Science Chapter 7 Pmr

5/24/2018 Science Chapter 7 Pmr

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Chapter 7: Electricity

Electrostatic.

1. The study of electric charges at rest or static electricity is known as electrolysis

2. Some common phenomenon that are due to static electricity:

a. You hear little crackles when you pull off a supermarket plastic bag from the roll.b. When ones hair is combed with a plastic comb! tufts of hair will be attracted to "arious

parts of the comb.

c. When a piece of transparency is placed on a piece of paper and rubbed againts the

underlying paper! it is difficult to seperate the transparency from the paper.

#. Static electric charges can be produced through friction. You can use eletroscope to detect

static charges.

$. There are two types of electrical charges

a. The type of charge that is present on a rubbered polythene strip is called nega"e charge. The

polythene strip gain e%tra electrones and becomes negati"ily charged.

b. The type of charge that is present on a rubbed cellulose acetate strip that is called

positi"e charge. The cellulose acetate looses electrones and becomes positi"ely charged.&. The electroscope uses repulsion of the gold leaf to indicate when something is charged.

Prepared by: Mr Suntharan Muniandy @ Your Science Coach D' Permai Tution Centre


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'. (ike charges repel while unlike charges attract.

). The attraction and repulsion effect of charged materials shows that a force e%ist between

electric charges. This force is known as electrostatic force.

*. E%amples of e"eryday phenomena that are caused by static electric charges.

a. + glass window that is cleaned with a dry cloth soon sttracts dust.b. When you switch on the tele"ision! the screen attracts dust and the hairs on you hand.

c. + dry nylon shirt can be easily charged when rubbed againts the body. This makes it

difficult to take the shirt off ass it is attracted to the body.

,. Electrostatic precipitator! electrostatic paint spraying and photocopier are a few useful

application of electrostatic in our e"eryday life.

1-. The bulding up of static electrical charges on a larger scale can be e%tremely dangerous.

(ightning is one e%ample.

11. louds are charged when they are blown around. The charge that builds up on one part of a

cloud can flow out rapidly as a flash of lightning to another cloud or to the ground.

(ightning produces tremedous heat and causes serious destruction.

12. (ightning can cause damage to tall buildings. (ightning conductors are installed on thebuildings to reduce the chances of a lightning strike.

1#. + lightning conductor consists of a copper strip or rod. /ne end of the rod consists of

se"eral metal spikes sticking up into the air. The other end is connected to a metal plate

buried in the ground. This lightning conductor pro"ides a route for electric charges to pass

into the ground if lightning does occur. This pre"ents lightning from striking the building.

1$. Safety measures that are used to o"ercome electrostatic charges ha0ards:

a. n a petrol tanker! charges can build up in the rubber tyres. These chargers can produce

electric sparks that can cause an e%plosion if petrol "apour is present in the air. Therefore! all

petrol tankers carry a metal chain or conduction strip to pass the charges to the ground.

b. The tyres of an aircraft are made of conducting rubber. harges that build up in the

body of the aircraft during flight are conducted to the ground upon landing.

c. n the operating theatre! all precautions are taken to pass any charges produced to the



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ground.

Electricity

Sources of electrical energy

1. /ur main source of electrical energy comes from huge generation in power stations.

2. There are other sources of electrical energy. These include electric cells! batteries! small

generators and solar cells.

#. + an de 3raaff generator is commonly used to in"estigate electrical current.

$. When the charge on the dome of the an de 3raaff generator is earthed "ia a gal"anometer!

the pointer of the gal"anometer deflects! indicating a new current flow.

&. Since the gal"anometer is not connected to any power supply or source! the electric currenty

detected by the gal"anometer must be due to the flow of charge when it is earthed.

'. When more charge accumulates on the dome! the deflection of the gal"anometer pointerbecomes larger when the charge is earthed. Thus! we can conclude that the rate at which the

charge flows is the measurement of the electric current.

). The flow of electric current in a conductor is known as elctricity. Electric current is defined

as the rate of flow of charge through a conductor.

*. Electric charge that is stored in the an de 3raaff generator is a source of potential energy. t

has a "ery high electrical potential.

,. + potential difference is built up between the an de 3raaff generator and the Earth when

the dome is earthed.

1-. The potential difference that cause the flow of the charge between the dome and the earth is

known as "oltage. oltage is defined as the potential difference across two points or the

electrical energy supply needed to mo"e each unit of charge from one point to another.

