemical mixture

Zinc wall

A dry cell with part of it cut away to show the inside. 31

V Insulators, Conductors,

and Current

F

I

THE TERMINALS

of a battery are joined to the

ends of a rod, the voltage supplied by the bat-

tery begins tugging at the electrons that are in the rod.

push them through the rod away from the negative terminal toward the positive terminal. But it does not always succeed in making the electrons move. Whether or not electrons will move through the rod depends on the kind of material that the rod is made of. It tries to

The

electrons in a rod are within the molecules of

the rod, where they surround positively charged nuclei.

In some materials there are electrons that are held loosely.

These

loosely held electrons are easily

made

move, even by a weak voltage. Materials of this kind are called conducting materials or conductors. When a battery is connected to the ends of a rod made of a conducting material, electrons flow through the rod. to

The

flow of electrons

is

called an electric current.

electric currents carry a large

32

number

Some

of electrons from

Cloth

Glass

Dry wood

Porcelain

These materials are good one place

to

another in a second. Some electric currents

carry only a small at

which an

electrical insulators.

number

in the

same time. The

electric current carries electrons

the intensity or strength of the current,

and

is

is

rate

called

measured

in units called amperes.

In some materials

the electrons are held very

all

tightly. In these materials,

%

an ordinary voltage obtained

from a battery or from an electrical socket in your house isn’t strong enough to pull any electrons loose and make them move. Materials like this are called insulating materials or insulators.

When

a battery

is

con-

nected to an insulator, no electric current flows through the insulator.

However,

nected to an insulator,

if it

a very high voltage

may

is

con-

pull the electrons hard

38

enough

to pry

some

loose

and produce an

electric cur-

1 he lowest voltage that will make a current flow through an insulator is called the breakdown voltage of the insulator. An insulator does not permit electric rent.

current to flow through across the insulator

is

All metals in solid

it

when

the voltage applied

breakdown voltage. or liquid form are conductors of less

than

its

electricity. Electric wires are usually

made

of the con-

ducting metal, copper. Non-metals are usually insulators. Glass,

common If a

porcelain, dry wood, cloth,

and rubber are

insulators.

person touches an exposed electric wire that

is

connected to a voltage source, he gets an electric shock.

A

strong shock

may

kill

him.

To

protect us from dan-

gerous shocks, electric wires are covered with insulating materials like cloth or rubber.

Whenever

electric wires

Plastic or rubber

insulating cover

1

Cloth insulating cover

Braided copper wires conduct electricity. Electric cord

34

used for lamps and small appliances

are supported on are

made

A

power poles or on

walls, the supports

of insulating materials like glass or porcelain.

made

gas

However,

if

of neutral molecules

some

an insulator.

is

of the molecides in the gas are broken

into electrically charged pieces, then

it

becomes a con-

ductor. For example, in a very hot gas the molecules

moving

may knock tron that

A

have violent

in the gas

is

The

collisions

electrons out of the molecules. Each elec-

knocked out

molecule that has

particle.

collisions.

The

is

a negatively

lost electrons

is

charged particle.

a positively

charged particles are called

contains ions, and

it is

charged

ions. If a gas

placed between the terminals of

makes the ions move. The negative ions move toward the positive terminal and

a voltage source, the voltage

the positive ions so that there

is

a

move toward

the negative terminal,

two-way flow of current

in the gas.

35

There

An

insulator for a

are

some

TV

lead-in wire

liquids that contain ions. For ex-

ample, water that has table

salt

compound made

dissolved in

it

contains

sodium atoms and chlorine atoms. When sodium atoms and chlorine atoms combine to form salt, each chlorine atom snatches an electron from a sodium atom. The chlorine atom, by gaining an electron, becomes a negative ion. The sodium atom, by losing an electron, becomes a positive ions. Salt

is

a

of

ion. In a crystal of table salt, the ions are

arranged in

the corners of tiny cubes, with each negative ion sur-

rounded by six positive ions, and each positive ion surrounded by six negative ions. When the salt is dis36

.

solved in water,

ence

of

many

of the ions separate.

ions in a licjuid

makes

it

a

the presence ol ions in a gas does. tains ions

The

pres-

conductor, just as

A

licpiid that con-

called an electrolyte

Ordinary water that we use for drinking, cooking, or bathing always has some ions in it, so it is a conductor of electricity. That is why it is dangerous to touch electrical appliances with wet hands or while standing in water. is

—

Electrolyte

The negative ions move towards the The

positive ions

positive terminal.

move towards the negative

The two-way flow

of current in

terminal.

an electrolyte 37

.

VI

Law

Resistance and Ohm’s

ELECTRICAL CONDUCTOR is like a road along which electrons may travel. But it is not an empty road. The molecules of which the conductor is made are scattered along the road. They are obstacles 'j

in

path of the electrons.

the

When

electrons

flow

through the conductor they collide with the molecules.

These

The

collisions tend to hold the flow of electrons back.

tendency of a conductor to hold back the flow of

an electric current in

it

resistance of a conductor

ohms.

A

e.m.f. of

makes

is

called

is

conductor has one 1

measured

ohm

resistance.

1

in units called

of resistance

ampere flow through

an electrical diagram

.

it.

.

represents a source of voltage or e.m.f. .

—yyyyy— 38

The if

an

volt connected to the ends of the conductor

a current of

In

its

.

.

.

.

