Selina Textbook Solutions: Household Circuits | Physics Class 10 ICSE PDF Download

 Page 1


Class X Chapter 09 – Electrical Power and Household circuits  Physics 
_____________________________________________________________________________ 
 
EXERCISE- 9 (A) 
 
Question 1:  
Write an expression for the electrical energy spent in flow of current through an electrical 
appliance in terms of current, resistance and time. 
Solution 1: 
Electrical energy, W = I
2
Rt joule  
 
 
Question 2: 
Write an expression for the electrical power spent in flow of current through a conductor in 
terms of  
(a) Resistance and potential difference, (b) current and resistance. 
Solution 2: 
(a) Electrical power, P = 
2
v
R
 
(b) Electrical power, P = I
2
R 
 
 
Question 3: 
Electrical power p is given by the expression P = (Q × V) ÷ time  
(a) What do the symbols Q and V represent? 
(b) Express the power P in terms of current and resistance explaining the meaning of symbols 
used there in. 
Solution 3: 
(a) Q represents Charge and V represents Voltage. 
(b)  Electrical Power, P = I
2
R 
       Where I : current  
        And R : Resistance  
 
Question 4: 
Name the S.I. unit of electrical energy. How is it related to Wh? 
Solution 4: 
The S.I. unit of electrical energy is joule. 
1Wh = 3600 J 
 
 
 
Page 2


Class X Chapter 09 – Electrical Power and Household circuits  Physics 
_____________________________________________________________________________ 
 
EXERCISE- 9 (A) 
 
Question 1:  
Write an expression for the electrical energy spent in flow of current through an electrical 
appliance in terms of current, resistance and time. 
Solution 1: 
Electrical energy, W = I
2
Rt joule  
 
 
Question 2: 
Write an expression for the electrical power spent in flow of current through a conductor in 
terms of  
(a) Resistance and potential difference, (b) current and resistance. 
Solution 2: 
(a) Electrical power, P = 
2
v
R
 
(b) Electrical power, P = I
2
R 
 
 
Question 3: 
Electrical power p is given by the expression P = (Q × V) ÷ time  
(a) What do the symbols Q and V represent? 
(b) Express the power P in terms of current and resistance explaining the meaning of symbols 
used there in. 
Solution 3: 
(a) Q represents Charge and V represents Voltage. 
(b)  Electrical Power, P = I
2
R 
       Where I : current  
        And R : Resistance  
 
Question 4: 
Name the S.I. unit of electrical energy. How is it related to Wh? 
Solution 4: 
The S.I. unit of electrical energy is joule. 
1Wh = 3600 J 
 
 
 
Class X Chapter 09 – Electrical Power and Household circuits  Physics 
_____________________________________________________________________________ 
 
Question 5: 
Name the S.I unit of electrical energy. How is it related to Wh? 
Solution 5: 
The power of an appliance is 100 W. It means that 100 J of electrical energy is consumed by the 
appliance in 1 second. 
 
 
Question 6: 
State the S.I. unit of electrical power. 
Solution 6: 
  The S.I. unit of electrical power is Watt.  
 
 
Question 7: 
(i) State and define the household unit of electricity. 
(ii) What is the voltage of the electricity that is generally supplied to a house? 
Solution 7: 
(i) The household unit of electricity is kilowatt-hour (kWh). 
One kilowatt-hour (kWh) is the electrical energy consumed by an electrical appliance of 
power 1 kW when it is used for one hour. 
(ii) The voltage of the electricity that is generally supplied to a house is 220 Volt. 
 
 
Question 8: 
Name the physical quality which is measured in  
(i) Kw     (ii) kWh 
Solution 8: 
(i) Electrical power is measured in kW and 
(ii) Electrical energy is measured in kWh. 
 
 
Question 9: 
Define the term kilowatt-hour and state its value in S.I unit 
Solution 9: 
One kilowatt-hour (kWh) is the electrical energy consumed by an electrical appliance of power 
1 kW when it is used for one hour. 
Its value in SI unit is 1kWh = 3.6 × 10
6
 J 
 
 
Page 3


Class X Chapter 09 – Electrical Power and Household circuits  Physics 
_____________________________________________________________________________ 
 
EXERCISE- 9 (A) 
 
Question 1:  
Write an expression for the electrical energy spent in flow of current through an electrical 
appliance in terms of current, resistance and time. 
Solution 1: 
Electrical energy, W = I
2
Rt joule  
 
