Suplemen Dynamic Electric

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Dynamic Electricity

Electricity influences human daily life in the whole world. Most of us depend on electricalinstruments or appliances to make our lives safer, healthier, easier and morecomfortable.

some electrical appliances or instruments that require electricity

Although this car is a toy, it needs electrical energy to operate. So do a calculator and an electricclock. There are electric currents flowing inside these appliances a clock and a small radio.rite down all electrical instruments that you use or you have ever seen. !lassify them

according to their function, i.e. which instruments produce light, sounds, heat, kinetics and soon."n a simple circuit. To turn on the bulb, you have to make a continuous path of conductingwires or cables, by connecting the cable from the negative terminal of the battery to the bulb

body that is a metal thread, and from the positive terminal of the battery to the bulb tip at the base as shown in Figure

hat do we need to produce electricity to power a bulb or a radio# $irst, we need electrons.Second, we need a battery as the electric energy source to move the electrons.Third, we need a cable as the path where the electrons move.$ourth, you have to do what you have done %ou have to provide a closed circuit for theelectrons. $or e&ample, a closed circuit.

The electricity you use is called dynamic electricity . 'ow look at second figure . The bulb isnot on because one of the wires is not connected to it. A circuit that is disconnected is called anopen circuit . "n the open circuit , there is no path to flow the electrons, and they cannot turn onthe bulb, rotate a fan or turn on a radio."f the bulb is on, the circuit is a closed one. Simple!ircuit Circuit is a closed path to flow electricity. hat is the direction of the electrons flowingaround the circuit# An electron flows from a place having more electrons, i.e. the negative poleof a battery, to a place having a fewer electrons, i.e. the positive pole of a battery.



$igure shows the electron current from the negative to positive poles. The electron flowingaround a circuit, results in electric current . So, electric current is flowing electrons. Scientistsformerly thought the current flows from the positive to the negative pole of the battery. (p tonow, this has been the consensus among the scientists on the direction of the electric current. So,the direction of electron flow is reversed to the direction of the electric current. figure shows thedirection of this electric current.

Open and Closed Circuits

A closed circuit is a circuit with all parts connected, so that its electric current can flow. )rovidea battery, a bulb and cables.

The *hm+s aw can be e&pressed by-

The unit of " current/ is ampere, 0 voltage/ is volt, and 1 resistance/ is ohm. *hm+s awe&plains how voltage, current and resistance in a circuit are related. *hm+s aw can be written inthree formulas. The formula that you use depends on the variable you need. *hm+s aw statesthat if the voltage across a resistor is higher, its current will also be higher2 and if it has lowervoltage, then the current will also be lower. $or e&ample, if the voltage is doubled, then thecurrent is doubled, too. This relationship is illustrated in the meters are showing voltage andcurrent and constant resistance.

*hm+s aw also shows that if the voltage is kept constant, lower resistance will produce highercurrent, and higher resistance will produce lower current. $or e&ample, if the resistance is halflowered, the current is doubled. $or constant resistance, if the voltage is higher, the current willalso be higher2 and vice versa. $or constant voltage, if the resistance of the circuit is higher,the current will decrease2 and vice versa. R increases, I decreases R decreases, I increases

Example 1(sing *hm+s aw , prove that the current flowing in 34 Ω ,5resistor is higher if the voltage isincreased from 6 0 to 74 0.

SolutionThe following calculation shows an increase of current flow made, from 4.6 A to 7 A.$or 0 8 6 0,$or 0 8 74 0,0 increases, " increases 0 decreases, " decreases%ou certainly have e&periences in utilising electricity. Every time you turn on the lights, radio,television or start driving, or turn on a flashlight, you are utilising electricity. 9ow can electricityflow# To answer this question, do the following observation.What you needTwo flashlights, each with three batteriesWhat to do3. )ut : batteries one by one in to flashlight 3 by putting the battery heads first.



7. )ut : other batteries in flashlight 7, in the same order as procedure 3, but the last battery isreversed.

:. Turn on both flashlights. 'otes- it is suggested that you use a battery which is insulated around its positive pole.Analysis3. hy is flashlight 3 on but flashlight 7 is not on#7. "f all batteries in flashlight 7 are taken out and arranged as in procedure 7, then the positive

pole is connected to the tail of a dc;bulb and the other pole is connected to the body of the bulb through a wire, will the light be on# hy#

A !onductor and an "nsulator A Conductor

A conductor is a material that enables electrons to move easilythrough it. Metals like copper and silver are made of atoms whoseelectrons are not tightly bound so that they move easily through amaterial made of these kinds of atom. Therefore, an electric wire,which is generally made of copper, is a good conductor. Silver can be avery good conductor too, but it is much more expensive than copper.

