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Chap 12 : Electricity

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CHAPTER 12
Electricity

ONE MARK QUESTIONS
1
...

(b) How is an ammeter connected in a circuit to
measure current flowing through it?
Ans :
[CBSE 2015]

If the charge on an electron be 1
...

Ans :
[CBSE 2016]

a
...

b
...


1
...
6 # 10-19
No
...
25 # 1018
2
...

Ans :
[CBSE 2016]
Resistance of a conductor:
a
...
(1)
R ?r
b
...


...


8
...
e
...


Volt is the SI unit of electric potential
...


When is potential difference between two points said
to be 1 volt?
Ans :
[Delhi 2016]

9
...

5
...

Ans :

[Delhi 2016]

If the physical conditions of a conductor remain
same then current through a conductor is directly
proportional to the potential difference b/w the two
ends of the conductor
...


7
...

Ans :
[CBSE 2015]

How does the resistivity of alloys compare with those
of pure metals from which they may have been formed?
Ans :
[All India 2014]
The resistivity of pure metals is lesser than resistivity
of alloys with which these alloys are made
...
Write SI unit of resistivity
...


State a difference between the wire used in the element
of an electric heater and in a fuse wire
...

Ans :
[CBSE 2015]

The wire used in element of electric heater has high
resistance and high melting point where as a fuse wire
has a low resistance and low melting point
...


12
...
Find the energy in joules

(a) Name the instrument/device used to measure

consumed by it in 1 s
...
happens to the resistance of a conductor when the

Energy = ^VI h t
E = P # t = 60 # 1 J
E = 60 J
13
...
5 V, 10 ohm resistor and 15 ohm resistor and
a plug key all connected in series
...


length of the conductor is reduced to half?
Ans :
[CBSE 2010]
Resistance is directly proportional to the length of the
conductor
...

20
...

21
...
5 V, 5 W ,
10 W and 15 W resistors and a plug key, all connected
in series
...
Why do we use copper and aluminium wire for

transmission of electric current?
Ans :

Schematic diagram of electric circuit con¬taining
cells, key and three resistances
...

15
...

Ans :

[CBSE 2011 ]

Gold, Silver, Copper etc
...

16
...
Distinguish between open and

closed circuit
...
In open
circuit there is no flow of current as the switch is off
...

17
...
In parallel combination:
1 = 1 + 1 + 1 + 1
R
R1 R2 R3 R 4
= 1+1+ 1 + 1
4 8 12 24
= 6 + 3 + 2 + 1 = 12 = 1
24
24 2
R = 2W

18
...
Why?
Ans :

[CBSE 2010]

Nichrome is used to make element of electric heater
because nichrome is an alloy which has high melting
point and high resistances
...
Out of the two, a toaster of 1 kW and an electric

heater of 2 kW, which has a greater resistance?
Ans :
[CBSE 2006]
2
As R = V
...
Then R ? 1 i
...
more
P
P
power, lesser be the resistance and vice versa
...


23
...

24
...
What is the resistance between two points at
the ends of any diameter of the circle?
Ans :
[CBSE 2005]
R = 5W
It can be assumed as two semi circular elements are
connected across diameter AB
...
Resistance of two
semicircular elements = 5 W
2
Both the elements are connected in parallel
...
Why is much less, heat generated in long electric

R = 20 ohm

cables than in filaments of electric bulbs?
Ans :
[CBSE 2005]
As heat produced = I2 Rt
Electric cables are made of thick good conductor
where as filaments are made of thin tungsten wire
whose resistances is high
...
So heat produced in cable is much
lesser than filaments
...
State which has a higher resistance a 50 W or a 25 W

lamp bulb and how many times?
Ans :
We have

[CBSE 2005]

P1 = 50 W

P2 = 25 W
Let their resistances are R1 and R2 respectively
...
The amount of charge passing through a cell in four

second is 12 C
...

Ans :
[CBSE 2016]
Given:

Q = 12 C

2

P = V or R = V
R
P
2

I =

R1 = 25 = 1
2
50
R2
R1 = 1 R2
2
1
i
...
50 W bulbs has resistance than 25 W bulb
...
Calculate the number of electrons that would flow per

second through the cross- section of a wire when 1 A
current flows in it
...
Define resistivity of a material
...
e
...

or
Resistivity of a material is the resistance of a conductor
of this material whose volume is unity
...
6 # 10-19
= 6
...
Name the device/instrument used to measure potential

difference
...
Voltmeter is connected in
parallel in an electric circuit
...
How much current will an electric bulb of resistance

1100 W draw from a 220 V source? If a heater of
resistance 100 W is connected to the same source
instead of the bulb, calculate the current drawn by
the heater
...
A cylinder of a material is 10 cm long and has a cross-

section of 2 cm2
...
2 A
100
R
33
...

Ans :
[All India 2015]

1 mA = 1 milli ampere = 10-3 A
1 mA = 1 micro ampere = 10-6 A

A — Bulb (load)
B — Rheostat
C — Cell
D — Ammeter (being in series)

34
...