11. Th eproperty of a material that resist the flow of electric current through it is known as

resistance. 4ifferent materials ha"e different resistance. opper is a good conductor as it has

a "ery low resistance.

12.



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Direction of the elctron flow

1. n a conductor there are millions of free electrons mo"ing randomly in all directions.

2. When a "oltage is applied across a conductor in an electric circuit! the elctrons are

pushed away from the negati"e terminal and attracted towards the positi"e terminal.

#. The flow of electrons causes an electric current flow from the positi"e terminal to

negati"e terminal of an energy source.



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$. The elctrons always flow in the direction opposite to the flow of electric current.

Measuring Electricity.

1. olatage! current and resistance of a circuit are the three important 5uantities for measuring

electricity.2. +ndre67arie +mpere disco"ered the unit for electric current! which is called ampere.

#. +lessandro olta disco"ered the unit for "oltage! which is "olt.

$. The unit for resistance! ohm! is named after 3erman scientist 3eorge Simon /hm.

&. The elctric path through the bulb! meter! battery and connecting wires is called a circuit. n a

complete circuit! current flows in the circuit and the bulbs lights up.

Relationship Between Current, Voltage and Resistance

1. urrent! "oltage and resistance are three 5uantities that are closely connected in a circuit.

2. hanges in the magnitude of one of the 5uantities will cause changes in the magnitude of the

rest of the 5uantities.#. + rheostat can be used to "ary the magnitude of the resistance os a circuit. The effecti"e

resistance of a circuit can be changed by sliding the contact of the rheostat.

$. hanges in the resistamce will change the amount of current and "oltage in a circuit.

&. Similarly! changes in the "oltage will change the amount of current an "oltage in a circuit.

'. Similarly! changes in the "oltage will change the amount of current and resistance in a

circuit. This shows that there is a close relationship between resistance! current and "oltage.



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3raph of current 8I) aaint! "oltae #$)

When a resistance of a circuit increases! the current in the circuit decreases.

When the "oltage across a conductor increases! the current that flows through it also

increases.

3raph of current 8I9 against length 8l9

Oh!s "aw

1. When a graph of "oltage 8$9 againts current 8I9 is plotted! a straight line graph through the

origin is obtained.



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2. This shows that $ I or $%I is a constant for the same conductor. This relationship is known

as /hms (aw.

#. /hms (aw states that:The current that flows through a conductor is directly propotional to the "oltage across the ends of

the conductor! pro"ided that the temperature and other physical conditions are unchanged.

$. onductors with a constant "alue for the ratio $%I are said to obey /hms law.

&. +ccording to /hms law! the ratio $%I is a measurement of resistance that acts againts the

flow of current through a conductor.

'. Thus! the relationship between "oltage 8$) current 8I) and resistance 8&9 can be written as:

$%I &

or

$ I&

#he concept of parallel and series circuits.

Componenet! (ound in an electric circuit

1. t is not easy to draw diagram of an electric circuit.

2. We need to use symbols to represent the different electrical components.



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#. ;sing the symbols! a circuit diagram can be easily drawn to show the connections of a

circuit.

$. + battery! a switch and a bulb that are connected with connecting wires form a simplecircuit. When the switch is closed! the circuit forms a complete circuit around which a

current flows.

&. +n electric circuit can be represented using a circuit diagram. ircuit symbols are used to

represent "arious components of a circuit.

'. There are two kinds of electrical circuits < series and parallel.

). When two bulbs are connected ne%t to each other in a circuit! they are said to be connected

in a series. The current flows through the two bulbs in a single path. There are no branches

in the circuit.

+ll the electrical components are connected end to end following one electrical path to the

ends of the battery

if one bulb is remo"ed! the circuit is broken. The other bulb does not light up.



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Each of the two bulbs glows dimly but are e5ually bright.

Series circuit

*. When two bulbs are connected seperately to the connected terminals of a battery! they are

said to be connected in parallel. The current from the battery flows through the two bulbs in

two seperate paths. There are branches in the circuit.

The electrical components are connected side by side following two or more electrical paths

to the ends of the battery.

f one bulb is remo"ed! there is still an unbroken circuit through the other bulb.=

Each bulb has direct connection to the battery so each bulb glows brightly.

>arallel circuit



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Science Chapter 7 Pmr

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