An InsulatorAn insulator is a material that does not enable electrons to move

easily through it. Beside plastic, some good insulators are wood,rubber and glass.

s there any electrostatic discharging when you touch a door handlemade of wood! "igure #$%& shows some conductors and insulators.

An Electric !ircuitSeries Circuit 'ust like a runner who has to touch each base in a softball game, in aseries circuit, electric current (ows from one part to another part. nthis circuit, electric current has only one path to (ow. f you have everinstalled lighting decoration, you might ever feel frustrated in trying to)nd a broken bulb Figure 3.30 . *ow could this happen+ if one bulb iso results in all bulbs to be o !

Because each part in a serial circuit is connected with each other,the amount of current is the same everywhere in the circuit. f there isa broken part in the circuit, there would be no current (owing in thecircuit. This is called an open circuit . -oes this explain why thebroken bulb causes the entire bulbs to be o !

Adi, ris and Ani are connecting two identical bulbs to a battery asshown in "igure



Before they connect the bulbs to the battery, the teacher asks thepupils to predict whether both bulbs will be on brightly or o . Theyknow that the brightness of a bulb depends on the amount of thecurrent (owing through it. Adi said that only the bulb located closer tothe positive terminal of the battery that will light up because all

currents will be consumed by the bulb. ris says that the second bulbwill light up, but it will be dimmer than the )rst one since some of theelectrical energy are converted into heat and light on the )rst bulb. Anisays that both bulbs will light up at the same brightness becausecurrent is a (ow of charges, and because the charges that pass the)rst bulb cannot (ow through any other part, both bulbs will have thesame amount of current. n your opinion, whose opinion is correct!

n a series circuit, electric current only has one path to (ow. Thismeans that the current is the same everywhere in the circuit. f youconnect three ammeters to the circuit as shown in "igure #$#%a, all of them will show the same value. This circuit, where all currents (owthrough each component of the circuit, is called series circuit .

n a series circuit, the voltage of the source, / source , is the same as that

across bulbs A and B, as in Figure 3.31.

V

source= V

A+ V

B

Because the amount of current I that (ows through the bulbs is thesame, then / A 0 1 A and / B 0 1 B. Therefore, / source 0 1 A 2 1 B or / source 0

31A 2 1 B4. The current (owing through the circuit can be calculated

using the following formula. V

source

I =

1 A 2 1 B

This e5uation applies for any number of series resistors, notonly for two as in Figure 3.32a . The same amount of current will(ow to a single resistor, 1, which has the same value as that of thetotal resistance of both bulbs. This resistance is called the equivalentresistance of the circuit. "or a series resistance, the e5uivalentresistance e5uals the total resistance of all resistors connected inseries.

R = R A

+ RB for two resistors connected

in series, and

R = R A

+ RB

+ RC for three resistors

connected in series, etc.



6otice that the e5uivalent resistance value is always higher thanthat of each of the resistors connected in series. To calculate thecurrent, , that is (owing in a series circuit, )rst calculate thee5uivalent resistance, 1, and then use the following e5uation tocalculate .

V source

I =

1Examples1esistors %7 Ω , #7 Ω , 87 Ω are connected in series with a 97/ powersupply. "ind the e5uivalent resistance of that series. "ind the current of that series. "ind the voltage di erence in the tips of each resistor.

Steps to problem solvingiven

As!ed$ The e5uivalent resistance, R

$ The current, I

$ The voltage of each resistance /A

,/B

,/:

Solution To )nd the e5uivalent resistance of the three resistorsR = 20 Ω + 30 Ω + 10 Ω

R = 60 Ω

The current in the serie

The voltage in each resistance

Exercises8. Two resistors, &7 Ω , and ;7 Ω are connected in series with voltage

%%7/. "ind the e5uivalent resistance, the current in the series andthe voltage in each resistance.%. 1esistors 877 Ω , <77 Ω , #77 Ω are connected in series with voltage

=/. "ind the e5uivalent resistance, the current in the serie and thevoltage in each resistance.



"arallel Circuit

>ook at the circuit shown in Figure 3.33a. ?hat is the amountof current (owing in the circuit! The current from a source can (ow inall three resistors. A circuit, where there are some di erent paths forthe current to (ow is called a parallel circuit . Three resistors areconnected in parallel and each end of the three parts is connected

together as in Figure 3.33b . n a model of mountain river for thecircuit, the three resistors circuit is described as three rivers goingdown a mountain. Some rivers might be (owing through large paths,while some others through smaller ones. *owever, no matter howmany branches or paths where water (ows are, the total amount of water (owing down the mountain before and after the water isdistributed into several paths is the same.