Tungsten used almost exclusively for filament of
electric lamp
...
Why do we use copper and aluminium wires for
transmission of electric current?
Ans :
[CBSE 2015]
a
...

b
...

35
...

36
...
Give one practical
application of this effect
...


39
...
Explain the statement with reason
...
Their average
velocity is zero
...
But when a potential difference is applied
across the ends of the conductor, it sets the electrons
to move in a direction
...

40
...
Which

one has greater resistance
...


37
...

Ans :
Voltmeter
1
...
D
...


Ans :
[CBSE 2014]

If X and Y are of same material
then length of wire X = l

Ammeter
It is used to
measure electric
current in an
electric circuit
...


Its resistance is very Its resistance is
high
...


3
...


An ammeter is
connected in series
in an electric
circuit
...
Given below is a circuit showing current flowing in it
...


[CBSE 2014]

Area of cross-section = A
RX = r l
A
Similarly,

RY = r 2l
A
RY = 2
RX

or
RY = 2RX
wire Y has two times resistance than that of wire X
...
Elements of electric toasters and electric iron are

made of an alloy rather than a pure metal
...

Ans :
[CBSE 2014]
a
...

b
...


their

42
...
Which one will have lower
resistance? Give reason
...
e
...
So thick wire has lower
resistance
...
Mention two special features of the material to be

used as element of an electric iron
...


ends of the conductor, charges (electrons) will move
in the conductor
...

48
...


44
...


Ans :

[CBSE2014]

We have, P = 100 W
V = 250 Volt
2

2

P = V or R = V
R
P

^250h
= 250 # 250 = 625 W
100
100
2

R =

Ans :

1W, 3W, 2W resistances are connected in series
...
Why do the wires connecting an electric heater to the

mains not glow while its heating element does?
Ans :
[CBSE 2011]
Connecting wire has low resistivity or good
conductivity, i
...
resistance of these wires is negligible
hence no heat is produced while heating element
is of an alloy whose resistivity is high, due to high
resistance heat is produced in the element
...
e
...

2
Voltage across B & C = 1 # 3 = 3 Volt
...

2

46
...
Which ammeter A1
or A2 will indicate higher reading for current? Give
reason
...
In the circuit diagram shown, the two resistance wires

A and B are of same length and same material, but
A is thicker than B
...

Ans :

[CBSE 2011]

Length of A is greater than B
...

Therefore resistance of A 2 resistance of B 6a R ? l @
...
Hence A2 will
give higher reading
...
A is thicker than B
...
e
...


47
...
Name the device which is used to
maintain this condition in an electric circuit
...


Tungsten is used almost exclusively for filaments of
electric bulb
...

Ans :
[CBSE 2011]
(1) high resistivity (2) high melting point and does
not oxidise at very high temperature
...
Three V-I graphs are drawn individually for two

resistors and their series combination
...
Give reason

a
...

b
...

54
...
Voltmeter is used to measure
potential difference across a conductor so it is
connected in parallel to it
...
The following table gives the resistivity of three

samples:
Sample

A

B

C

Resistivity 1
...
2 # 10-5 Wm 100 # 10-4 Wm
Which of them is suitable for heating elements of
electrical appliances and why?
Ans :
[CBSE 2010]
Ans :
More slope of V-I graph means more resistance, slope
of C is maximum
...

So it is for series combination of two resistors
...

56
...


Give reason why tungsten is used for making
filament of electric lamps
...
The elements of heating electrical appliances are
made-up of an alloy rather than pure metal
...


Due to (1) high resistivity and high melting point
tungsten is used for making filaments of electric
lamp
...
high resistivity, and high melting point than their
constituents pure metals which do not oxidise at
high temperature
...
State the factors on which the resistance of a cylindrical

conductor depends
...

When conductor is stretched its radius decreases but
the volume of the conductor in both the cases will be
same
...


57
...
When all the three bulbs glow, a
current of 3 A is recorded by the ammeter A
...
What happens to the glow of the other two bulbs
when the bulb B1 gets fused?
b
...
(a) What material is used in making the filament of

an electric bulb?
(b) Name the characteristics which make it suitable
for this
...


[CBSE2010]

The other two bulbs will glow even if B1 gets
fused
...

Here V = 4
...
5
3
I

RP = 1
...
5
R R R

Q = 0
...
2 # 60 # 60 A - s

1 = 3 & R = 4
...
5
R
4
...
5

For B 2 ,

1A = I
4
...
5 1A = I

For B 3 ,
Current in Ammeter,

Q
or Q = I # t
t

1 = 1 + 1 + 1
RP
R1 R2 R3

^Ah = 1A + 1A = 2A

= 720 C
61
...
5 A when the

voltage is 200 volt
...

Ans :
[CBSE 2004]
We have

I = 0
...
5 # 3600 A - s = 1800 C

62
...
4 A when

the voltage is 200 volt
...

Ans :
[CBSE 2004]
Q = It
58
...

(b) In a circuit, if the two resistors of 5 ohm and 10
ohm are connected in series, how does the current
passing through the two resistors compare?
Ans :
[C8SE 2006)

Given,

I = 0
...
4 # 3600 C
Q = 1440 C

THREE MARKS QUESTIONS
a
...
Show how would you join three resistors, each of

b
...