 
Question 2: 
Write an expression for the electrical power spent in flow of current through a conductor in 
terms of  
(a) Resistance and potential difference, (b) current and resistance. 
Solution 2: 
(a) Electrical power, P = 
2
v
R
 
(b) Electrical power, P = I
2
R 
 
 
Question 3: 
Electrical power p is given by the expression P = (Q × V) ÷ time  
(a) What do the symbols Q and V represent? 
(b) Express the power P in terms of current and resistance explaining the meaning of symbols 
used there in. 
Solution 3: 
(a) Q represents Charge and V represents Voltage. 
(b)  Electrical Power, P = I
2
R 
       Where I : current  
        And R : Resistance  
 
Question 4: 
Name the S.I. unit of electrical energy. How is it related to Wh? 
Solution 4: 
The S.I. unit of electrical energy is joule. 
1Wh = 3600 J 
 
 
 
Class X Chapter 09 – Electrical Power and Household circuits  Physics 
_____________________________________________________________________________ 
 
Question 5: 
Name the S.I unit of electrical energy. How is it related to Wh? 
Solution 5: 
The power of an appliance is 100 W. It means that 100 J of electrical energy is consumed by the 
appliance in 1 second. 
 
 
Question 6: 
State the S.I. unit of electrical power. 
Solution 6: 
  The S.I. unit of electrical power is Watt.  
 
 
Question 7: 
(i) State and define the household unit of electricity. 
(ii) What is the voltage of the electricity that is generally supplied to a house? 
Solution 7: 
(i) The household unit of electricity is kilowatt-hour (kWh). 
One kilowatt-hour (kWh) is the electrical energy consumed by an electrical appliance of 
power 1 kW when it is used for one hour. 
(ii) The voltage of the electricity that is generally supplied to a house is 220 Volt. 
 
 
Question 8: 
Name the physical quality which is measured in  
(i) Kw     (ii) kWh 
Solution 8: 
(i) Electrical power is measured in kW and 
(ii) Electrical energy is measured in kWh. 
 
 
Question 9: 
Define the term kilowatt-hour and state its value in S.I unit 
Solution 9: 
One kilowatt-hour (kWh) is the electrical energy consumed by an electrical appliance of power 
1 kW when it is used for one hour. 
Its value in SI unit is 1kWh = 3.6 × 10
6
 J 
 
 
Class X Chapter 09 – Electrical Power and Household circuits  Physics 
_____________________________________________________________________________ 
 
Question 10: 
Distinguish between kilowatt and kilowatt-hour. 
Solution 10: 
Kilowatt is the unit of electrical power whereas kilowatt-hour is the unit of electrical energy. 
 
 
Question 11: 
Complete the following: 
(a) 1Kwh = 
1volt 1am pere ..............
1000
??
 
(b) 1kWh = …….. J 
Solution 11: 
(a) 1Kwh = 
1volt 1am pere 1hour
1000
??
 
(b) 3.6 × 10
6
J 
 
 
Question 12: 
What do you mean by power rating of an electrical appliance? How do you use it to calculate 
(a) the resistance of the appliance, and (b) the safe limit of current in it, while in use? 
Solution 12:12 
An electrical appliance such as electric bulb, geyser etc. is rated with power (P) and voltage 
(V) which is known as its power rating. For example: If an electric bulb is rated as 50W-220V, 
it means that when the bulb is lighted on a 220 V supply, it consumes 50 W electrical power. 
(a) To calculate the resistance of the appliance, the expression is: 
       Resistance, R = 
2
v
P
 
(b) The safe limit of current I is : 
P
I
v
? 
 
 
Question 13: 
An electric bulb is rated ‘100 W, 250 V’. What information does this convey? 
Solution 13: 
It means that if the bulb is lighted on a 250 V supply, it consumes 100 W electrical power (which 
means 100J of electrical energy is converted in the filament of bulb into the light and heat energy 
in 1 second). 
 