Similar to this model, in a parallel circuit, the total current is thesum of current in each path. But the voltage along each path is thesame.

?hat is the current (owing in each resistor! This depends on theresistance of each resistor. "or example, in Figure 3.3# , the voltageof each resistor is 8%7 /. The current (owing in a resistor is 0 /@1, sothe current (owing in %& ohm resistor is 0 38%7 /4 @ 3%& ohm4 0 = A.

To calculate the current (owing to the other two resistors, we have tocalculate the same way. ?ith appropriate resistance on them.

?hat will happen if the < ohm resistor is disconnected from the

circuit! s the current (owing on %& ohm resistor di erent! -oes thecurrent depend only on voltage and its resistors! ?ill there be thesame case for a ohm resistor! ach branch in a parallel circuit isindependent. n "igure #.#&, the total current of the generator is &7 A.

A single resistor as the e5uivalent resistor will have &7A current if 8%7/ voltage is applied across the resistor. To measure the resistance, thee5uation applied is+



6ote that this resistance is smaller than the resistance of eachresistor connected in parallel in the circuit. Adding two or moreresistors in parallel circuit always reduces the amount of e5uivalent

resistance of the circuit. The decreasing of the resistance caused byeach new resistor adds new current path and increases the totalcurrent while the voltage keep constant.

To measure the e5uivalent resistance of a parallel circuit, )rst we haveto know that the total current is the sum of each branch current.

f A, B and : are the currents through branches and is the totalcurrent, then 0 A 2 B 2 : .

Cotential across each ends of the resistor is similar, so that thecurrent of each resistor can be measured. "or example, 1 A can be

measured from A 0 / @ 1 A. So+

C B A R

V

R

V

R

V

R

V ++=

By dividing both sides to /, there is an e5uation for the e5uivalentresistance of the three parallel resistors, i.e.

C B A R R R R

3333++=

This e5uation can be applied to calculate the e5uivalent of anumber of resistance in a parallel circuit.

"roblem Three parallel resistors & Ω , <7 Ω , 8% Ω are connected serially with a#/ battery. "ind the e5uivalent resistance of that and the current of that serie.

Steps to solve

iven

R A

= 4 Ω , R B = 6 Ω , dan R

C = 12 Ω pararelled conected

V source

= 3 V

As!ed$ The e5uivalent resistance, R

$ The current, I

Solution To )nd the e5uivalent resistance of the three resistors, the formula isapplied+



the negative terminal of the battery to the bulb.. The )lament in thebulb becomes very hot and glown.

The current (ows through wires in the circuit from one end of thebattery to the other end of the battery. Culling the switch backward toits normal position will reopen the circuit and turn the bulb o .$o% to Calculate t&e 'oltage and Current in aSerial Circuit(

n a series circuit, the voltage of the source, / source , is the same as

that across bulb A and B, as in Figure 3)31.

V source

= V A

+ V B

Because current I (owing through the bulbs is the same, then / A 0 1 A and / B 0 1 B. Therefore, / source 0 1 A 2 1 B or / source 0 31 A 2 1 B4. Thecurrent (owing through the circuit can be calculated using thefollowing formula.

V source

I =

1 A 2 1 B

This e5uation applies for any number of series resistors, notonly for two as in Figure 3)32a . The same amount of current will

(ow to a single resistor, 1, which has the same value as that of thetotal resistance of both bulbs. This resistance is called the equivalentresistance of the circuit. "or a series resistance, the e5uivalentresistance e5uals the total resistance of all resistors connected inseries.





"arallel Circuit

>ook at the circuit shown in Figure 3)33 ?hat is the amount of

current (owing in the circuit! The current from source can (ow in allthree resistors. A circuit, where there are some di erent paths for thecurrent to (ow is called a parallel circuit . Three resistors areconnected in parallel and each end of the three parts is connectedtogether as in Figure 3)33 . n a model of mountain river for thecircuit, the three resistors circuit is described as three rivers goingdown a mountain. Some rivers might be (owing through large paths,while some others through smaller ones. *owever, no matter howmany branches or paths where water (ows are, the total amount of water (owing down the mountain before and after divided into severalpaths is the same, and the vertical distance between water falling fromthe top of the mountain to the ground is the same.

Similar to this model, in a parallel circuit, the total current is thesum of current in each path, and the voltage along each path is thesame.

*ow much is the current (owing in each resistor! This dependson the resistance of each resistor. "or example, in Figure 3)3# , the

voltage di erence of each resistor is 8% /. The current (owing in aresistor is 0 /@1, so the current (owing in & ohm resistor is 0 38% /4@ 3& ohm4 0 7,%= A. To calculate the current (owing to the other tworesistors, we have to calculate the total current generated by thevoltage generator, i.e. being e5uivalent to the amount of current(owing to all three paths. n this case, it is 8,;= A.