59
...
0 V, 100 mA
...

Ans :
Rating of bulb,

[CBSE 2006]

V = 5
...


I = 100 mA
I = 100 # 10-3 A

resistance 9 W so that the equivalent resistance of the
combination is (i) 13
...

(b) Two bulbs, one rated 100 W; 220 V, and the other
60 W; 220 V are connected in parallel to electric
mains supply
...

Ans :
[CBSE 2017]

I = 0
...


Power of bulb = V # I
P = 5
...
1 W = 0
...
0 W = 50 W
0
...


i
...

1 =1+1 =2W
`
9 9
9
Rp
or

60
...
2 A when the

voltage is 220 volts
...

Ans :
[CBSE 2004]
Given:

I = 0
...

Q =?

Rp = 9 W
2

Now R1 and R p are in series
...
5 W
ii
...

Rs R3

resistance of A is more and its length also
...

and

Req
...
What is meant by “electrical resistance” of a

a
...
Given two bulbs of 100 W–200 V and 60W–220 V
...
45 A
220
11
V
Current in second bulb
...
27 A
220
11
V
64
...

(b) Why are metals good conductors of electricity
whereas glass is a bad conductor of electricity?
Give reason
...

Ans :
[CBSE 2017]
a
...
Resistance is directly proportional to length
...
Resistance is inversely proportional to area of
cross-section
...
Metal are good conductor due to having large
number of free electrons and their low resistivity
...

c
...

65
...
If both the wires are made of the same material
and are of equal thickness, which of the two is of more
length? Give justification for your answer
...
Hence

conductor? State how resistance of a conductor is
affected when
a
...
a heavy current passes through it for about 30
seconds
...
It is equal to the ratio of the potential difference
applied across its ends to the current flowing through
it
...
When a low current is passed for a short duration,
through a conductor, heat produced is almost
negligible and hence no appreciable change in its
resistance
...
When heavy current is passed through the
conductor for 30 s
...

67
...
Calculate the

number of electrons that flow through a conductor in
1 second to constitute a current of 1 ampere
...
6 # 10-19 coulomb)
Ans :
[CBSE 2016]
SI unit of current is Ampere (A)
q
I =
t
If q = 1 C, t = 1 s
then
I = 1A
If 1 C charge flows in 1 s in a conductor then magnitude
of current is said to be 1A
...
6 # 10
= 6
...
Electric current flows through three lamps when

arranged in (a) a series (b) a parallel
...
Explain what happens to the
other two lamps in both the cases
...


In series combination if the filament of one lamp
breaks then the circuit will be broken and hence
other lamps stops glowing
...

Ans :
Given:

[All India 2015]

V = 20 Volt
W = 40 J
W = P#t
= V#I#t
Q
= V# #t
t

b
...


or

W = V#Q
40 = 20 # Q
Q = 2C

or

ne = Q
Q
2
=
n =
e
1
...
25 # 1019
71
...
Study the V-I graph for a resistor as shown in the

figure and prepare a table showing the values of I (in
amperes) corresponding to four different values of V
(in volts)
...

How can we determine the resistance of the resistor
from this graph?

Name and define SI unit of resistance
...
8 V
...
The resistance of a conductor is said to be 1 ohm
if a current of one amp flows through it when a
potential differences of 1 volt is applied across it
...
8
c
...


72
...

(b) Differentiate between conductor, resistor and
resistance
...


When a current is passed through a conductor,
the atoms or molecule of the conductor produce
an hindrance in the path of flow of electron
...

b
...

Resistor : A conductor having some value of
resistance is called a resistor
...

73
...
Find the number of electrons transferred between two

points kept at a potential difference of 20 V if 40 J of

of 12 V
...
Now, the same wire is redrawn by stretching it
to double its length
...

Ans :
[CBSE2014]
Given:

R = 6W
V = 12 Volt
I =?

I = V = 12 A & I = 2A
6
R
When length becomes twice = 2 L
area of cross-section = A2
then

R2 = 7
...
Find the equivalent resistance of the following circuit:

L1 A1 = L2 A2
LA1 = 2LA2
A2 = 1
2
A1

or
or

R1 = r L
A1
R2 = r 2L & R2 = 2 # A1
R1
A2
A2
R2 = 2 2 = 4
#
R1

Ans :
In the given circuit 2 W, 2 W resistances are in parallel
...

74
...
How will its:
a
...
Resistivity change?
Ans :
[CBSE 2010, 2014]
a
...

When length is stretched two times, let its area
of cross-section becomes Al
...


1W, 1W resistances also are in parallel
...
5 W
p
The circuit can be reduced as,

Al = Al2l
Al = 1 or Al = A
2
2
A

or

^2l h
= r 2l # 2
A
Al
= 4r l
A

Rl = r

=4R
b
...

75
...

Ans :
[CBSE 2014]

Now all resistances are in series combination
...
5 = 7
...


A hot plate of an electric oven connected to a 220 V
line has two resistors A and B each of 22 W resistance
...
Find the current flowing in all the three
cases
...
Current in series combination

a
...


