12405942 Chapter 7 ElectricityTeachers Guide 2009

8/12/2019 12405942 Chapter 7 ElectricityTeachers Guide 2009

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JPN Pahang Physics Module Form 5

Teachers Guide Chapter 7: Electricity

CHAPTER 7: ELECTRICITY

7.1 CHARGE AND ELECTRIC CURRENT

Van de Graaf

1. What is a Van de Graaff generator? Fill in each of the boxes the name of the part shown.

A device thatproducesandstore electric chargesat high voltage on its dome

- 1 -

dome

ruer elt

Metal dome

roller

roller

motor

+ ++

+

+

+

++

+

+


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2. ow are electrical charges prod!ced b" a Van de Graaff generator? And what t"pe of

charges is !s!all" prod!ced on the dome of the generator?

When the motor of the Van de Graaff generator is switched on# it drives therubberbelt.

$his ca!ses the r!bber belt to against the rollerand hence becomes charged.

$he charge is then carried b" the moving belt !p to the metal domewhere it is

collected.

A large amo!nt of chargeis b!ilt !p on the dome

Positivecharges are !s!all" prod!ced on the dome of the generator.

%. What will happen if the charged dome of

the Van de Graaff is connected to the earth

via a micrometer? &xplain.

$here is a deflectionof the pointer of

the meter.

$his indicates an electric c!rrentflow.

- 2 -


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'. (redict what will happen if a discharging

metal sphere to the charged dome.

When the discharging metal sphere isbro!ght near the charged dome#

sparklingocc!rs.

An electric c!rrentflow!

). (redict what will happen if hair of a

st!dent is bro!ght near to the charged

dome. Give reasons for "o!r answer.

$he metal dome attracts the hair and

the hair stand upright!

$his is beca!se of each strand of hair

receives positive charges and repels

each other.

*. $he flow of electrical charges prod!ces electric current.

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Electric Crrent

1. &lectric c!rrent consists of aflow of electrons

2. $he more charges that flow thro!gh a cross

section within a given time# the largeris the

c!rrent.

%. Electric crrentis defined as the

rate of flow of electric charge

'. +n s"mbols# it is given as,

where I electric current

Q charge

t time

i/ $he 0+ !nit of charge is Ampere Coulomb Volt/ii/ $he 0+ !nit of time is min!te second ho!r/

iii/ $he 0+ !nit of c!rrent is Ampere! o!lomb Volt/ is e3!ivalent to

s -1s Cs-1/

iv/ 4" rearranging the above form!la# 5 "It !t

" !

"

t #

'. +f one co!lomb of charge flows past in one second# then the c!rrent is oneampere.). 1) amperes means in eachsecond# 15co!lomb of charge thro!gh a cross section of a

cond!ctor.

*. +n a metal wire# the charges are carried b" electrons.

6. &ach electron carries a charge of 1.6 1!-1"C.

7. 1 of charge is 6.#5 1!1$electrons.

- ' -

I= Q

&ach second# 1) co!lombs of charge cross theplane. $he c!rrent is + 15amperes. 8neampere is oneco!lomb per second.


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Electric $ield

a/ An electric fieldis a regionin which an electric chargeexperiences aforce.

b/ An electric field can be represented b" a n!mber of lines indicate both the magnitudeand

directionof the field

c/ $he principles involved in drawing electric field lines are ,

i/ electric field lines alwa"s extend from apositivel%-chargedob9ect to a

negativel%-chargedob9ect to infinit"# or from infinit%to a negativel%-charged ob9ect#

ii/ electric field lines never crosseach other#

iii/ electric field lines are closerin a strongerelectric field.

De%& 1 : T& 'td( t)e electric field and t)e effect' &f an electric field.

Apparat!s : materials

&xtra high tension &..$/ power s!ppl" ; < ) =V/# petri dish# electrodes with different

shapes pointed electrode and plane electrode/# two metal plates# talc!m powder# coo=ing oil#

pol"st"rene ball coated with cond!cting paint# thread and candle.

>ethod

DE*A/

1. 0et !p the apparat!s as shown in the above fig!re

2. 0witch on the &..$. power s!ppl" and ad9!st the voltage to ' =V

%. 8bserved the pattern formed b" the talc!m powder for different t"pes of electrodes.

'. raw the pattern of the electric field lines.