?hat will happen if the 8% ohm resistor is disconnected from thecircuit! s the current (owing on & ohm resistor di erent! -oes thecurrent depend only on voltage di erence and its resistors! ?ill therebe the same case for a %& ohm resistor! ach branch in a parallelcircuit is not interdependent. n picture 8&$8&, the total current of thegenerator is 8.;= A.

A single resistor as the e5uivalent resistor will have 8.;= A current if 8% / voltage is applied across the resistor. To measure the resistance,the e5uation applied is+



=== A

V

I

V R

@6.3

37

6ote that this resistance is smaller than the resistance of each

resistor connected in parallel in the circuit. Adding two or moreresistors in parallel circuit always reduces the amount of e5uivalentresistance of the circuit. The resistance is declining because each newresistor adds new current path and increases the total current whilethe voltage di erence does not change.

C B A R

V

R

V

R

V

R

V ++=

xercises8. Two resistor, &7 Ω , and ;7 Ω are connected in series with voltagedi erence %%7/. "ind the e5uivalent resistance, the current in theserie and voltage di erence in each resistance.

%. 1esistors 877 Ω , <77 Ω , #77 Ω are connected in series with voltagedi erence =/. "ind the e5uivalent resistance, the current in the serieand voltage di erence in each resistance.

By dividing both sides to /, there is an e5uation for the e5uivalentresistance of the three parallel resistors, i.e.

C B A R R R R

3333 ++=

xamples1esistors & Ω , <7 Ω , 8% Ω are serial pararelled connected with a #/battery. )nd the e5uivalent resistance of that and the current of thatserie.

Steps to problem solving

*&at are !no%n(

R A

= 4 Ω , R B = 6 Ω , dan R

C = 12 Ω pararelled conected

V source

= 3 V

*&at are questioned($ The e5uivalent resistance, R

$ The current, I

Ans%er To )nd the e5uivalent resistance of the three resistors



Ω=Ω

=

Ω=

Ω+

Ω+

Ω=

Ω+

Ω+

Ω=

++=

7<

37

37<

373

377

37:

373

<3

>3

3333

!?A

R

R R R R

To )nd the current in the serie

A6,37

0:S =Ω

== R

V I

Exercises8. Two resistors, 87 Ω , and 8= Ω are pararelled connected with battery

9 / . "ind the e5uivalent resistor and the current of that serie.%. 1esistors 8% Ω , 8= Ω , %7 Ω are serial connected with voltage

diference 8% /. "ind the e5uivalent resistance, and the current passesthrough that resistor.

Review

8. ?hat does electrical current means! ?here does the electrical current happen!

%. ?hat does closed circuit mean!#. Based on Dhm >aw, what will happen if the current in a

serie de creases!&. n a serie, there is &77 resistance, if the voltage source is

8%/, *ow many ammeter is the current in that serie!=. Three resistance %7?,#7?, =7? are serial connected

with the 8%/ voltage source, calculate the e5uivalentresistance and current in that circuit.

<. 1esistors <7?, &7?, %7? are paralleled connected withpower supply 87/. "ind the substitute resistor and thecurrent in that circuit.

Eou certainly have experiences in utilising electricity. very timeyou turn on the lights, radio, television or start driving, or turn on a(ashlight, you are utilising electricity. *ow can electricity (ow! Toanswer this 5uestion, do the following observation.

An Electromotive $orce To maintain the (ow of the current in a circuit, a device to maintain

the potential di erence is needed. This device is called electromotive+orce ,em+- source . The most common examples of emf source are abattery and a generator. A battery transforms chemical energy intoelectrical energy. A generator transforms mechanical@kinetic energyinto electrical energy. The principle of these devices will be discussedin a later section.



So, an electromotive force source is a device that convertschemical, kinetic or other forms of energy to electrical energy neededto maintain a steady (ow of electric charges. The function of thissource in an electric circuit is similar to that of a water pumpmaintaining a steady (ow of water through a pipe.

Electricit +rom /agnetism

inetic energy can be converted into electrical energy. Moving a

magnet in a coil wire can generate electric current. A device

consisting of a coil wire and a magnet to generate electricity is called

a generator . A generator converts kinetic energy into electrical

energy.

*&at ou need Two (ashlights, each with three batteries

*&at to do8. Cut # batteries one by one in to (ashlight 8 by putting the battery

heads )rst.%. Cut # other batteries in (ashlight %, in the same order as procedure

8, but the last battery is reversed.#. Turn on both (ashlights.

6otes+ it is suggested that you use a battery which is insulatedaround its positive pole.