R2 = r 3l = 3 r l
4 A
4A
R2 = 3 R = 3 # 10 = 15 W
2
4
4

&

RS = 22 + 22 = 44 W
IS = 220 = 5 A
44
Current in parallel combination
1 = 1 + 1 = 2 W
22 22
22
RP
RP = 11 W

IP = 220 = 20 A
11

so first bulb will consume more energy
...
Draw the nature of V–I graph for a nichrome wire
...
An electric kettle of 2 kW works for 2 h daily
...
00 per unit
...
If the resistivity of the metal is 4
...

Ans :
[CBSE 2013]
V–I graph for nichrome wire

Electric energy, E = P # t = 2 # 2 = 4 kWh
(b) Total energy consumed in month of June (having
30 days)
Electric kettle = ^4 # 30h kWh = 120 kWh
= 120 units
...
Two lamps, one is rated 100 W at 220 V, and the

other 60 W at 220 V, are connected in parallel to a
220 V supply
...

Ans :
[CBSE 2014]
Given: Lamp one 100 W, 220 V, Lamp 2 60 W, 220 V
...

1 = 1 + 3
484 2420
R
R = 605 W
2

Given:

l = 625 mm = 0
...
8 # 10-7 Ohm-m
A =?
R = r l or A = r l
R
A
A = 4
...
625
4
= 0
...
Derive the relation R = R1 + R2 + R3 when three

resistors R1, R2 and R3 are connected in series in an
electric circuit
...

Therefore current in each resistor is same
...


I = V = 220 # 2 A = 0
...
Two bulbs A and B are rated as 90W–120V and

60W–120V respectively
...
Find the current in each bulb
...
75 A
160
R1
Second Bulb: 60W–120V
2
R2 = V = 120 # 120
60
P2

R2 = 240 W
I2 = V = 120
240
R2
I2 = 0
...
(a) Nichrome wire of length L and radius R has

resistance of 10 Q
...

(b) Why element of electrical heating devices are
made-up of alloys?
Ans :
[CBSE 2012]

a
...

i
...
When only diameter is doubled
...
5 W
4
b
...

84
...
62 # 10-8 Wm and 520 # 10-8 Wm respectively
...
filament of electric bulb
...
wires for electrical transmission lines
...

Ans :
[CBSE 2012]
r A = 1
...
For filament of electric bulb resistivity must be
higher i
...
520 # 10-8 Wm
...

b
...


b
...
The
resistivity of manganin is more than copper
...

87
...
How does
the heat produced by the combination of resistance
change?
Ans :
[CBSE 2010]
Let the resistances of two wires are R each
...
Study the following circuit and answer the following

questions:

85
...
Calculate the resistance of a

conductor, if the current flowing through it is 0
...
8 V
...

V = RI
I = 0
...
8 Volt
...
8 W
0
...
(a) Why are copper or aluminium wires generally

used for electrical transmission and distribution
purposes?
(b) Two wires, one of copper and other of manganin,
have equal lengths and equal resistances
...

Ans :
[CBSE 2011]
a
...


a
...

b
...
What would be the ammeter reading?
Ans :
[CBSE 2010]
a
...

b
...

I in 10 W resistor = 3 = 0
...
2 A
15
I = 0
...
2 = 0
...
Define electric current and state its SI unit
...

Ans :
[CBSE 2010]
Electric current may be defined as the rate of flow of
charge through a circuit
...

According to Ohm’s law V = IR
R =V
I
If
V = 1 Volt, I = 1 A then R = 1 ohm
...


P = 25 W

90
...

Ans :
[CBSE 2010]
a
...
Rn
RS = R + R
...
1
RP
R1 R2
Rn
= 1 + 1 + 1 +
...
n times
R
1 =n
RP
R

or

b
...
(a) How is the direction of electric current related to

the direction of flow of electrons in a wire?
(b) Calculate the current in a circuit, if 500 C of
charge passes through it in 10 minutes
...


The direction of flow of electrons is opposite to
the direction of conventional current
...
Q = 500 C , t = 10 mts = 10 # 60 = 600 s
Q
= 500 A
I =
t
600
I = 0
...
(a) Define the term ‘volt’
...

Calculate the potential difference between the
two terminals of a battery, if 100 joules of work
is required to transfer 20 coulombs of charge from
one terminal of the battery to the other
...


R1 = 3 W
R2 = 4 W
R3 = 12 W
1 = 1 + 1 + 1
R
R1 R2 R3
=1+1+ 1
3 4 12

Potential difference b/w two points in an electric
field is said to be 1 volt if the amount of work
done in bringing a unit positive charge from one
point to another point is 1 J
...
Given: W = 100 J , Q = 20 C , V = ?
As
V = W & V = 100 JC-1
20
Q
V = 5 JC-1

= 4+3+1 = 8 = 2
12
12 3
R = 3 W = 1
...
(a) State Ohm’s law
...


(b) Write symbols used in electric circuits to represent:
(i) variable resistance
...

(c) An electric bulb is rated 220 V and 100 W
...