- ) -


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ELECTRIC $IELD ARUND A P,ITIVE CHARGE

ELECTRIC $IELD ARUND A NEGATIVE CHARGE

ELECTRIC $IELD ARUND A P,ITIVE AND NEGATIVE CHARGE

ELECTRIC $IELD ARUND T- NEGATIVE CHARGE,

- * -


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ELECTRIC $IELD ARUND T- P,ITIVE CHARGE,

ELECTRIC $IELD ARUND A NEGATIVE CHARGE AND A

P,ITIVELY CHARGED PLATE

ELECTRIC $IELD ARUND A P,ITIVE CHARGE AND A

NEGATIVELY CHARGED PLATE

ELECTRIC $IELD ET-EEN T- CHARGED

PARALLEL PLATE,

- 6 -


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E$$ECT $ AN ELECTRIC $IELD N A PLY,TYRENE ALL

1. (lace the pol"st"rene ball between the

two metal plates.

2. 0witch on the &..$ and displace the

pol"st"rene ball slightl" so that it

to!ches one of the metal plates

/'er0ati&n:

&he pol%st%rene ball oscillated between thetwo plates' touching one plate after

another.

E2lanati&n:

(hen the pol%st%rene ball touches the

negativel% charged plate' the ball

receives negative charges from the plate

and eperiences a repulsive force.

&he ball will then move to the positivel%

charged plate.

(hen the ball touches the plate' the

ball loses some of its negative charges

to the plate and becomes positivel%

charged.

It then eperiences a repulsive force.

&his process continues.

- 7 -


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E$$ECT $ AN ELECTRIC $IELD N A CANDLE $LA*E

/

1/ 0witch of the &..$ and replace the

pol"st"rene ball with a lighted candle.2/ 0=etch the flame observed when the

&..$. is switched on.

/'er0ati&n:

&he candle flame splits into two portions in

opposite direction. &he portion that is

attracted to the negative plate is ver% much

larger than the portion of the flame that is

attracted to the positive plate.

E2lanati&n:

&he heat of the flame ioni)es the air

molecules to become positive and

negative charges.

&he positive charges are attracted to the

negative plate while the negative

charges are attracted to the positive

plate.

&he flame is dispersed in two opposite

directions but more to the negative

plate.

&he positive charges are heavier than

the negative charges. &his causes the

uneven dispersion of the flame.

C&ncl'i&n

1. &lectric field is a region where an electric charge eperiences a force.

2. @i=e charges repeleach other b!t opposite charges attracteach other.

%. &lectric field lines are lines of forcein an electric field. $he direction of the field

lines is frompositiveto negative!

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Eerci'e 7.1

1. ) of charge flows thro!gh a wire in 1; s. What is the c!rrent in the wire?

2. A charge of %;; flow thro!gh a b!lb in ever" 2 min!tes. What is the electric

c!rrent in the b!lb?

%. $he c!rrent in a lamp is ;.2 A. alc!late the amo!nt of electric charge that passes

thro!gh the lamp in 1 ho!r.

'. +f a c!rrent of ;.7 A flows in a wire# how man" electrons pass thro!gh the wire in one

min!te? Given, $he charge on an electron is 1.* x 1;-1/

). An electric c!rrent of 2;; mA flows thro!gh a resistor for % seconds# what is the

a/ electric charge

b/ the n!mber of electrons which flow thro!gh the resistor?

- 1; -

# $ "t

" $ #%t

$ 5 % &'$ '!5 (

# $ "t

" $ #%t

$ )'' % &*'

$ *!5 (

# $ "t

$ '!* +,' - ,'.

$ 7*' C

# $ "t

$ '!/ +,'. Con0ert:& minute $ ,'s$ 1/ C

&!, - &' 2&3C o4 charge & electron!

ence6 1/ C o4 charges is rought y 1/ C $ ) - &'*'electrons

&!, - &' 2&3C

a. # $ "t

$ *'' - &'2)+).

$ '!, C

. &!, - &' 2&3C o4 charge & electron!

ence6 '!, C o4 charges is '!, C $ )!75 - &'&/electrons

&!, - &' 2&3C


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Idea' &f P&tential Difference

a/

"/#

(ress!re at point ( isgreaterthan the press!re

at point 5

Water will flow fromPto #when the valve is

opened.

$his d!e to the di44erencein the press!re of

water

Gravitational potential energ" at B isgreaterthan

the gravitational potential energ" at C.

$he apple will fall fromto 8when the apple is

released.

$his d!e to the di44erencein the gravitational

potential energ".

c/ 0imilarl"#

(oint A is connected topositi0eterminal

(oint 4 is connected to negati0eterminal

&lectric potential at A isgreater than the electric potential at

4.

&lectric c!rrent flows from A to 4# passing the b!lb in the

circ!it and lights upthe b!lb.

$his is d!e to the electricpotential di44erencebetween the two

terminals.

As the charges flow from A to 4# wor= is done when electrical

energ" is transformed to lightand heatenerg".

$hepotential di44erence6 9between two points in a circ!it is

defined as the amo!nt of wor= done# (when one co!lomb of

charge passes from one point to the other point in an electric

field.