Anal sis8. ?hy is (ashlight 8 on but (ashlight % is not on!%. f all batteries in (ashlight % are taken out and arranged as in

procedure %, then the positive pole is connected to the tail of a dc$bulb and the other pole is connected to the body of the bulb througha wire, will the light be on! ?hy!



This e5uation can be applied to calculate the e5uivalnet of anumber of resistance in a parallel circuit.

D Electric Current Sources

To maintain the flow of the current in a circuit, a device to maintain the potential difference is

needed. This device is called electromotive orce !em " source . The most common e&amples ofemf source are a battery and a generator. A battery transforms chemical energy into electricalenergy. A generator transforms mechanical kinetic energy into electrical energy. The principle of these devices will be discussed in a later section.So, an electromotive force source is a device that converts chemical, kinetic or other forms ofenergy to electrical energy needed to maintain a steady flow of electric charges. The function ofthis source in an electric circuit is similar to that of a water pump maintaining a steady flow ofwater through a pipe.

Electricity rom #agnetismBinetic energy can be converted into electrical energy. Moving a magnet in a coil wire cangenerate electric current. A device consisting of a coil wire and a magnet to generate electricity iscalled a generator . A generator converts kinetic energy into electrical energy.

Electricity rom the Sun9ave you ever used a calculator powered by solar energy# Solar energy supplies power to thecalculator. A solar cell converts sun rays to electrical energy. hen the sun rays hit the solar cell,electric current is produced. A solar cell generates electricity without damaging theenvironment. The use of solar energy can create no pollution at all. This is unlike energy

produced by burning coals. "t creates air pollution. (sing nuclear energy to produce electricalenergy can also create water and land pollution.

Electricity rom a $atteryA battery is a device that generates electricity. "t converts chemical energy into electrical energy.A battery is mainly made of electric cells. Each cell consists of two different materials calledelectrodes and electrolyte . Electrolyte is a chemical solution which produces chemicalreactions. Those chemical reactions release electric charge. Electric cells can be dry or wet cells,depending on the type of the electrolyte used. "n a wet cell, as in a car battery accumulator/, theelectrolyte is liquid. "n a dry cell, as in the battery of a flashlight, the electrolyte is adry solution like paste.

%rimary CellThis kind of cell cannot be recharged. hen the cell is used up it has to be thrown away. "t is

because the chemical reactions inside the cell cannot be renewed. Some e&amples of primarycells are battery used for flashlights, toys, etc.

Secondary CellThis cell can be recharged because its chemical reactions can be renewed. hen the cell suppliescurrent to a load such as a bulb, the cell releases charges. The current in the cell tends toneutraliCe charges in electrodes. 9owever, a flow of current can be reversed to regain thecharges in the electrodes. This process is called cell charging . This charging process must besupplied by a dc direct current/ voltage source. ?ecause the secondary cell can be charged, it isalso called storage cell source .9ave you ever noticed that the work of a tape player is caused by the energy generated from the

battery# 9ow does the battery work in such a tape or other appliances#The following section is the e&planation to that question.A Dry Cell $atteryThe battery most common by used is a dry cell

battery. ook at a dry cell shown in Figure &'&( . The outesCinc container of a dry cell encloses the humid chemical

paste in the cell. "n the middle of the paste these shell is asolid carbon rod. The carbon functions as a positive poleand the Cinc functions as a negative pole. A dry cell



works like an electron pump due to its potential difference between the positive and negative poles. hat is the causefor this potential difference#"f both poles of the dry cell are connected to a closedcircuit, as in a flashlight, a chemical reaction involvingcarbon, Cinc, and some chemicals in the paste will occur.As a result, the carbon loses electrons so it becomes

positively charged D/, and forms the positive pole D/ of the dry cell. *n the other hand, Cinc gains more electronsand these electrons form the negative pole ;/ of the drycell. The potential difference between both poles causesthe current to flow through a closed circuit, as when youturn on the radio.Figure &'&("mportant elements of a drycellDry cell produces potential differencedue to the chemical reactionthat involves Cinc andchemical substances in the

paste.A $atteryScience Words

batteryO)*ectives; +o explain the structureand the principle of primaryand secondary electriccellscarbon rod as

positive poleCinc as thenegative polechemical

paste plasticwrap144 / Student Book Grade IX Chapter 3%ou may connect two or more cells in a series to

produce higher voltages. These series of two or more cellsare called )attery as shown in Figure &'&, . !an youmention electrical appliances at home or at school thatrequire more than one dry cells to operate#A battery may also be a series of wet cells. A wet cell,as shown in Figure &'&- , contains two plates made of different metals that are immersed in sulphuric acid.9ave you ever noticed a wet cell or usually called anaccumulator in a car# Most car accumulators consist of aseries of si& wet cells made of lead and lead dio&ide platesimmersed in sulphuric acid. The chemical reaction of eachcell produces a potential difference around 7 0. "f there areseries of < wet cells in an accumulator, then in total it

provides 37 0 potential difference. hen the car runs, thecar dynamo helps to recharge the accumulator so that itdoes not run out quickly.A Wet Cell $atteryFigure &'&,A )attery dry cell.