Ohm’s Law states that if the physical conditions of
a conductor are kept constant then current passing
through a conductor is directly proportional to
the potential difference across its ends
...

94
...
Also, show this relationship by drawing a graph
...
35 ampere when the
potential difference across it is 1
...
e
...

Graph b/w V and I :

V ? I or V = RI
b
...


A variable resistor
Given rating of bulb 220 V – 100 W
R=?
Power consumed = ? when V = 110 Volt
2
R = V = 220 # 220 = 484 W
100
P
Power at 110 volt
2

P = V = 110 # 110
484
R

Given:

I = 0
...
4 Volt
...
4 Ohm
0
...
The value of current (I) flowing through a given

resistor of resistance (R), for the corresponding values
of potential difference (V) across the resistor are as
given below :
V

0
...
0

1
...
0

2
...
0

4
...
0

I

0
...
2

0
...
4

0
...
6

0
...
8

Plot a graph between current (I) and potential
difference (V) and determine the resistance (R) of the
resistor
...
40 W–240 V

W VIt
25 W–240 V
2

R2 = V 2
P2

2

R2 = 240 # 240 W
25

P1 = V 1
R1
R1 = V 1
P1

2

R1 = 240 # 240 W
40
R1 = 25 or R1 = 5
40
R2
R2 8
R1 < R2 i
...
, Resistance of 25 W bulb is more than 40
W bulb
...
Draw a labelled circuit diagram showing three resistors

R1, R2 and R3 connected in series with a battery
(E), a rheostat (Rh), a plug key (K) and an ammeter
(A) using standard circuit symbols
...
List two precautions you would observe
while performing the experiment
...
Every time the magnitude of current is
found same i
...
, in series combination the current in
every part of the circuit, remains same
...
a
...
Why does the connecting cord of an electrical
heater not glow while the heating element does?
c
...
60 # 10-8 Wm

Copper

1
...
2 # 10-8 Wm

Iron

10
...
0 # 10-8 Wm

Nichrome

100 # 10-6 Wm

Answer the following questions in relation to them:
i
...
Which material would you advise to use in
electrical heating device? Why?
Ans :
[CBSE 2016]
Precaution:
a
...

b
...

c
...

97
...

(b) One electric bulb is rated 40 W and 240 V and
other 25 W and 240 V
...


Let R is the resistance of the resistor in which I
current is passed for a time t by applying a p
...
(
V ) across the resistor
...


If the amount of work done in bringing a unit
positive charge from one point to another is 1 J in
an electric field then potential difference b/w the
two points is said to be 1 volt
...
Connected cord of an electrical heater does not
glow because this is of copper metal, which has
low resistivity and good conductivity
...

c
...

(ii) The resistivity of nichrome is maximum in the
given table so nichrome will be advised to use

in electrical heating device
...
(a) Name an instrument that measures electric

current in a circuit
...

(b) What are the following symbols mean in an
electric circuit?
(i)

of charge and work
...

(b) Two electric circuits I and II are shown below “

(ii)

(c) Draw a closed circuit diagram consisting of 0
...
5 V and a plug key
...


Ammeter
...

b
...


(i) Which of the two circuits has more resistance?
(ii) Through which circuit more current passes?
(iii) In which circuit, the potential difference
across each resistor is equal?
(iv) If R1 > R2 > R3 in which circuit more heat
will be produced in R1 as compared to other
two resistors?
Ans :
[CBSE 2014]
a
...
What is meant by resistance of a conductor? Name

and define its SI unit
...
How is the
resistance of a wire affected if:
a
...
its radius is doubled?
Ans :
[CBSE 2015]
Property of any conductor by virtue of which it
opposes the flow of current in the conductor is called
its resistance
...
If by applying a potential
difference of 1 volt the current in the conductor is 1 A
...

Factor affecting resistances:
a
...

b
...

101
...
Name two devices based on
heating effect of current
...
Production of heat in a conductor on
passing the current is called heating effect of current
...
During the
collision there is a loss of kinetic energy
...
E
...
Electric heater, 2
...

102
...
Define
the unit of potential difference in terms of SI unit

Voltmeter
The amount of work done in bringing a unit
positive charge from one point to another in an
electric field is said to be potential difference
V =W
Q
(i)
(ii)

Variable resistance
Closed key
...
(i) In series combination resistance is more than
parallel combination
...
e
...

(iii) In parallel combination (II)
(iv) More heat in (I) across R1
...
When a high resistance voltmeter is connected directly

across an electric bulb, its reading is 2 V
...
4 ampere (measured by
an ammeter) in the electric circuit
...
Draw the circuit
...
Find the resistance of the electric bulb,
c
...
If a graph is plotted between V and I,
show the nature of the graph obtained
...


b
...
4 A
R = V = 2 ohm or R = 5 ohm
0
...


Ohm’s law: If the physical conditions of a conductor
is kept constant then current in the circuit is
directly proportional to the potential difference

applied across the ends of the conductor
...

Since V ? I
Hence graph between V and I is a straight line
for a conductor
...

104
...
This potential difference maintains a current
in the circuit
...
(i) The resistance b/w XZY and XTY are in
parallel combination
...