.

represents a resistance.

If

makes a current of 1 ampere

an e.m.f.

.

of 1 volt

Q-f

-1-

©-

1

1

.

.

.

flow through

a conductor.

. .

> ^

the conductor has a resistance of 1 ohm.

The

depends on the malength, and its cross-section

resistance of a conductor

terial that

it is

made

of, its

area.

two threads are made the same length and with the same cross-section area, and one is made of carbon while the other is made of copper, the carbon thread will have a high resistance and the copper thread will If

^

have a low resistance.

two wires are made of the same material and have the same cross-section area but have different lengths, the longer of the two wires has a higher resistance. If two wires are made of the same material and have If

the same length, but have different cross-section areas, the thinner of the two wires has a higher resistance.

39

Copper

Carbon

A copper

thread has a lower resistance than a carbon thread of the same length and width.

These two wires are made of the same material and have the same width. The longer wire has the higher resistance.

These two wires are made of the same material and have the same length. The thinner wire has the higher resistance.

40

WHien

a

given e.m.f. pushes a current through a wire,

the higher the resistance of the wire intensity of the current

is.

The

is,

the smaller the

strength of a steady

current can be calculated by means of a rule

known

as

Ohm's Law: Fhe number of amperes in the current is e(|ual to the number of volts in the e.m.f. divided by the number of ohms in the resistance of the wire. For example,

if

a steady e.m.f. of 110 volts

is

connected to

an appliance that has a resistance of 50 ohms, the current produced has an intensity of 110

50

=

2.2

amperes.

made by joining many

parts

each of which has a definite electrical resistance.

The

Electrical appliances are

way

combine to form the total resistance of the appliance depends on how the parts are joined. If two parts are placed one after the other in

which the

resistances

in the path of the electrons,

we

say that they are joined

\

These two lamps are joined

in series.

41

These two lamps are joined

in parallel.

combine by simple 10-ohm resistance and a 15series form a 25-ohm resist-

in series. In this case the resistances

addition. For example, a

ohm

resistance joined in

ance. If

the two parts are placed side by side in the path of

the electrons, so that the current divides into

streams to flow through them,

we

two

say that they are

joined in parallel. In this case the resistances combine

according to a different rule.

To

calculate the com-

bined resistance, represent the two resistances by the lengths of two vertical lines standing on a horizontal Join the top of each vertical line to the bottom of the other by drawing a straight line. The two lines line.

drawn

The line

in this

way

will intersect, or cross each other.

height of the intersection above the horizontal is

combined resistance. If a 15-ohm resistance are joined

the measure of the

10-ohm resistance and a in parallel, the combined resistance 42

is

6 ohms.

10-ohm

6-ohm

15-ohm

resistance

resistance

resistance

Finding the combined resistance of two resistances in

parallel

\

48

VII Direct and Alternating

Current

T

here are two tric

different types of flow of elec-

current. In direct current, the voltage

is

steady and pushes the electrons in one direction only.

The

current supplied by a dry

cell

or by an automobile

direct current. In alternating current, the

battery

is

voltage

is

unsteady.

direction,

and then

It first

pushes the electrons in one

in the opposite direction, over

over again with a regular rhythm.

The house

supplied by your electric power company

is

and

current usually

alternating current.

The

path of an electric current

is

called a circuit.

A

circuit for direct current has to be a closed loop of

conductors joined to a voltage source, diagram.

If

there

is

a break in the loop,

shown in the no current will

as

.

.

Switch open no current flows. .

.

Switch closed current flows.

.

.

.

.

flow. If

we put

roadway.

.

a switch into the circuit,

the current on or in a

.

off.

When

The

switch

the switch

is

.

we can turn

drawbridge open, there is a gap is

like a

and no current flows. When the switch is closed, it bridges the gap, and the current flows again as long as an e.m.f. keeps pushing it. The drawings below and on page 46 show an ordinary wall toggle switch and how it works. in the circuit,

Front view

Current flows to switch through wire

Toggle \

Side view

A toggle

switch with the wall plate removed

1 is

..

.

.

When

the toggle pushed to the left

4 .

... so that

it

touches these copper strips

.

.

.

5 2 ...

it

.

.

.

which are connected

pushes

this spring

.

.

.

to the wire

3

.

.

this

which pushes copper strip

through which the current flows.

.

.

.

This

1

When

Is

pushed

the right

is

the

ON

position.

4

the toggle .

.

.

.

away from

these copper strips

to

.

.

.

.

5

.

.

.

breaking the circuit.

2 ...

it

pulls

this spring

3

.

.

.

which pulls this copper .

.

.

strip

.

.

This

46

.

is

the

OFF

position.

A

circuit for

an alternating current doesn’t have to

be a closed loop of conductors. is

supplied, current will flow back and forth in the

circuit is

an alternating e.m.f.

If

even

if

there

is

a

gap

in

a capacitor across the gap.

it,

A

provided that there

simple capacitor, for

example, consists of two metal plates separated by an air space. Electrons flowing back and forth in such a circuit

first

away from the first

np on one

pile

plate while they are

drawn

drawn away from the np on the other, and so on,

other, then are

plate while they pile

over and over again. As in the case of direct current, a switch in the circuit can be used for turning the current

on and off. \Vhen electrons are being pushed onto a plate of a capacitor, the more crowded they become the more they push back. This backward push tends to interfere

©