$he 2&tential difference3* between the two points will be

given b",

where W is work or energ% in +oule ,+

5 is charge in Coulomb ,C

- 11 -

A B

Bulb

9ech#uantityo4

:or;

arg

#

:

P 4


12/60


13/60



1. Dame the device !sed to meas!re

electrical c!rrent.

(n ammeter

2. a/ What is the 0+ !nit for c!rrent?

(mperes

b/ What is the s"mbol for the !nit of

c!rrent?

%. ow is an ammeter connected in an

electrical circ!it?

"n series

'. $he positive terminal of an ammeter is

connected to which terminal of the dr"

cell?

Positi0e

). What will happen if the positive terminal

of the ammeter is connected to the

negative terminal of the dr" cell?

The ammeter needle


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E2 1: T& in0e'ti6ate t)e relati&n')i2 /eteen crrent and 2&tential difference

f&r an &)%ic c&ndct&r.

a/ b/

Fig!re a/ and fig!re b/ show two electrical circ!its. Wh" do the ammeters show different

readings? Wh" do the b!lbs light !p with different intensit"?

Eeferring to the fig!re a/ and b/#

i/ >a=e one s!itable inference.

ii/ 0tate one appropriate h"pothesis that co!ld be investigated.

iii/ esign an experiment to investigate the h"pothesis.

a/ +nference The current 4lo


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>ethod ,

&! >et up the apparatus as sho


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a/ What is the shape of the V-+ graph?

The graph o4 9 against " is a straight line that passes through origin

b/ What is the relationship between V and +?

This sho


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8hms law states

that the electric current6 " 4lo


18/60



1. $he resistance of a cond!ctor is a meas!re of the abilit" of the cond!ctor toresist!

allow/the flow of an electric c!rrent thro!gh it.

2. From the form!la V +E# the c!rrent + is directl" in0ersely# proportional to the

resistance# E.

%. When the val!e of the resistance# E is large# the c!rrent# + flowing in the cond!ctor is

small large/

'. What are the factors affecting the resistance of a cond!ctor?

a. the length o4 the conductor

. the cross2sectional area o4 the conductor

c. type o4 material o4 the conductor

d. the temperature o4 the conductor

). Write down the relevant h"pothesis for the factors affecting the resistance in the table

below.

Factors iagram "pothesis Graph

@engthofthe

nd!

ctor#

l

The longer the conductor6 the

higher its resistance

@esistance is directl%

proportional to the length o4 a

conductor

$hecross-sectional

tor#

The igger the cross2sectional

area6 the lo


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Eesistance of a cond!ctor# E length

Eesistance of a cond!ctor# E 1

cross2sectional area

ence# resistance of a cond!ctor# E length

cross2sectional area

8r E l or E l where 9 re'i'ti0it( &f t)e

( ( '/'tance

Eerci'e 7.

- 1 -


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1. $ic= / the correct answers

$r!e False

a/ Hnit of potential difference is I -1

b/ I -1J volt# V

c/

$he potential difference between two points is 1 volt if 1 9o!le

of wor= is re3!ired to move a charge of 1 co!lomb from one

point to another.

d/2 volt is two 9o!les of wor= done to move 2 co!lomb of charge

from one to another in an electric field.

e/ (otential difference J Voltage

2. i/ &lectric charge# # "tt

"

"

t /

ii/ Wor= done# #9#

9

9

#/

iii/ 4ase on "o!r answer in 2i/ and ii/ derive the wor= done# in terms of"6 9and t.

$ #9

$ "t9

%. +f a charge of ).; flows thro!gh a wire and the amo!nt of electrical energ" convertedinto heat is 2.) I. alc!late the potential differences across the ends of the wire.

$ #9

*!5 $ 5!' +9.

9 $ '!5 9

'. A light b!lb is switched on for a period of time. +n that period of time# ) of chargespassed thro!gh it and 2) I of electrical energ" is converted to light and heat energ". Whatis the potential difference across the b!lb?

$ #9

*5 $ 5 +9.

9 $ 5 9

). $he potential difference of 1; V is !sed to operate an electric motor. ow m!ch wor= isdone in moving % of electric charge thro!gh the motor?

$ #9

- 2; -


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$ ) +&'.

$ )' J

*. When the potential difference across a b!lb is2; V# the c!rrent flow is % A. ow m!ch wor=done to transform electrical energ" to light andheat energ" in ); s?

$ 9"t

$ *' +). +5'.

$ )''' J

6. What is the potential difference across a light b!lbof resistance ) when the c!rrent that passes

thro!gh it is ;.) A?

9 $ "@

$ '!5 +5.

$ *!5 9

7. A potential difference of %.; V applied across a resistor of resistance E drives a c!rrent of2.; A thro!gh it. alc!late E.

9 $ "@

)!' $ *!' +@.