1 =1+1
2 6
Rxy
Rxy = 3 = 1
...
An ammeter
is put in the circuit to measure the electric current
through the resistors and a voltmeter is connected so
as to measure the potential difference across the 12
W resistor
...

Ans :
[CBSE 2014]

(ii) Net resistance of the circuit
Hence,

I = 2
...
D
...
5 = 3
...
0 = 1
...

Hence,

I in 6 W resistor = 3
...
5 A
6

RS = R1 + R2 + R3

= ^5 + 8 + 12h W

RS = 25 W
As

a
...
4 A
Ammeter reading is 0
...


R = 1
...
5 = 3
...
(a) State the commercial unit of electric energy and

find its relation with its SI unit
...
Calculate how it will affect
the heat produced in the resistor
...

Ans :
[CBSE 2014]
a
...
6 # 106 J
b
...
V = IR
Potential drop across 12 W resistance
V = 0
...
8 Volt
...
(a) Explain how does a cell maintain current in a

circuit
...

(ii) Find the current in the main circuit
...



...
e
...
(2)

2

H2 = 9I Rt
In later case, heat generated is 9 times the initial heat
generated
...
If another conductor of 2R is connected in series
then total resistance = R + 2R = 3R
...

107
...


Also, draw the schematic diagram of the circuit
...

Ans :

[CBSE 2013]

(i) Symbol is of variable resistor and it is used to
regulate the current
...
When plug key is closed
current flows through the circuit
...
Define resistance of a conductor
...


which resistance of a conductor depends
...


If the equivalent resistance is RS

Calculate the resistance of 50 m length of wire of
cross-sectional area 0
...

Ans :
[CASE 20131

then,

V = IRS

But,

V = V1 + V2 + V3
RS = R1 + R2 + R3

Resistance is the opposition offered in the path of flow
of current by the atoms or molecules of a conductor
...
length R ? l
b
...
nature of material
...


b
...
01 mm2 = 0
...
01 # 10-6
R = 250 W

110
...


Rp = 3 W
108
...
Also,
express this law mathematically
...
Justify your
answer
...

(i)
Ans :
a
...

If the physical conditions of a conductor is kept
constant then current through it is directly
proportional to the potential difference applied
across it
...
Since V ? I so a graph b/w V and I is a straight
line
...

Ans :
[CBSE 2012]
a
...
1 - 0
TI
R = 20 W

1 = 1 + 1 + 1
R
R1 R2 R3
b
...

c
...

112
...
Resistance of the circuit to carry a current of 5A
on 220V
...

176 = 44 or n = 176
n
44
a
...

(i) When this experiment is repeated by inserting
another nichrome wire of the same thickness
but twice the length (2L), what changes are
observed in the ammeter reading?
(ii) State the changes that are observed in the
ammeter reading if we double the area of
cross-section without changing the length in
the above experiment
...

(b) “Potential difference between points A and B in
an electric field is 1V”
...


2
P = W = I Rt
t
t

P = I2 R But I = V
R
2

2

P = V 2 R or P = V
R
R

111
...
Draw
suitable circuit diagram
...

(b) Why are electric bulbs filled with chemically
inactive nitrogen or argon?
(c) What is meant by the statement that the rating of
a fuse in a circuit is 5 A?
Ans :
[CBSE 2012]
a
...


(i) The resistance of two times long wire also
becomes two times so current decreases in the
circuit
...

b
...

113
...
Also, connect a
voltmeter to record the potential difference across the
15 W resistor and calculate:
a
...
potential difference across 5 W resistor when the
key is closed
...


Net effective resistance of the circuit
...
33 A

b
...
D
...
33 # 5 = 1
...

V = 1
...

114
...

Charge on an electron is 1
...

(c) Draw an electric circuit for studying Ohm’s law
...

Ans :
[CBSE 2012]
a
...

b
...
6 # 10
c
...
25 # 1018 electrons
K–Plug key, (A)–Ammeter, (V)–Voltmeter, Rh–
Rheostat

If by applying a potential difference of 1 volt across a
conductor, the current is 1A then the resistance of the
conductor is said to be 1 ohm
...

a
...

b
...

117
...
State the difference between 1 watt and

1 watt hour
...
An electric
heater rated 1000 W/220 V operates 2 hours daily
...
00 per kWh
...

1 watt is the power consumed by an electric appliance
that carries 1A current when a potential difference of
1 volt is applied across it
...
is the unit
of electric energy, which is consumed by an electric
appliance of 1 W power in one hour
...

1 kWh = 1000 W # 60 # 60 s
1 kWh = 3
...
(a) Define potential difference between two points in

a conductor
...
How is it connected?
(c) A current of 2 A passes through a circuit for
1 minute
...


Electric potential is the amount of work done in
bringing a unit positive charge from one point to
another
...
Voltmeter
...

c
...
= 60 s , V = 3 V
W = VQ = V ^It h
W = 3 # 2 # 60 J
W = 360 J
116
...
Write the mathematical
representation of Ohm’s law
...
List two disadvantages of connecting
different electrical appliances in series
...

