CHAPTER 7-Electric Form 3

8/11/2019 CHAPTER 7-Electric Form 3

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CHAPTER 7 : ELECTRICITY

ELECTRICITY

CURRENT, VOLTAGE

& RESISTANCE

ELECTROMAGNETISM

ELECTROSTATICS

UNDERSTANDING ELECTRICITY

MEASURING ELECTRICITY

MAGNETISM

TYPES OF CIRCUIT

magnetic field

use of magnet

electromagnet

application

types & properties of charges

detect static electrical charges

everyday phenomena

safety measures to be taken

parallel & series

advantages & disadvantages

comparison

relationship

Ohms Law

effect

examples of sources

define the current, voltage and resistance

current

voltage

Measurement

& unit

1.1ELECTROSTATIC

What is electrostatic.

The study of static electrical charges.

Build up of electrical charges that do not move.

This charges can be positive or negative

How can static electrical charges can be produced

Produced when two objects or materials are rubbed together. Both object or material must be made up of insulators.

Examples :-

Glass, ebonite, and plastic such as acetate and polythene

Two types of charges :-

Positive charges

Negative charges

Neutral charges => has the same number of positive charges and negativecharges

***electron it call for negative charges

Free to move from one object to another

*** proton it call for positive charges Did not move

Neutral charges loses electron

It become positive charged Neutral charges gains electron

It become negative charged


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Properties of static electrical charges

Like charges repel each other

Unlike charges attract each other

Formation of charges

Charging by friction

o

When you rub a Perspex rod with a piece of silk cloth, the frictional force of therubbing causes electrons to flow from the rod to the silk cloth. Therefore, the rod willhave positive charges and the silk cloth will have negative charges.


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Formation of a negative charge

Rubbing a polythene rod with a piece of silk cloth causes electrons to flow from the cloth to

the rod. The polythene rod becomes negatively charged while the silk cloth becomes

positively charged

Detection of static electrostatic charges

Instrument

Electroscope

Electric charges on an object can be detected with an electroscope.

The main parts of an electroscope consist of a metal disc which is connected to a metal rod.

A gold leaf is attached to the other end of the metal rod.

The gold leaf is protected by a metal case with a glass front The metal disc and the rod are insulated from the metal case


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When a positively charged object is brought near the metal disc of a neutral electroscope,

the gold leaf will diverge

This is because the positive charges on the object attract the negative charges on the

metal disc and gold leaf of the electroscope. As a result, only positive charges remain

on the fold leaf and the metal rod

The gold leaf diverges because of the repulsion between similar charges on the goldleaf and the metal rod.

When a negative charged object is brought near the metal disc of a neutral electroscope, the

gold leaf will diverge.

This is because the negative charges on the object repel the electrons in the metal

disc to the gold leaf of the electroscope. As a result, the gold leaf and the metal rodbecome negatively charged.

The repulsive force between the remaining negative charges on the gold leaf and the

metal rod causes the divergence of the gold leaf

When a charged object is brought near the metal disc of a charged electroscope, an

increase or a decrease of the divergence of the gold leaf will indicate the type ofcharge on the object.

The table below shows the results obtained when a charged object is brought near the

metal disc of a charged electroscope.

Charges on

electroscope

Charges on

object

Divergence of

the gold leaf

Positive (+) Positive (+) Increase


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Positive (+)

Negative (-)Negative (-)

Negative (-)

Positive (+)Negative (-)

Decrease

DecreaseIncrease

When the charges on the object and the electroscope is similar, there will be an

increase in the divergence of the gold leaf When the charges on the object and the electroscope are opposite, there will be a

decrease in the divergence of the gold leaf.

Charging an electroscope by induction

When a negatively charged object is brought near the metal disc of a neutral electroscope,

the negative charges on the object will repel the negative charge on the metal disc of theelectroscope towards the gold leaf.

When the electroscope is earthed (by touching the metal disc with your finger), the negative

charges will flow out of the gold leaf through your body into the earth.

After removing your finger, followed by the negative charged object, the electroscope isnow positively charged

When a positively charged object is brought near the metal disc of a neutral electroscope,

the positive charges on the object will attract the negative charges on the metal disc of theelectroscope, leaving positive charges on the gold leaf

When the electroscope is earthed, negative charges will flow from the earth through your

body into the gold leaf, neutralizing its charge After removing your finger, followed by the positively charged object, the electroscope is

now negatively charged

Phenomena caused by static electrical charges Safety measures related to static electric charges

Lightning Produced by a discharge of electrical

charges from one cloud to another orbetween a cloud and the earth.

Positive static electrical charges build up

on the clouds during a storm as strong

wind rubs against water particles in thecloud causing lightning.

Most clouds floating in the air contain

more negative charges

When clouds that have negative charges

Lightning conductor The roof of high buildings are

usually installed with lightningconductors to protect these buildings

from being struck by lightning

The end of a lightning conductor that

is branched and sharp directs theelectric charges of the lightning to

the earth (earthed)

The other end of the lightning

conductor is buried in the ground to

direct the electric charges from thelightning to the earth without

damaging the building


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move closer to the Earths surface, these

clouds will cause materials on theEarths surface to become positively

charged

Charges jump from the clouds to theEarths surface because of electrostatic

attractive force

Hence, lightning is produced

Lightning can also occur when two

groups of clouds of opposing chargescome close to one another.

The base of a huge cloud is usually

charged negatively while the tip of acloud is charged positively

Electric charges will jump from one

group of clouds to another

Hence, lightning is produced.