 
Page 4


Class X Chapter 09 – Electrical Power and Household circuits  Physics 
_____________________________________________________________________________ 
 
EXERCISE- 9 (A) 
 
Question 1:  
Write an expression for the electrical energy spent in flow of current through an electrical 
appliance in terms of current, resistance and time. 
Solution 1: 
Electrical energy, W = I
2
Rt joule  
 
 
Question 2: 
Write an expression for the electrical power spent in flow of current through a conductor in 
terms of  
(a) Resistance and potential difference, (b) current and resistance. 
Solution 2: 
(a) Electrical power, P = 
2
v
R
 
(b) Electrical power, P = I
2
R 
 
 
Question 3: 
Electrical power p is given by the expression P = (Q × V) ÷ time  
(a) What do the symbols Q and V represent? 
(b) Express the power P in terms of current and resistance explaining the meaning of symbols 
used there in. 
Solution 3: 
(a) Q represents Charge and V represents Voltage. 
(b)  Electrical Power, P = I
2
R 
       Where I : current  
        And R : Resistance  
 
Question 4: 
Name the S.I. unit of electrical energy. How is it related to Wh? 
Solution 4: 
The S.I. unit of electrical energy is joule. 
1Wh = 3600 J 
 
 
 
Class X Chapter 09 – Electrical Power and Household circuits  Physics 
_____________________________________________________________________________ 
 
Question 5: 
Name the S.I unit of electrical energy. How is it related to Wh? 
Solution 5: 
The power of an appliance is 100 W. It means that 100 J of electrical energy is consumed by the 
appliance in 1 second. 
 
 
Question 6: 
State the S.I. unit of electrical power. 
Solution 6: 
  The S.I. unit of electrical power is Watt.  
 
 
Question 7: 
(i) State and define the household unit of electricity. 
(ii) What is the voltage of the electricity that is generally supplied to a house? 
Solution 7: 
(i) The household unit of electricity is kilowatt-hour (kWh). 
One kilowatt-hour (kWh) is the electrical energy consumed by an electrical appliance of 
power 1 kW when it is used for one hour. 
(ii) The voltage of the electricity that is generally supplied to a house is 220 Volt. 
 
 
Question 8: 
Name the physical quality which is measured in  
(i) Kw     (ii) kWh 
Solution 8: 
(i) Electrical power is measured in kW and 
(ii) Electrical energy is measured in kWh. 
 
 
Question 9: 
Define the term kilowatt-hour and state its value in S.I unit 
Solution 9: 
One kilowatt-hour (kWh) is the electrical energy consumed by an electrical appliance of power 
1 kW when it is used for one hour. 
Its value in SI unit is 1kWh = 3.6 × 10
6
 J 
 
 
Class X Chapter 09 – Electrical Power and Household circuits  Physics 
_____________________________________________________________________________ 
 
Question 10: 
Distinguish between kilowatt and kilowatt-hour. 
Solution 10: 
Kilowatt is the unit of electrical power whereas kilowatt-hour is the unit of electrical energy. 
 
 
Question 11: 
Complete the following: 
(a) 1Kwh = 
1volt 1am pere ..............
1000
??
 
(b) 1kWh = …….. J 
Solution 11: 
(a) 1Kwh = 
1volt 1am pere 1hour
1000
??
 
(b) 3.6 × 10
6
J 
 
 
Question 12: 
What do you mean by power rating of an electrical appliance? How do you use it to calculate 
(a) the resistance of the appliance, and (b) the safe limit of current in it, while in use? 
Solution 12:12 
An electrical appliance such as electric bulb, geyser etc. is rated with power (P) and voltage 
(V) which is known as its power rating. For example: If an electric bulb is rated as 50W-220V, 
it means that when the bulb is lighted on a 220 V supply, it consumes 50 W electrical power. 
(a) To calculate the resistance of the appliance, the expression is: 
       Resistance, R = 
2
v
P
 
(b) The safe limit of current I is : 
P
I
v
? 
 
 
Question 13: 
An electric bulb is rated ‘100 W, 250 V’. What information does this convey? 
Solution 13: 
It means that if the bulb is lighted on a 250 V supply, it consumes 100 W electrical power (which 
means 100J of electrical energy is converted in the filament of bulb into the light and heat energy 
in 1 second). 
 
 
Class X Chapter 09 – Electrical Power and Household circuits  Physics 
_____________________________________________________________________________ 
 
Question 14: 
List the names of three electrical gadgets used in your house. Write their power, voltage rating 
and approximate time for which each one is used in a day. Hence find the electrical energy 
consumed by each in a day. 
Solution 14: 
Appliance Power 
(in watt) 
Voltage 
(in volts) 
Time 
( hours) 
Electrical energy   
(E = p × t) 
Fluorescent tube 
Television set 
Refrigerator 
40 
120 
150 
220 
220 
220 
12 
4 
24 
0.48 kWh 
0.48 KWh 
3.6 kWh 
 
 
Question 15: 
Two lamps, one rated 220 V, 50 W and the other rated 220 V, 100 W, are connected in series 
with mains of 220 V. Explain why does the 50 W lamp consume more power. 
Solution 15: 
Resistance of 220 V, 50 w lamp is 
22
1
1
v 220
R 968
p 50
? ? ? ? 
Resistance of 220 V, 100 lamp is 
22
2
2
v 220
R 484
p 100
? ? ? ? 
Since the two lamps are connected in series  
So same current I passes through each lamp. 
Power consumed in 220 v, 50 w lamp is 
2
11
p I R ? 
Power consumed in 220 v, 100 w lamp is 
2
22
p I R ? 
Since R1 > R2,  P1 > P2 
i.e. 50 w lamp consumes more power. 
 