Mathematical representation of Ohm’s law V = IR
...
(a) Why is the series arrangement not used for

domestic circuits?
(b) Why is the tungsten used almost exclusively for
filament of electric lamps?
(c) Why are the conductors of electric heating devices
such as bread toasters and electric irons made of
an alloy rather than a pure metal?
(d) Why are copper and aluminium wires usually
employed for electricity transmission?
(e) Why does the cord of an electric heater not glow
while the heating element does?
Ans :
[CBSE 2010]
a
...
In series combination current remains
same
...

b
...

c
...

d
...

e
...


119
...
(a) (i) What is meant by saying that the potential

current in a circuit
...

(b) What do the following symbols mean in circuit
diagrams?

difference between two points is 1 volt?
(ii) How much energy is given to 5 coulomb of
charge passing through a 12 V battery?
(b) Describe an activity with necessary electric circuit
drawn to study the factors on which the resistance
of a conducting wire depends
...
5 m long
nichrome wire XY, an ammeter, a voltmeter, four
cells of 1
...

(i) Draw a diagram of this electric circuit to study
the relation between the potential difference
maintained between the points X and Y and
the electric current flowing through XY
...
8 V, 1
...
6 V
respectively? What conclusion do you draw from these
values?
Ans :
[Delhi 2008]
Ammeter: Ampere is the unit of current
...
(A)
b
...


c
...


(i) Potential difference b/w the two points is 1
volt mean if we bring a unit positive charge (+
1C) from one point to other point then amount of
work done is 1 J, W = VQ
...
Activity to study the factors of affecting
resistance of the conductor—Take a cell, an
ammeter, manganin wires of different length and
different area of cross- section and key
...

(i)
...

(ii)
...

(iii)
...
You
will find that current in the circuit in each
case is different
...


(i)
121
...

(i)

(ii)

(b) Draw a schematic diagram of a circuit consisting
of a battery of 12 V, three resistors of 5 W , 10
W and 20 W connected in parallel, an ammeter
to measure the total current through the circuit,
a voltmeter to measure the potential difference
across the combination of resistors
...

Ans :
[CBSE 2008]
a
...

R = V = 1
...
6 = 2
...
6 - 0
0
...

(ii) Wires crossing each other which are not
connected together
...


Given:

V = 1
...

R = 30 W
I = V = 1
...
05 A
30
20
R

b
...
When two resistors of resistances R1 and R2 are

c
...
In parallel combination if
one device fails others keep working
...


connected in parallel, the net resistance is 3 W
...
Calculate the
values of R1 and R2
...

1 = 1 + 1
Rp
R1 R2
R1 # R2 = 3
R1 + R2
When R1 and R2 are in series combination
...
An electric iron has a rating of 750 W; 200 V
...
the current required
...
the resistance of its heating element,
c
...

Ans :
[CBSE2016]

R1 + R2 = 16
Solving eq
...
(2)

a
...

c
...
3 Volt
...
75
I
Energy consumed in 2 hr = P # t

^R1 - 4h^R1 - 12h = 0
R1 = 4 W, 12 W
R2 = 12 W or 4 W
126
...
5 A is drawn by a filament of an electric

bulb for 10 minutes
...

Ans :
[CBSE 2015]
Given:

123
...
The potential difference across the terminals of a

cell is 1
...
If it is connected with a resistance of
30 ohms, calculate the current flowing through the
circuit
...
5 A
t = 10 # 60 s
Q
or Q = It
I =
t
Q = 0
...
Compare the new resistance
with the original resistance
...
(2)

R 12 - 16R1 + 48 = 0

Rating of iron is 750 W–200V
...

P = VI or I = P
V
I = 750 = 3
...
(1)

Q = 300 C
127
...
18 A

(b)

V = 220 V
R = 100 ohm
I = V = 220 A
100
R
I = 2
...
The potential difference between the terminals of an

electric heater is 60 V when it draws a current of 4 A
from the source
...
When heating is at maximum rate

the potential difference is increased to 120 V?
Ans :

P = 840 W
I = 840 = 3
...
7 W
3
...
When heating is at minimum rate
and

V = 120 V , R = 15 W
Il = V = 120
15
R

when
then,

P = 360 W
I = 360 = 1
...
Resistance of a metal wire of length 1 m is 26 W at

20°C
...
3 mm, what will
be the resistivity of the metal at that temperature?
Ans :
We have

l = 1m
R = 26 ohm
t = 20cC

and,

132
...
Calculate the total energy consumed by the
tube lights and press in a month of 30 days
...
15 mm

Power of 5 tubes = 5 # 40 W = 200 W
Energy consumed by 5 tubes in 5 hr
...
34
...
64

R =rl
A
2
r = R # A = R # pr
l
l

= 200 # 5 = 1000 Wh
Energy consumed by electric press per day

^0
...
84 # 10-6 Wm

= 2000 Wh
Total energy consumed per day
= ^1000 + 2000h Wh

130
...
Calculate

the new resistance of the wire
...
(1)
R1 = r l1
A1

133
...
total resistance in arm CE,
b
...
current in each arm, i
...
, AB and CE of the circuit
...
(2)

l
2

2A

R2 = r l
4A
Dividing (3) by eq
...
(3)

R2 = R1 = 4 = 1
4
4
R2 = 1 W
131
...
The voltage is 220 V
...


