Science Form 3

Unit 7

Electricity

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ELECTROSTATICS

7.1

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Electrostatics

Electrostatics is the study of static electrical

charges.

Electrical charges are produced in material

through friction.through friction.

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Example of phenomena happens as a

result of electrical charges

When a plastic pen is rubbed with some wool, the pen will attract dust and small pieces of paper.

When you clean a mirror or glass window with a dry cloth on a hot, dry day, dust and fine threads will stick to the mirror or glass.mirror or glass.

When you comb your hair with a plastic comb, the plastic comb may pull against your hair.

When you remove your nylon or terylene shirt on a dry day, you will hear a crackling sound.

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Types of electrical charges and their properties

All substances are made up of atoms.

An atom contains

a. protonsprotons that are positivelypositively charged

b. neutrons that are neutralb. neutrons that are neutral

(do not have charges)

c. electrons that are negatively

charged and orbit around

the nucleus of an atom.

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Types of electrical charges and their properties

There are two types of electrical charges

a. positive charges and

b. negative charges

A neutral material has the same number of same number of A neutral material has the same number of same number of

protons and electrons. protons and electrons.

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Types of electrical charges and their properties

When two different materials are rubbed together, electrons

will be transferred from one material to another.

a. The material that loses electrons will

become positively charged.

b. The material that receives electrons will b. The material that receives electrons will

become negatively charged.

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Rub the plastic rod with fur to negatively

charge the rod.

Rub the glass rod with silk to positively charge

the rod.

Electrostatic ForceThe effects of electrostatic force:

a. Object with same type of charges repel (push part) each

other.

b. Object with different type of charges attract each other

The strength of the electrostatic force depends on the

quantity of charges that are present in the object.quantity of charges that are present in the object.

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Objects will

repel each other

if they have the

same charge

Objects will attract

each other if they

have different

charges

Producing and detecting electrical charges

Table below shows the type of electrical

charges that produced when two different

materials are rubbed against each other.

Negatively charged Positively charged

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Negatively charged Positively charged

Polythene Wool

Wool Cellulose acetate

Ebonite Animal fur

Silk Glass

Amber Animal fur

Electroscope An electroscope is a device that

is used to detect the electrical

charges.

An electroscope consists of a

metal cap and a gold leaf.metal cap and a gold leaf.

The gold leaf is attached to the

end of the metal rod.

When a charged substance is

brought close to the metal cap,

the gold leaf repels from the

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Study electrostatic ~

Van de Graaff generator

The Van de Graaff

generator is a highly

efficient electrostatic

generator. generator.

It produces electrical

charges which are stored

on the metal dome.

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Natural phenomena related to electrostatic

LightningLightning

a. Lightning is an electric is an electric

discharge.discharge.

b. During a thunderstorm, b. During a thunderstorm,

electrical charges are

produced when air that

flows quickly rubs against

the rain clouds.

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c. Negative charges build up at the bottom of the

cloud.

d. The top of the cloud becomes positively charged.

e. The Earth's surface and the buildings underneath

the clouds are positively charged.

f. Lightning occurs

i. when the electrons that collect ati. when the electrons that collect at

the bottom of the cloud move to the

positively charged areas of the cloud.

ii. when sparks jump between clouds

and the surface of the Earth.

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Phenomena related to electrostatic

I. Petroleum tankers

Petroleum tankers may become charged as a result of

friction.

i. between the tankers and the surrounding air on a dry day.

ii. between the exhaust gases and the exhaust pipe.

These charges can heat up the tanker and result in an

explosion.

An iron chain is usually hung from the tanker to enable the

electrical charges to flow to the ground.

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Phenomena related to electrostatic

II. Aeroplanes

Aeroplanes become charged as they fly in the air. The body of

the aeroplane comes in contact with the hot, dry air.

The tyres of aeroplanes are made of carbon and this enables

the charges to flow to the ground during landing.

Aeroplanes are also fitted with strips of metal conductor to

enable charges to flow to the ground during landing

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ELECTRICITY

7.2

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Source of electrical energy

Electrical is generated from two main source:

a. electric cells,

b. generators.

An electric generator converts kinetic energy and potential An electric generator converts kinetic energy and potential

energy into electrical energy.

Electric cells convert chemical energy into electrical energy.

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Examples of electric cells

1. dry cells,

2. mercury cells,

3. lithium cells,

4. nickel- cadmium cells,4. nickel- cadmium cells,

5. lead acid accumulator.

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Electric Current, Voltage and

Resistance

7.3

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Current

There are two forms of electricity

i. electrical charges (electrostatic)

ii. electric current.

Electrostatic charges are stationary and have Electrostatic charges are stationary and have

limited use.

Electric current is the flow of electrons through a

circuit and it can defined as the rate of flow of

charges.

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Measuring electric current

The quantity of electric current is measured

using an ammeter.

An ammeter has to be connected in

series in an electric circuit.

The S.I. unit for electric current is amperes (A), The S.I. unit for electric current is amperes (A),

While measuring electric current,

a. the positive terminal of the ammeter must be connected to

the positive terminal of the electrical supply.

b. the negative terminal of the ammeter must be connected

to the negative terminal of the electrical supply.