A wet cell produces potential



difference as a result of itschemical reactions which involvelead and lead dioxide

plates immersed in sulphuricacid.Figure &'&-A car battery is a series of wet cells.

positive polelead plate aslead dio&ide plate negative poleas positive polenegative

pole Student Book Grade IX Chapter 3/ 145An ammeter is an instrument used to measure currentthat passes through a node or another part of a circuit. "f youwant to measure the current in a resistor, you have to placean ammeter in series with the resistor. The current passingthrough the circuit will decrease when the device has higher resistance. Therefore, the resistance of the ammeter resistanceshould be very small.Figure &'&. shows an ideal ammeter where a Ceroresistor is placed in parallel to 4.43 ohm resistor. The resistanceof the ammeter is much smaller than that of the shunt. Thecurrent will drop from 3.4 A to 4.===6 A. Another instrumentis a voltmeter. "t is used to measure voltage that passes acrossa circuit. To measure the voltage across a resistor, connectthe voltmeter in parallel to the resistor. The resistance of thevoltmeter should be very large so that it enables to detectsmall current and voltage changes in the network. ook atthe circuit shown in Figure &'&. . An ideal voltmeter consistsof a Cero resistor in the meter connected in series to a 34 kiloohm resistor. hen this resistor is connected in parallel to 1?,the total parallel resistance becomes much lower than 1?.Therefore, the total resistance of the circuit decreases. "tincreases the current.#easuring Current and /oltageFigure &'&.An ammeter to measurecurrent is always connectedin series in a circuit..Ammeter146 / Student Book Grade IX Chapter 31A is constant, and the current is going up, whichresults in the increasing voltage across 1A. ?ecause thevoltmeter is connected to the ends of the resistor 1?, thevoltage across the resistor 1? decreases. "ncreasingresistance in the voltmeter reduces the voltage. (sing avoltmeter with a 34,444 ohm resistor will change thevoltage across 1? tips from 34 0 to =.===6 0. A modernelectronic multimeter has very large resistance, i.e. 34@ohm and produces a very small voltage change.Reading Electric Current in an AmmeterSteps in reading electric current measured in anammeter are as follows- 3/ reading the scale shown bythe needle, 7/ dividing the value on the scale by the



ma&imum scale, and :/ multiplying the result of step7/ by the selected ma&imum scale. $or e&ample, look at

the illustration in Figure &'0 . "f the selected ma&imumscale is 34 A and the ma&imum value measured is 34 A,the electric current measured by the ammeter as shown

by the needle is-> 6/ & 34 A 8 A

or 4. 3/ & 34 A 8 AFigure &'&2A laboratory voltmeter tomeasure potential difference.0oltmeter is connected in

parallel.34 6 4A4 34,> 4,< 4,4,77 :4 63 >Figure &'0An ammeter

Student Book Grade IX Chapter 3/ 147 Reading Electric /oltage in a /oltmeter1eading the voltage measured by a voltmeter can

be done by dividing the value shown by the needle byits respective ma&imum value. The result is thenmultiplied by the selected ma&imum scale. $or e&ample, look at the illustration in Figure &'01 . "f theselected ma&imum scale is 36 0, the electric voltagemeasured by the voltmeter is

37 36/ & 36 0 8 37 0or

> 6/ & 36 0 8 37 0#easuring /oltageTo measure voltage, connect the voltmeter to the componentwhose voltage is to be measured. Thisconnection is a parallel connection. The negative terminalis connected to the negative side of the circuitand its positive terminal to the positive one. Figure &'03shows a voltmeter connected in a circuit to measurethe voltage across a resistor.36 6 404 67 : >3< =4 : 3637Figure &'01A voltmeter.Figure &'03An e&ample of voltmeter connection to measure thevoltage of a simple circuit.148/Student Book Grade IX Chapter 3Electrical %ower and