@ $ &!5

. What is the val!e of the resistor in the fig!re# ifthe dr" cells s!ppl" 2.; V and the ammeterreading is ;.) A?

9 $ "@

*!' $ '!5 +@.

@ $ 1

1;. +f the b!lb in the fig!re has a resistance of * #what is the reading shown on the ammeter# if the

dr" cells s!ppl" % V?

- 21 -

3 A

20

Bulb


22/60



9 $ "@

)!' $ " +,.

" $ '!5 (

11. +f a c!rrent of ;.) A flows thro!gh the resistor of% in the fig!re# calc!late the voltage s!pplied

b" the dr" cells?

9 $ "@

$ '!5 +).

9 $ &!5 9

12. $he graph shows the res!lt of an experiment todetermine the resistance of a wire. $he resistanceof the wire is

From 92" graph6 resistance $ gradient

$5

*!&

$ '!*1

1%. An experiment was cond!cted to meas!re thec!rrent# + flowing thro!gh a constantan wire whenthe potential difference V across it was varied.$he graph shows the res!lts of the experiment.What is the resistance of the resistor?

From 92" graph6 resistance $&% gradient

$&% + 1

&'-/ )

.

$&%+ *!' -

&'2).

$ 5''

1'.Eeferring to the diagram on the right# calc!late

- 22 - 12 V

) +

9V

"A; )

1.2


23/60



a/ $he c!rrent flowing thro!gh the resistor.

9 $ "@

&* $ " +5.

" $ *!1 (

b/ $he amo!nt of electric charge that passesthro!gh the resistor in %; s

# $ "t

$ *!1 +)'.

$ 7* C

c/ $he amo!nt of wor= done to transform theelectric energ" to the heat energ" in %; s.

$ #9 or $ 9"t

$ 7* +&*. $ &*+*!1.+)'.

$ /,1 J $ /,1 J

1). Fig!re shows a torchlight that !ses two 1.) V dr"cells. $he two dr" cells are able to provide ac!rrent of ;.% A when the b!lb is at its normal

brightness. What is the resistance of the filament?

9 $ "@

)!' $ '!)+@.

@ $ &'

1*. $he diagram shows fo!r metal rods of (# 5# E

and 0 made of the same s!bstance.

a/ Which of the rod has the most

resistance?

P

b/ Which of the rod has the least

resistance?

>

- 2% -

1.) V+ ; 1.) V+ ;


24/60



16. $he graph shows the relationship between the

potential difference# V and c!rrent# + flowing

thro!gh two cond!ctors# B and C.

a/ alc!late the resistance of cond!ctor B.


$*

/

$ 1

b/ alc!late the resistance of cond!ctor C.


$**

$ &

c/ +f the cross sectional area of B is ).; x 1; -*

m2# and the length of B is 1.2 m# calc!late its

resistivit".

17. $he graph shows a graph of"against 9for three

cond!ctors# (# 5 and E.

i/ 5ompare the resistance of cond!ctor (# 5 and E.

@@ @#@p

ii/ &xplain "o!r answer in a/

From "29 graph6 resistance $ &%gradientThe greater the gradient6 the lo


25/60



1. Fig!re shows a wire ( of length# lwith a cross-

sectional area# A and a resistance# E. Another

wire# 5 is a cond!ctor of the same material with

a length of %land twice the cross-sectional area

of (. What is resistance of 5 in terms of E?

Conductor P @ $(

l

Conductor # @ $E(

El+notes: P and @ ha0e the same resisti0ity6 D.

$(*

.l)+

9*

)@

2;. (5# is a piece of !niform wire of length 1 m

with a resistance of 1;. 5 is connected to an

ammeter# a 2 resistor and a % V batter". Whatis the reading on the ammeter when the 9oc=e"

is at B?

@esistance in the


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21. Fig!re shows the circ!it !sed to investigate the relationship between potential

difference# V and c!rrent# + for a piece of constantan wire. $he graph of V against +

from the experiment is as shown in the fig!re below.

a/ What 3!antities are =ept constant in this experiment?

ength %% cross2sectional area %% type o4 material %% temperature o4 the


27/60



7.< ,ERIE, AND PARALLEL CIRCUIT,

Crrent $l& and P&tential Difference in ,erie' and Parallel Circit

,ERIE, CIRCUIT PARALLEL CIRCUIT

1 the c!rrent flows thro!gh each b!lbresistor isthe same

"$ "&$ "*$ ")

the potential difference across eac)b!lb

resistor depends directl" on its resistance. $he

potential difference s!pplied b" the dr" cells is

shared b" all the b!lbs resistors.