[CBSE 2013]

Total resistance in arm CE

= ^2 + 4h W = 6 W
b
...

1 = 1+1
effective resistance,
b6 6l
R
R = 3W
Total current in the circuit 6a V = 3V given@
I = V = 3 = 1A
3
R
c
...

I = 0
...
An electric heater rated 1200 W operates 2 hour per

day
...
00 per unit
...


= 60 hrs
Energy consumed in one month (30 days)
= P # t = 1200 # 60
= 72000 Wh
= 72 kWh
Cost of energy consumed in 30 days
= `72 # 5 = `360
135
...
Calculate
the total current flowing through the circuit
...
625 A
136
...
Calculate the ratio of power
consumed in the two cases
...

Net resistance = 5 = 2
...
5
2
...


I = 1A
138
...
Find the cost to operate it for 30 days at Rs
...
00
per unit
...
of days = 30
Energy consumed by the heater
= P#t
= 600 W # 6 hr # 30 days
day
= 108000 Wh
= 108 kWh
Total cost @ ` 3/- = `108 # 3 = `324
139
...
Ammeter and voltmeter are connected in
the circuit
...
What is the current
flowing and potential difference across 10 W resistance?
Ans :
[CBSE 2012]
Effective resistance of the circuit
R = 5 + 10 + 15 = 30 W

PS = 36 W
(ii) In parallel combination 1 = 1 + 1
Rp 2 2
RP = 1 W
2
PP = V = 12 # 12 W
1
RP

PP = 144 W
137
...
Find the equivalent resistance across the two ends A

resistance of coils

and B of this circuit
...


= 30 + 30 = 60 W
I = 220 = 3
...
67 A
15

142
...


equal parts,
a
...
If the four parts are placed in parallel, how will
the joint resistance compare with the resistance of
the original wire?
Ans :
a
...
Effective resistance in parallel combination of
these four pieces of resistance R4 each
...
A wire is 1
...
2 mm in diameter and has

a resistance of 10 W
...

Ans :
[CBSE 2011]
L = 1
...
2 mm
r = D = 0
...
1 mm
2
2

Hence network can be reduced further as

r = 0
...
A hot plate of an electric oven, connected to a 220 V

line
...
Find the value of the current required in each
of the three cases
...

Then current drawn by each
I = V = 220 = 7
...
When two coils are connected in series then total
a
...
14 # ^0
...
0

r = 3
...
The rating of an electric heater is 1100 W, 220 V
...

Also, calculate the energy consumed in kWh in the
month of November, if the heater is used daily for four
hours at the rated voltage
...
(i
...
30
days) @ 4 hr per day
...
5 A

= 1100 # 4 # hr # 30
= 1100 # 120 Wh
E = 132 kWh
145
...

(ii) total resistance of the circuit
...


Ans :
a
...
Find out the following in the electric circuit given in

figure:
a
...

b
...

c
...

d
...

e
...


Ans :
a
...

(i) I in 30 W resistor,
=V = 6 = 1 A
30
5
R

8 W and 8 W resistors are in parallel
Rp = 8 # 8 = 4 W
8+8

= 0
...
(i) Voltage across each appliance remains
(ii) If any appliance fails to work other appliances
continue to work
...
Three 2 W resistors, A, B and C, are connected as

shown in figure
...
Find the maximum current that can flow
through the three resistors
...
Total resistance of the circuit
= ^4 + 4h W = 8 W
Current in 4 W resistor
I = 8 = 1A
8
c
...

d
...


P =4W
Both the ammeters are in series in the circuit so
current will be same in both ammeters
...


148
...

from

P = I2 R or I2 = P
R
I2 = 18 = 9
2
I = 3A

allows a 10 A current when connected to a 220 V
power source
...

Resistance of bulb = V = 220 = 22 W
10
I
149
...
It is used

for 5 hours daily
...

Ans :
[CBSE 2007]
Electric Iron 100W –– 220 V
Resistor 2 W, 2 W are equal and in parallel combination
so current of 3 A will be distributed equally
...
daily

2

R = V = 220 # 220 = 484 W
100
P
(ii) Energy consumed by iron per day
(i)

= P # t = 100 # 5 Wh
= 500 Wh = 0
...
A torch bulb is rated 5 V and 500 mA
...

Ans :
[CBSE 2005]
Given: 5V––500 mA
Power = V # I = 5 # 500 # 10-3

(i)

= 2
...

= 2
...
s
...
6 # 10 4 J
151
...
the total effective resistance of the arrangement
and
b
...


Ans :
a
...

1 = 1 + 1 = 2
30 30
30
Rp

(ii)

R p = 15 W
I = V = 12 = 0
...
An electric bulb is rated at 200 V–100 W
...
What is
the electrical energy consumed? Calculate the cost, if
the rate is 50 paise unit
...
by one bulb
...
4 kWh
Total energy consumed by 5 similar bulb
= 5 # 0
...
50 per unit = `2 # 0

---