Energy3. Take the battery out of a radio. !onnect one end of acable to the battery using a cellotape or rubber band.7. !onnect the other end of the cable to the battery inthe radio.:. !onnect one end of the second cable to one terminalof an ampere meter and the other end to the battery.>. (se the third cable to connect another terminal of theampere meter to the other tip of the battery.6. Turn on the radio and measure the current by writingthe value in the ampere meter.<. Fetermine the voltage of the battery. "f two batteriesare connected in a series, do not forget to sum both

batteries+ voltage to obtain the total voltage.Analysis3. (se the voltage and the current to calculate theelectrical power in watt.7. ill the electrical power change if you turn on theradio with a different volume# Try to turn on theradio with different volumes. hen is the higest

power used#What is the power needed to turn on a taperecorder4Figure 1('1An observation to calculateelectrical power of a radio"f you have a tape recorder, you have to spare someof your money to buy batteries needed. 9ow much is theelectrical power needed by the tape recorder# Make anobservation that is suggested below.3 :73Science Wordselectrical power energykilo watt hour O)*ectives; +o explain therelationship betweenelectrical power,electrical energyand their units k hand Goule/.; +o apply theconcept of energyand electrical power in the measurementof electricity usage

based on theamount shown ink h meter.; +o show someappliances usingelectrical energy.E

Student Book Grade IX Chapter 3 / 149hat do you have in your mind if you hear the

word power # The word power has many different



meanings. the chapter that deals with Energy andork, you have learned that power is the capacity of

doing work. Electricity can work for us. Electricalenergy is easily changed into other kinds of energy.$or e&ample, the leaves of a fan can spin and cool youwhen the electrical energy is changed into mechanicalenergy. An electric iron changes electrical energy intoheat energy. Electrical power is the speed of electricalenergy to be changed into other forms of energy.Fifferent electric appliances use differentconsumption of electric energy. These appliances areoften advertised by showing their power consumption,which depends on the amount of energy needed byeach to work. The electric heating elements in anelectric iron and an electric stove have high electrical

power. 9owever, they are not used at all the time.Things that are used all the time, e.g. a refrigerator,usually uses higher energy. +a)le &51 shows the power consumption of some electric equipment, and Figure&'00 shows the label with power information on it.Electrical power is measured in watt / unit or kilowatt k /. The amount of power used by electricequipment is equivalent to the potential difference andthe electric current, and it can be measured by multiplyingthem.%ower 6 voltage x currentWatt 6 volt x ampere% 6 / x 7nat5alat 8istri9 Rumah#easuring %owerElectrical %ower

att ! " or k#lowatt !k "is the unit in which electrical

power is measured.15$/Student Book Grade IX Chapter 3+anggaEnergy :sed )y ;ousehold Electric E<uipmentElectric E<uipment 8ength o :sage :sage o :sage o !;our=Day" %ower !watt" Energy !9Wh=day"9airdryer 4,76 3444 4,76Microwave *ven 4,64 @44 4,:6Stereo 7,6 34= 4,7@1efrigerator $reeCer 34,44 <36 <,36Television colour/ :,76 744 4,<6344;watt light bulb < 344 4,<4>4;watt neon lamp 3 >4 4,4>Figure &500Most electric equipment has labelsshowing the amount of

power that is consumed.F+a)le &51

Student Book Grade IX Chapter 3 / 151%ro)lem4.43 A current flows in a calculator. "t works in = 0 potentialdifference. Fetermine the power used by the calculator#Steps to solve3. Hivencurrent, " 8 4.43 A voltage, 0 8 = 0





in a second. *ne kilowatt;hour is equal to 3444 wattof power used in an hour. The electricity companygives you a bill every month for every kilowatt;hour of power that you use. %ou can count the amount of the bill by multiplying the amount of energy usedand the price of every kilowatt;hour. +a)le &53shows some e&amples of the price of running electricequipments.+a)el &53Energi yang Diguna9an Alat5alat Rumah5tangga;airdryer Stereo Colour +/Average power in watt 3.444 watt 34= watt 744 wattFaily using hours 4,76 hour :,4 hour 7,6 hour Monthly using hours @,6 hour =4,4 hour @6,4 hour Monthly watt ; hour @. 644 att;hour =. 34 watt;hour 36.444 watt;hour Monthly k h usage @,6 k h =, 3 k h 36,44 k hTariff per k h 1p 744 1p 744 1p 744Monthly bill 1p 3,644 1p 3 <7 1p :.444Electric E<uipment154/Student Book Grade IX Chapter 3#easuring Electrical EnergyExampleA refrigerator is one of the main electrical power appliances at home. "f it uses :44 and runs for 74 hours a day, what is the energy in k h/#Steps to %ro)lem Solving3. Hiven power, ) 8 :44 8 4.: ktime, t 8 74 hours7. Asked energy, E:. !hoosing equation E 8 ) & t>. )roblem solving E 8 4.: k & 74 hours 8 < k hExerciseA 344 light bulb is on for 6.6 hours. hat is the energy k h/ used#mary3. hat is electrical power#7. A television uses : A current on 774 0 voltage. "t is on for 7 hours. !alculate the

power used in k and the energy in k h.:. Critical +hin9ing - ?ased on the data in Table 6;7, how many k h energy is neededto enIoy the stereo everyday in May# 9ow much will you pay if the price per k h is1p 744,44#Chapter reviewSummary