V 9& 9* 9) where V is the potential

difference across the

batter"

< +f 8hms law is applied separatel" to each b!lb

resistor# we get ,

V 9& 9* 9)

+E "@& "@* "@)

+f each term in the e3!ation is divided b" +# weget the effective resistance

E @& @* @)

1 the potential difference is the same across eachb!lbresistor

9$ 9&$ 9*$ 9)

the c!rrent passing thro!gh each b!lb resistor is

inversel" proportional to the resistance of the

resistor. $he c!rrent in the circ!it e3!als to the

s!m of the c!rrents passing thro!gh the b!lbs

resistors in its parallel branches.

+ "& "* ") where + is the total c!rrent

fromthe batter"

< +f 8hms law is applied separatel" to each b!lb

resistor# we get ,

+ "& "* ")

+f each term in the e3!ation is divided b" V# we

get the effective resistance

Identif( 'erie' circit &r 2arallel circit

- 26 -

VE

9

@

9

@

9

@

1E

&

@

&

@ +&

@

I

V


28/60



a/ b/ c/

d/

>eries Parallel (6 H 2 series #6 > 2 parallel

A%%eter readin6 = Crrent

V&lt%eter readin6 = P&tential difference = V&lta6e

- 27 -


29/60



Effecti0e re'i'tance3 R

a/@ $ *' &' 5$ )5

b/&%@ $ I &%5 &%&' $ 1%5

E44ecti0e @ $ &!*5

c/&%@ $ &%/ &%/$ &%/

@ $ 1

E44ecti0e @ $ *' &' 1 $ )1

d/&%@ $&%&, &%/ &%/$5%&,

E44ecti0e @ $ )!*

e/&%@ $ &%1 &%*$)%1

@ $ &!))

E44ecti0e @ $ &!)) & $ *!))

f/&%@ $ &%1 &%&*$&%)

@ $ )

E44ecti0e @ $ ) * $ 5

g/E44ecti0e @ $ *5)&'

$ *'

h/&%@ $ &%*' &%*'$&%&'

@ $ &'

E44ecti0e @ $ &' &' 5 $* 5

i/&%@ $ &%5 &%&'$)%&'

@ $ )!))

9/&%@ $ &%&' &%2'$3%2'

R = 20/3 + 8

Effective@ $ 14.67

- 2 -


30/60



,&l0e 2r&/le%' 'in6 V 9 IR

Eerci'e 7.ustitute in : E $ 9 "r

&!5 $ &!)5 '!)+r.

r $ '!5

2. A circ!it contains a cell of e.m.f %.; V and internal resistance# r. +f the external resistence has a val!e of

1;.; and the potential difference across it is 2.) V# find the val!e of the c!rrent# + in the circ!it and the

internal resistance# r.

E $ )!' 96 @ $ &' 6 9 $ *!5 9

Calculate current : 9 $ "@ 6 " $ '!*5 (

Calculate internal resistance : E $ "+@ r.6 )!' $ '!*5+&'r.

r $ *!'

% A simple circ!it consisting of a 2 V dr" cell with an internal resistance of ;.). When the switch is

closed# the ammeter reading is ;.' A.alc!late

a/ the voltmeter reading in open circ!it

The 0oltmeter reading $ e!m!4! $ * 9

b/ the resistance# E c/ the voltmeter reading in closed circ!it

E $ "+@ r. 9 $ "@

* $ '!1+@ '!5. $ '!1 +1!5.

@ $ 1!5 $ &!/ 9

' Find the voltmeter reading and the resistance# E of the

resistor.

E $ 9 "r

&* $ 9 '!5 +&!*.

9 $ &&!1 9

- % -

e.%.f.


40/60



9 $ "@

&&!1 $ '!5 +@.

@ $ **!/

) A cell of e.m.f.# & and internal resistor# r is connected

to a rheostat. $he ammeter reading# + and the

voltmeter reading# V are recorded for different

resistance# E of the rheostat. $he graph of V against +

is as shown.

From the graph# determine

a/ the electromotive force# e.m.f.# & b/ the internal resistor# r of the cell

E $ 9 "r r $ 2 gradient

@earrange : 9 $ E 2 " r $ 2 +, 2 *.

ELui0alent : y $ m- c *

ence6 4rom 9 K " graph : E $ c $ intercept o4 92a-is $ *

$ , 9

*$he graph V against + shown was obtained from an experiment.

a/ 0=etch a circ!it diagram for the experiment

b/ From the graph# determine

i/ the internal resistance of the batter" ii/ the e.m.f. of the batter"

r $ 2gradient E $ c $ intercept o4 92a-is

$ '!*, $ &!5 9

6 A graph of E against 1+ shown in fig!re was obtained

from an experiment to determine the electromotive force#

e.m.f.# & and internal resistance# r of a cell. From the

graph# determine

a/ the internal resistance of the cell

E $ "+@ r.