 
Question 16: 
Name the factors on which the heat produced in a wire depends when current is passed in it, and 
state how does it depend on the factors stated by you. 
Solution 16: 
When current is passed in a wire, the heat produced in it depends on the three factors: (i) on 
the amount of current passing through the wire, (ii) on the resistance of wire and (iii) on the 
time for which current is passed in the wire. 
 (i) Dependence of heat produced on the current in wire: The amount of heat H produced in the 
wire is directly proportional to the square of current I passing through the wire,  i.e., H ? I
2
 
Page 5


Class X Chapter 09 – Electrical Power and Household circuits  Physics 
_____________________________________________________________________________ 
 
EXERCISE- 9 (A) 
 
Question 1:  
Write an expression for the electrical energy spent in flow of current through an electrical 
appliance in terms of current, resistance and time. 
Solution 1: 
Electrical energy, W = I
2
Rt joule  
 
 
Question 2: 
Write an expression for the electrical power spent in flow of current through a conductor in 
terms of  
(a) Resistance and potential difference, (b) current and resistance. 
Solution 2: 
(a) Electrical power, P = 
2
v
R
 
(b) Electrical power, P = I
2
R 
 
 
Question 3: 
Electrical power p is given by the expression P = (Q × V) ÷ time  
(a) What do the symbols Q and V represent? 
(b) Express the power P in terms of current and resistance explaining the meaning of symbols 
used there in. 
Solution 3: 
(a) Q represents Charge and V represents Voltage. 
(b)  Electrical Power, P = I
2
R 
       Where I : current  
        And R : Resistance  
 
Question 4: 
Name the S.I. unit of electrical energy. How is it related to Wh? 
Solution 4: 
The S.I. unit of electrical energy is joule. 
1Wh = 3600 J 
 
 
 
Class X Chapter 09 – Electrical Power and Household circuits  Physics 
_____________________________________________________________________________ 
 
Question 5: 
Name the S.I unit of electrical energy. How is it related to Wh? 
Solution 5: 
The power of an appliance is 100 W. It means that 100 J of electrical energy is consumed by the 
appliance in 1 second. 
 
 
Question 6: 
State the S.I. unit of electrical power. 
Solution 6: 
  The S.I. unit of electrical power is Watt.  
 
 
Question 7: 
(i) State and define the household unit of electricity. 
(ii) What is the voltage of the electricity that is generally supplied to a house? 
Solution 7: 
(i) The household unit of electricity is kilowatt-hour (kWh). 
One kilowatt-hour (kWh) is the electrical energy consumed by an electrical appliance of 
power 1 kW when it is used for one hour. 
(ii) The voltage of the electricity that is generally supplied to a house is 220 Volt. 
 
 
Question 8: 
Name the physical quality which is measured in  
(i) Kw     (ii) kWh 
Solution 8: 
(i) Electrical power is measured in kW and 
(ii) Electrical energy is measured in kWh. 
 
 
Question 9: 
Define the term kilowatt-hour and state its value in S.I unit 
Solution 9: 
One kilowatt-hour (kWh) is the electrical energy consumed by an electrical appliance of power 
1 kW when it is used for one hour. 
Its value in SI unit is 1kWh = 3.6 × 10
6
 J 
 
 
Class X Chapter 09 – Electrical Power and Household circuits  Physics 
_____________________________________________________________________________ 
 
Question 10: 
Distinguish between kilowatt and kilowatt-hour. 
Solution 10: 
Kilowatt is the unit of electrical power whereas kilowatt-hour is the unit of electrical energy. 
 
 
Question 11: 
Complete the following: 
(a) 1Kwh = 
1volt 1am pere ..............
1000
??
 
(b) 1kWh = …….. J 
Solution 11: 
(a) 1Kwh = 
1volt 1am pere 1hour
1000
??
 