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Voltage

The voltage ( potential difference ) between

two points is the energy needed for the

electrical charges to flow between the two

pointspoints

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Measuring voltage

The potential difference (voltage) is measured with a

voltmeter.

The S.I. unit for voltage is volts (V).

A voltmeter is always connected in parallel across the

electrical components in a circuit. electrical components in a circuit.

The positive and negative terminals of the voltmeter are

connected to the positive and negative terminals of the

electrical component respectively.

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Resistance

Resistance is the property of a substance that

opposes the flow of electric current through

it.

Resistance influences the size of the electric Resistance influences the size of the electric

current that flows in a circuit.

The S.I. unit for resistance is ohms (Ω),

named after Georg Simon Ohm (1787- 1854),

a German physicist.

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The Relationship between

Current, Voltage and Resistance

7.4

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Ohm’s Law

Ohm's law states that the current ( I ) that

flows through a metal conductor is directly

proportional to the potential difference or

voltage (V) across it, if the temperature if the temperature voltage (V) across it, if the temperature if the temperature

remains constant. remains constant.

Ohm's law is expressed in the following

formula:

V is the voltage in volt, V.

I is the current in ampere, A.

R is the resistance in ohm, Ω.

Electric Circuits

7.5

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Electrical circuits and components

• The symbols of electrical component make it

easier for us to draw electric .

• An electric circuit is a complete pathway formed from connecting wires and other electrical components that allow the flow of electric current.

• A complete electric circuit consists of :

a. an electrical source such as dry cells,

b. connecting wires such as copper wires,

c. a switch that controls

current flow,

d. other electrical

components such as

ammeters, voltmeters,

resistors or bulb

Series circuits and parallel circuits

• Electrical components in a circuit can be connected in

series or parallel.

Electrical components

in a series circuit are

connected one after

another in one path

Electrical components

in a parallel circuit are

connected in several

path.

Series circuits

7.6

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The advantages of a series circuit

One switchOne switch can control all the electrical appliances,

such as fans and lights, in the circuit.

A battery that supplies more powermore power can be built A battery that supplies more powermore power can be built

from cells connected in series. Example, car battery.

When more cells are connected in series, the voltage

across the circuit and the current increases.

The disadvantages of a series circuit

If one of the bulbs fuses or burns out, the other bulbs will not light up because the current flows along one path only.

Each bulb in a circuit does not receive the full Each bulb in a circuit does not receive the full voltage from the electrical source. If more bulb are added, the bulbs will become dimmer.

The current that flows in a circuit decreases if more electrical devices are connected in series.

Parallel circuits

7.7

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The advantages of a parallel circuit

o If one bulb fuses, the other bulbs in the parallel circuit can still light up because current can flow through the other complete paths.

o Each bulb receives the full voltage from the electrical source and has the same brightness.

o If more bulbs are added in parallel, the brightness of the bulbs will remain the same.

o Electrical appliances are controlled individually through their own switches.

o Cells connected in parallel supply energy for a longer timecompared to one cell of the same voltage.

The disadvantages of parallel circuits

o Each electrical appliance is controlled by its

own switch.

It is not suitable for use in places, like a large

hall, that have many lights and fans. hall, that have many lights and fans.

o If too many resistors are connected in parallel,

a high current flows through the main circuit.

This is dangerous as it may cause fires.

Similarities of series circuit and parallel

circuit

Differences of series circuit and parallel circuit

Differences of series circuit and parallel circuit

Magnetism

7.8

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Magnetic Fields

The area surrounding a magnet where

magnetic forces act is known as a magnetic

field.

The pattern of a magnetic field consists of The pattern of a magnetic field consists of

curved lines of forces called magnetic field

lines

Characteristics of the magnetic field lines

1. begin from the north pole and end at the south pole.

2. do not cut across each other.

3. from the same pole repel each other. 3. from the same pole repel each other.

4. are strongest at the

poles of a magnet.

5. is closer if the magnetic

field is stronger.

The compass

The compass is useful:

a. in finding directions as it always points towards the magnetic north pole ,

b. for navigation by sailors and pilots,b. for navigation by sailors and pilots,

c. In finding and mapping magnetic fields.

d. To detect metal pipes hidden underground.

The compass has a magnetised needle that fixed at its centre of gravity

ELECTROMAGNETISM

7.9

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Electromagnetism

Electromagnetism is the combination combination of an electric field

and a magnetic field and their interaction to produce a

forceforce.

The magnet produced when current flows through a

conductor is known as an electromagnet. electromagnet. conductor is known as an electromagnet. electromagnet.

An electromagnet is a temporary magnettemporary magnet.

The direction of the magnetic field is determined by the

direction of the flow of the current.flow of the current.

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The magnetic field of a straight wire

carrying electric current

The direction of the magnetic field through a

straight wire is determined by :

a. Maxwell's corkscrew rulea. Maxwell's corkscrew rule

b. The right- hand grip rule.

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Maxwell's corkscrew rule

If a cockscrew is turned

clockwise, the direction

of the screw represents of the screw represents

the direction of the

magnetic field.

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Right-hand grip rule

• When a straight wire is

gripped with the right

hand, the thumb points

in the same way as the

current.current.

• Your curved fingers fingers

show the direction of direction of

the magnetic fieldthe magnetic field

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