- '; -

E

1.%

- ;.2

;.)1 A-1/+

A

! V

&%(

9 % 9

1.)

;.2

)


41/60



@earrange : @ $"

E2 r6

ence6 r $ 2gradient $ 2+2'!*. $ '!*

b/ the e.m.f. of the cell

e!m!4! $ gradient $ ) 9

7.? ELECTRICAL ENERGY AND P-ER

Electrical Ener6(

1. &nerg" onversion

"a# "/#

2. When an electrical appliance is switched on# the currentflows and the electrical energy

s!pplied b" the so!rce is trans4ormedto other forms of energ".

%. $herefore# we can define electrical energ"as , The energy carried y electrical charges


42/60



1. P&tential difference# V across two points is the ener6(3Edissipated or transferred b" a

c&l&%/ &f c)ar6e# 5 that moves across the two points.

2. $herefore#

%. ence#

'. P&er is defined as the rate of energ" dissipated or transferred.

). ence#

Electrical Ener6(3 E Electrical P&er3 P

From the definition of potential

difference# V

(ower is the rate of transfer of electrical energ"#

&lectrical energ" converted# &

N where 5 +t

ence# N where V +E

ence# N where + V E

ence#

0+ !nit ,+oule ,+ 0+ !nit ,+oule per second 22 + s-122 (att,(

P&er Ratin6 and Ener6( C&n'%2ti&n &f Vari&' Electrical A22liance'

1. $he amo!nt of electrical energ" cons!med in a given period of time can be calc!lated b"

&nerg" cons!med (ower rating x $ime

& (t where energ"# & is in Io!les

power# ( is in watts

- '2 -

4*

3 *4

3 *I t

3 I#t

*# t

tP 9

*4t

P

P *I

I#P

P I#

3lectrical energ% dissipated' 3

Charge' 4

Potential difference' *

3 *4

3nerg% dissipated' 3time' t

Power P


43/60



time# t is in seconds

2. $he !nit of meas!rement !sed for electrical energ" cons!mption is the

;ilo


44/60



Eefrigerator 1 ';; '!1 2' ho!rs 3!,

Oettle 1 1);; &!5 % ho!rs 1!5

+ron 1 1;;; &!' 2 ho!rs *&lectricit" cost, E> ;.27 per =Wh

$otal energ" cons!med# & +*!1 3!, 1!5 *!'.

&/!5=Wh

ost &/!5 =Wh x E> ;.27

E> 5!&/

C&%2arin6 Vari&' Electrical A22liance' in Ter%' &f Efficient U'e &f Ener6(

1. A t!ngsten filament lamp changes electrical energyto

!sef!l lightenerg" and !nwanted heatenerg"

2. A fl!orescent lamp or an Penerg" saving lamp prod!ces

less heat than a filament lamp for the same amo!nt of

light prod!ced.

%. a/ &fficienc" of a filament lamp ,

E44iciency $ utput po


45/60



b/

2.

alc!late

a/ the c!rrent# + in the circ!it b/ the energ" released in E 1 in 1; s.

b/ the electrical energ" s!pplied b" the batter" in 1; s.

2. A lamp is mar=ed Q12 V# 2' WR. ow man" 9o!les of electrical energ" does it cons!me

in an ho!r?

- ') -

E $ Pt

$ *1 +& - ,' - ,'.

$ /, 1'' J

) V

E 1;

E 1;

@12

@2' @%'

9 1)V "

P $ 9*

@

$ 5*% 5

$ 5

Total resistance6 @ $ +* 1 1.

$ &'

9 $ "@

" $ 9%@

$ &5 % &'

$ &!5 (

E $ "*@t

$ +&!5.*+&'.+&'.

$ **5 J

E $ "*@t

$ +&!5.*+*.+&'.

$ 15 J


46/60



%. A c!rrent of )A flows thro!gh an electric heater when it is connected to the 2' V mains

s!ppl". ow m!ch heat is released after 2 min!tes?

'. An electric =ettle is rated 2'; V 2 =W. alc!late the resistance of its heating element and

the c!rrent at normal !sage.

). An electric =ettle operates at 2'; V and carries c!rrent of 1.) A.

a/ ow m!ch charge will flow thro!gh the heating coil in 2 min!tes.

b/ ow m!ch energ" will be transferred to the water in the =ettle in 2 min!tes?

c/ What is the power dissipated in the =ettle?

*. An electric =ettle is labeled % =W# 2'; V.

a/ What is meant b" the label % =W# 2'; V?

The electric ;ettle dissipates electrical po


47/60



b/ What is the c!rrent flow thro!gh the =ettle?

c/ etermine the s!itable f!se to be !sed in the =ettle.

&) (

d/ etermine the resistance of the heating elements in the =ettle.