Student Book Grade IX Chapter 3 / 155<. ightning shows the discharge of JJa. proton

b. electronc. neutrond. atom nucleus@. Static electricity moves from one

place to another because JJa. the electrons are attracted byother electrons

b. the potential energy is attractedto kinetic energyc. the electrons are attracted to thenatural magnetic pole of the Earthd. the electrons are attracted to

positive charges. An electric circuit needs JJ



a. an electron source, a way for theelectrons, and a closed switch

b. a proton source, a way for theelectrons, and a closed switchc. a proton source, a way for theelectrons, and an open switchd. an electron source, a way for theelectrons, and an open switch=. The difference of potential energy per charge unit between two electrodes ismeasured in JJa. ampere

b. coulombc. ohmd. volt!hoose the appropriate word or e&pression to complete the followingsentences.3. Something becomes positively charged if it JJ.a. loses electrons

b. loses protonsc. receives electronsd. receives protons7. "f two negative charges attract eachother, they will JJa. repel each other

b. attract each other c. not attract or repel each other d. remove the charges into the ground:. "f an atom loses an electron, it will JJa. be positively charged

b. be negatively chargedc. stay neutrald. become an isotope>. An e&ample of a good insulator is JJa. copper

b. silver c. woodd. salt water 6. !onnecting something having a charge tothe ground in order to remove the charge intothe ground is called JJJa. charging

b. groundingc. flowingd. induction156/Student Book Grade IX Chapter 334. The difference of the energy brought

by electrons in different nodes in acircuit determines JJa. voltage

b. resistancec. currentd. power 33. The resistance of an electric wirecauses its electrical energy to changeinto JJa. chemical energy



b. nuclear energyc. heat energyd. sound37. hich of the following wires tends tohave the lowest resistance#a. long wire

b. glass;fiber wirec. heated wired. thick wire3:. Electrical energy is measured in JJa. volt

b. 'ewtonc. kilowattd. kilowatt;hour Answer the ollowing <uestions in yournote)oo9 using complete sentences'3. Mention the differences between staticelectricity and dynamic electricity.7. hat is the difference betweenelectrons of a conductor and those of an insulator#:. hy do electrons to flow in a circuit#>. 9ow does lightning happen#6. hat is the relationship betweenresistance and potential difference withthe amount of the current flowing in acircuit#3. A lightning conductor is a conductor that is placed on the roof of a buildingand is grounded. 9ow does theconductor protect the building fromlightning#7. E&plain how an electroscope can be usedto recogniCe something negativelycharged.:. A toy car has 3.6 A current and 7 ohmresistance. Fetermine the voltagerequired#>. The electric current in an electricequipment that is connected to a 774 0source is 3 A. 9ow many kilowatt;hour of electrical energy is needed for a

period as long as > hours#6. %ou are asked to connect a stereo,television, tape recorder and a lamp ina circuit. ill you connect them in

parallel or series# 9ow can you preventa fire caused by a short circuit# E&plainyour answer.

Student Book Grade IX Chapter 3 / 157 are connected in a series. hat is your hypothesis# hat are the independentvariables# hat are the dependentvariables# hat are the controlvariables#6. Classi ication - Make a list of 34 electricappliances you have ever seen. !lassifythese appliances according to their current source, dc or ac.



Electroscope$rought closerCausesCausesAnd they<. )laying football and standing under atall tree during rain with thunder isdangerous. hy#3. #a9ing and using a chart - The resistanceof a 3 cm copper wire in differenttemperatures is shown be low.Resistance +emperature!#icro ohm" !oC"7 64: 7446 >@6Make a line chart of the data above."s copper a better conductor in highor low temperature#7. 7nterpreting Data - ook at the power consumption of electric equipment in+a)le &53 and calculate the currentused in each from a 774 0 source.

hich equipment absorbs the mostcurrent#:. Concept #ap - Make a conceptmap that organiCes the events inan electroscope that is attracted to

positively and negatively chargedthings. Fo not forget to show the flowof the electrons and the flow of charges, and also the movement of theelectroscope.>. :sing /aria)le? Constant andControl - Fesign an e&periment to testthe effect of current and voltage in acircuit if two batteries of equal voltage158/Student Book Grade IX Chapter 3)repare a concept map that showssteps in determining the amount of energyspent to operate appliances withknown voltage and current for a known

period of time.Concept #ap

Suplemen Dynamic Electric

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