Oil tanker Friction between a moving vehicle (oil

tanker) and the air charges the vehicle

electrostatically. Too much electriccharges produced may cause fire and

explosions

A metal chain To avoid such a spark, a metal chain is hung

from the body of the lorry so that is touches

the ground. Any charge produced by frictionwill flow to the ground through the chain

and thus prevent the accumulation of charges

on the lorrys body and reducing the risks offires or explosions

Aeroplane

Friction between an aeroplane and the aircharges the aeroplane electrostatically.

Sparks due to discharge of electrostatic

charges may develop when the planelands

Tyres made from slightly conducting rubber

Aeroplane tyres are specially made todirect electric charges to the earth when

the plane lands

Another Electrostatic Phenomena in our daily life

Spark plug

A spry of sparks is produced when a car engine is started


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Electric charges jump across spark space on the plug to produce sparks

These sparks cause fuel to burn to produce energy

Nylon clothes A crackling sound is heard when we take off our nylon clothes on a dry day

This because static electric charges are produced when the nylon clothes rub against

our body

Combing hair Dry hair will stand erect or will be attracted towards the comb on a dry day

This is because hair becomes charged when it rubs against the comb

Electronic lighter Electric are produced when an electronic lighter is lit produce sparks

The sparks will then ignite the gas


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What is electrical energy (electricity)

Electrical energy (electricity) is the flow of electrical charges (electron) in one direction

The continuous flow of electron forms an electric current.

The strength depend on the quantity of electron flow in a unit time

Defined of electric current is

The rate of flow of electrons (electrical charges) in a conductor

Observation the flow of electric current

Electric current is a flow of electrons through a conductor An electrical conductor allows the flow of electrons or electric charges through it. Examples of

electrical conductors are metal and carbon

An electrical insulator does not allow electrons or electric charges to flow through it. Examplesof electrical insulators are wood, rubber and plastic.

A Van de Graff generator shows that electric current is a flow of electrons When the motor of the generator is started, the dome becomes positively charged

When a wire is connected from a gas pipe, to the galvanometer and the dome of the Van deGraff generator in series, a deflection will be observed in the galvanometer.

This shows that there is a small current flowing through the wire and the galvanometer from the

dome to the gas pipe. A current is produced when electrons are attracted from the earth to neutralize the positive

charges on the dome

Electric current is said to flow from the dome to the earth. In reality, electrons flow from theearth to the dome.

What is Voltage

Is electrical energy that is needed to enable electrons to flow from one point to another in aconductor or

Potential different (p.d) or voltage is the difference in electrical potential energy between

any two points in a circuit

The energy must be supplied by source of electrical energy Example :- dry cell

What is resistance


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Resistance is the measure of how much an electrical component restricts current flow

The characteristic of a material that opposes (menghalang) the flow of electrons

Because electron that flow through a conductor constantly (sentiasa) experience(mengalami) obstruction(rintangan)

Direction of electron flow and electric current in an electric circuit

Electronsflow from the negative terminal to the positive terminal Electric currentflow from the positive terminal to the negative terminal

MEASURING ELECTRICAL ENERGY

The Instrument For Measuring Current

By using ammeter

The symbol for current isI

The SIunit is the ampere (A)1 ampere (A) = 1 000 miliamperes (mA)

1 miliamperes (mA) = 1 000 microamperes (A)

The ammeter is connected in series to a circuit when measuring current

If the ammeter needle does not move, this shows that no current is flowing in the circuit

The positive terminal of the ammeter should be connected to the positive terminal of the electricsource in an electric circuit

The Instrument For Measuring Voltage

By using a voltmeter

The symbol for voltage is VSI unit is the volt (V)


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The voltmeter is connected in parallel (selari) to a circuit when measuring voltage

Thepositive terminalof the voltmetershould be connectedto thepositive terminalof the

electric sourcein an electric circuit

The unit of resistance

The unit of resistance is ohmThe symbol for resistance isR

SIunit is the ohm ()Two types of resistors:

Fixed resistors with fixed resistance

Variable resistors (rheostats) with variable resistance

A good electrical conductor has low resistanceA poor electrical conductor has high resistance

The resistance in a wire depends on the following factors:

The length of the wire : the longer the wire, the more resistance it has.The thickness of the wire : the thicker the wire, the more resistance it has

The type of metal/ wire : different types of metals have different resistance

The relationship between current, voltage and resistance

The relationship between resistance and current

o In order to study the relationship between resistance and current, we have to keep the voltage

fixed, then manipulate the resistance and observe the changes in the current.

o The graphs shows that when resistance increases, current decreases and when resistancedecreases, current increases.

Current (I)

Graph of current against resistance

Resistance (R)


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The relationship between voltage and current

o When studying the effect of the change in voltage on current, the resistance in a circuit

fixed. Then, manipulate the voltage used and observe the changes in current.

o The higher the voltage in a circuit, the greater the current that flows in the circuit.

o The graph shows that the current flow through a standard resistor is directly proportional

to the voltage of the fixed resistor.

Voltage (V)

Gradient =I

V =R

Graph of voltage against current

Current (I)

o The relationship between the current in a resistor and voltage across the resistor is known as

Ohms Lawo Ohms Law states

o The current (I), passing through a conductor is directly proportional the potential

difference (V), across the conductor if the resistance remains constant

o Therefore, Ohms Law is expressed as follows:

R =I

VwhereRis a constant known as the resistance of the conductor.

V= the voltage in volt (V)

I = the current in ampere (A)

R = the resistance in ohm ()

Parallel and Series Circuit

Component of an electric circuit symbol

SymbolElectric

componentSymbol

Electric

component

Rheostat Variable resistor

Resistor

Ammeter

Bulb

Voltmeter Dry cell

V

I R


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SymbolElectric

componentSymbol

Electric

component

Fuse Earth

Power supply Galvanometer

Switch Battery

Circuit diagrams

A closed circuit An opened circuit

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CHAPTER 7-Electric Form 3

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