6. $able below shows the power rating and energ" cons!mption of some electrical appliances

when connected to the 2'; V mains s!ppl".

Appliance 5!antit" (ower rating W $ime !sed per da"

Oettle 9!g 1 2;;; 1 ho!r

Eefrigerator 1 ';; 2' ho!rs

$elevision 1 2;; * ho!rs

@amp ) *; 7 ho!rs

&lectricit" cost, E> ;.217 per =Wh

alc!late

a/ &nerg" cons!med in 1 da"

Energy consumed $ #uantity - Po ;.217

E> 33!1&

- '6 -

P $ "9

)''' $ " +*1'.

" $ &*!5 (

P $ "*@

)''' $ +&*!5.*@

@ $ &3!*


48/60



7. A vac!!m cleaner cons!mes 1 =W of power b!t onl" delivers ';; I of !sef!l wor= per

second. What is the efficienc" of the vac!!m cleaner?

. An electric motor is !sed to lift a load of mass 2 =g to a height ) m in 2.) s. +f the s!ppl"

voltage is 12 V and the flow of c!rrent in the motor is ).; A# calc!late

a/ &nerg" inp!t to the motor

b/ Hsef!l energ" o!tp!t of the motor

c/ &fficienc" of the motor

Reinf&rce%ent Eerci'e C)a2ter 7

Part A: /ecti0e 4e'ti&n'

1. What is the !nit of electric charge?A. Ampere# A

4. =elvin#O . o!lomb#

- '7 -

E44iciency $ utput po


49/60



. Volt# V

2. Which of the following diagrams

shows the correct electric field?

A.

4.

.

%. Which of the following graphs shows

the correct relationship between the

potential difference# V and c!rrent# +

for an ohmic cond!ctor?

A.

4.

.

.

'. A small heater operates at 12 V# 2A.ow m!ch energ" will it !se when it isr!n for ) min!tes?A. ; I

4. 12; I

. 17;; I

. 62;; I

- ' -

& V+t 122/)x *;/ 62;;I


50/60



). $he electric c!rrent s!pplied b" abatter" in a digital watch is %.; x 1;-)

A. What is the 3!antit" of charge thatflows in 2 ho!rs?

A. 2.) x 1;-64. 1.) x 1;-)

. *.; x 1;-)

. %.* x 1;-%

&. 2.2 x 1;-1

*. Which of the following circ!its can be!sed to determine the resistance of the

b!lb?

A.

4.

.

.

6. Wh" is the filament made in the

shape of a coil?

A. $o increase the length and prod!ce

a higher resistance.

4. $o increase the c!rrent and prod!ce

more energ".

. $o decrease the resistance and

prod!ce higher c!rrent

. $o decrease the c!rrent and prod!ce

a higher potential difference

7. Which of the following will not

affect the resistance of a cond!cting

wire.

A. temperat!re

4. length

. cross-sectional area

. c!rrent flow thro!gh the wire

. $he potential difference between two

points in a circ!it is

A. the rate of flow of the charge from

one point to another

4. the rate of energ" dissipation in

moving one co!lomb of charge

from one point to another

- ); -

5+t %.; B1;-)2 B *; B *;/ ;.21* 2.2 B 1;-1


51/60



. the wor= done in moving one

co!lomb of charge from one point

to another

. the wor= done per !nit c!rrent

flowing from one point to another

1;. An electric =ettle connected to the

2'; V main s!ppl" draws a c!rrent

of 1; A. What is the power of the

=ettle?

A. 2;; W

4. 2;;; W

. 2';; W

. %*;; W

&. '7;; W

11. An e.m.f. of a batter" is defined as

A. the force s!pplied to 1 of charge

4. the power s!pplied to 1 of

charge

. the energ" s!pplied to 1 of

charge

. the press!re exerted on 1 of

charge

12. Which two resistor combinations have

the same resistance between B and C?

A. ( and 5

4. ( and 0

. 5 and E

. E and 0

&.

1%. +n the circ!it above# what is the

ammeter reading when the switch 0

is t!rned on?

A. 1.; A

4. 1.) A

. 2.; A

. .; A

&. 1;.; A

- )1 -

( +V 1;2';/ 2';; W

E 1K E ;.'K

E 2.)K E 1K

E * K

+ VE 12*

2 A


52/60



1'. A 2 =W heater ta=es 2; min!tes to

heat a pail of water. ow m!ch

energ" is s!pplied b" the heater to

the water in this period of time?

A. 1.2 x 1;*I

4. 1.7 x 1;*I

. 2.' x 1;*I

. %.* x 1;*I

&. '.7 x 1;*I

1). All b!lbs in the circ!its below are

identical. Which circ!it has the

smallest effective resistance?

A.

4.

.

.