(b) 3.6 × 10
6
J 
 
 
Question 12: 
What do you mean by power rating of an electrical appliance? How do you use it to calculate 
(a) the resistance of the appliance, and (b) the safe limit of current in it, while in use? 
Solution 12:12 
An electrical appliance such as electric bulb, geyser etc. is rated with power (P) and voltage 
(V) which is known as its power rating. For example: If an electric bulb is rated as 50W-220V, 
it means that when the bulb is lighted on a 220 V supply, it consumes 50 W electrical power. 
(a) To calculate the resistance of the appliance, the expression is: 
       Resistance, R = 
2
v
P
 
(b) The safe limit of current I is : 
P
I
v
? 
 
 
Question 13: 
An electric bulb is rated ‘100 W, 250 V’. What information does this convey? 
Solution 13: 
It means that if the bulb is lighted on a 250 V supply, it consumes 100 W electrical power (which 
means 100J of electrical energy is converted in the filament of bulb into the light and heat energy 
in 1 second). 
 
 
Class X Chapter 09 – Electrical Power and Household circuits  Physics 
_____________________________________________________________________________ 
 
Question 14: 
List the names of three electrical gadgets used in your house. Write their power, voltage rating 
and approximate time for which each one is used in a day. Hence find the electrical energy 
consumed by each in a day. 
Solution 14: 
Appliance Power 
(in watt) 
Voltage 
(in volts) 
Time 
( hours) 
Electrical energy   
(E = p × t) 
Fluorescent tube 
Television set 
Refrigerator 
40 
120 
150 
220 
220 
220 
12 
4 
24 
0.48 kWh 
0.48 KWh 
3.6 kWh 
 
 
Question 15: 
Two lamps, one rated 220 V, 50 W and the other rated 220 V, 100 W, are connected in series 
with mains of 220 V. Explain why does the 50 W lamp consume more power. 
Solution 15: 
Resistance of 220 V, 50 w lamp is 
22
1
1
v 220
R 968
p 50
? ? ? ? 
Resistance of 220 V, 100 lamp is 
22
2
2
v 220
R 484
p 100
? ? ? ? 
Since the two lamps are connected in series  
So same current I passes through each lamp. 
Power consumed in 220 v, 50 w lamp is 
2
11
p I R ? 
Power consumed in 220 v, 100 w lamp is 
2
22
p I R ? 
Since R1 > R2,  P1 > P2 
i.e. 50 w lamp consumes more power. 
 
 
Question 16: 
Name the factors on which the heat produced in a wire depends when current is passed in it, and 
state how does it depend on the factors stated by you. 
Solution 16: 
When current is passed in a wire, the heat produced in it depends on the three factors: (i) on 
the amount of current passing through the wire, (ii) on the resistance of wire and (iii) on the 
time for which current is passed in the wire. 
 (i) Dependence of heat produced on the current in wire: The amount of heat H produced in the 
wire is directly proportional to the square of current I passing through the wire,  i.e., H ? I
2
 
Class X Chapter 09 – Electrical Power and Household circuits  Physics 
_____________________________________________________________________________ 
 
 (ii) Dependence of heat produced on the resistance of wire: The amount of heat H produced in  
       the wire is directly proportional to the resistance R of the wire, i.e., H ? R 
(iii) Dependence of heat produced on the time: The amount of heat H produced in the wire is 
directly proportional to the time t for which current is passed in the wire, i.e., H ? t 
 
 
MULTIPLE CHOICE TYPE: 
 
Question 1: 
When a current I flows through a resistance R for time t, the electrical energy spent is given by: 
(a) IRt       (b) I²Rt    (c) IR
2
t      (d) I
2
R/t 
Solution 1: 
I²Rt 
Note: Electrical energy (W) = 
2
vt
I²Rt VIt
R
?? 
 
 
Question 2: 
An electrical appliance has a rating 100 W, 120 V. the resistance of element of appliance when 
in use is: 
(a) 1.2O   (b) 144 O   (c) 120 O   (d) 100 O 
Solution 2: 
144O  
Solution: 
Given, power (p) = 100 w 
Potential difference, v = 120 volt  
? Resistance, r = 
? ? ? ?
22
v 120
144
P 100
? ? ? 
 
 
NUMERICALS: 
 
Question 1: 
A current of 2 A is passed through a coil of resistance 75O for 2 minutes. (a) How much heat 
energy is produced? (b) How much charge is passed through the resistance? 
Solution 1: 
Given, current (I) = 2 A 
Resistance, R = 75 O 
Time, t = 2 min = 120s 
(a) Heat produced, H = 
2
I Rt