1*. An electric motor lifts a load with a

potential difference 12 V and fixed

c!rrent 2.) A. +f the efficienc" of the

motor is 7;S# how long does it ta=e

to lift a load of *;; D thro!gh a

vertical height of ' m

A. 2; s

4. '; s

. *; s

. 7; s

&. 1;; s

16. $he =ilowatt-ho!r =Wh/ is a !nit of

meas!rement of

A. (ower

4. &lectrical energ"

. &lectromotive force

17. $he circ!it above shows fo!r

identical b!lbs to a cell * V. Which

b!lb labeled A# 4# and is the

brightest?

- )2 -

& (t 2 x 1;%x 2; x *; 2';; x 1;%

2.' x 1;*I 7; *;; x ' x 1;; t 2.) x 12 / t *;;; x ' B 1;; 2.) x 12 7; 1;;s


53/60



1. A 2' resistor is connected across

the terminals of a 12 V batter".

alc!late the power dissipated in the

resistor.A. ;.) W

4. 2.; W

. '.; W

. *.; W

&. 7.; W

2;. Which of the following 3!antities can

be meas!red in !nits of I-1

A. Eesistance

4. (otential difference

. &lectric c!rrent

Part : ,trctred 4e'ti&n'

1.

- )% -

E 2'V 12

( V2E 1''2' *.;W


54/60



$he fig!re above shows a graph of electric c!rrent against potential difference for three

different cond!ctors B# C and T.

a/ Among the three cond!ctors# which cond!ctor obe"s 8hms law?

Conductor 8

b/ 0tate 8hms law.

The potential di44erence across a conductor is directly proportional to the current that

4lo


55/60



$he table below shows the maxim!m electric c!rrent that is able to flow thro!gh

wires of vario!s diameters.

diameter of wire mm maxim!m c!rrent A

;.7; 7

1.;; 1;

1.2; 1%

1.'; 1)

a/ What is the c!rrent flowing thro!gh the cable when the =ettle is switched

on?

P $ "9

" $ P%9 $ *5'' % *1' $ &'!1 (

b/ Eeferring to the table above#

i. What is the smallest diameter wire that can be safel" !sed for this

=ettle?

&!*' mm

ii. &xplain wh" it is dangero!s to !se a wire thinner than the one selected

in bi/

(s resistance is in0ersely proportional to cross2sectional area6

a thinner


56/60



Bo not operate ;ettle hort circuit might occur i4 the insulating materials o4 the


57/60



Fig!re 2 shows the reading of the same voltmeter

a/ What is meant b" electromotive force e.m.f./ of a batter"?

b/ Eeferring to fig!re a/ and fig!re b/# compare the state of the switch# 0# and

the readings of the voltmeter. 0tate a reason for the observation on the

readings of the voltmeter.

c/ raw a s!itable simple electric circ!it and a s!itable graph# briefl" explain

how the e.m.f. and the 3!antit" in "o!r reason in b/ can be obtained.

d/

$he fig!re above shows a dr" cell operated torchlight with metal casing

i/ What is the p!rpose of the spring in the torchlight?

ii/ Wh" it is safe to !se the torchlight altho!gh the casing is made of metal?

iii/ What is the p!rpose of having a concave reflector in the torchlight?

7uggested Answers

&! +a. The


58/60



+. 2 >


59/60



Fo!r different cables and their characteristic of the cables were given. $he length and

diameter of all the cables are similar.

a/ efine the resistance of a cond!ctor.

b/ $he table below shows the characteristic of the fo!r cables# A# 4# and .

Eesistivit" m

>axim!m loadbefore brea=ing

D

ensit" =gm-%

Eate ofexpansion

A ;.;2; );; 27;; @ow

4 ;.;)* %;; %2;; @ow

;.;%1 ';; )*;; >edi!m

;.;7) 2;; %7;; igh

4ase on the above table,

i/ &xplain the s!itabilit" of each characteristic of the table to be !sed for a long

distance electricit" transmission

ii/ etermine the most s!itable wire and state the reason

c/ 0!ggest how three similar b!lbs are arranged effectivel" in a domestic circ!it.

raw a diagram to explain "o!r answer. Give two reasons for the arrangement.

d/ An electric =ettle is rated 2.; =W.

i/ alc!late how long wo!ld it ta=e to boil 1.) =g of water from an initial

temperat!re of 27;.

Uspecific heat capacit" of water '2;; I =g -1;-1

ii/ What is the ass!mption made in the calc!lations above?

>uggested (ns


60/60



Characteristics E-planations

( lo< resisti0ity Energy loss during transmission is reduced

Ma- load e4orera;ing is high

>tronger and long lasting

( lo< densityMass or

Documents

12405942 Chapter 7 ElectricityTeachers Guide 2009