Selina Textbook Solutions: Electricity | Physics Class 8 ICSE PDF Download

 Page 1


8. Electricity 
? ELECTRICITY: “Is the rate of flow of electrons”. i =  
? To keep electrons move, potential difference is needed. This is done by a cell or battery. 
? Potential difference “is the amount of work done in moving a unit positive charge from one 
point to other.” 
Potential difference V = Work done (W) / charge moved (Q) 
Or W = QV but charge = it 
Hence, W = VIt or Electrical energy = VIt 
? Power is “Rate of doing work” 
P = W/t = VIt/t = VI 
Power is measured in watt or J S
-1
 
1 Watt = 1 Volt × 1 Ampere 
? S.I. unit of charge is coulomb (C). 
S.I. unit of current is Ampere (A). 
S.I. unit of P.D. is volt. 
S.I. unit of electrical energy is Joule (J) and of power is watt (W) 
1 kWh-3600000J = 3.6 x 10
6
 J 
? ELECTRIC power is generated at the GENERATING STATION at 11000 volt, or 11 kV as 
these stations are at very far off place from areas where it is to be used. The voltage 
(A.C.) is of 50 HZ frequency. 
? AT GRID SUB-STATION this alternating current (A.C) voltage is stepped up from 11 kV to 
132 kV to minise the loss of energy in transmission line wires. 
? At MAIN-SUB-STATION this voltage is stepped down from 132 kV to 33 kV and 
transmitted to city SUB-STATION. 
? At CITY SUB STATION, it is further stepped down from 33 kV to 220 V for supply to 
hourses for consumers. 
? Colour coding: Live wire — Red or Brown 
Neutral—Black or light blue 
Earth wire — Green or yellow 
? 1 kWh = 1 unit: Power Rating on an appliance 100 W – 220 V means the appliance when 
worked on a 220 V will consume 100 W electricity power 
? OVER LOADING: is the condition of Electric circuit, when it draws more current than it is 
designed for i. e. when a number of appliances are switched on at a time i.e. geyser, A.C. 
Electric motor etc. or a large number of plugs are put in the same socket. 
? EARTHING: is done in a house near the kWh meter. Earthing is a safety device which 
puts the appliance at zero potential. 
? SHORT CIRCUITING: If the insulation on the wire of cable used f in the wiring (or used 
with an appliance) breaks. The LIVE WIRE 
COMES IN CONTACT WITH THE NEUTRAL WIRE, this result in SHORT CIRCUITING 
? FUSE: “Is a device used to limit the current in an electric circuit”. The use of fuse protects 
the appliance in circuit from being damaged Fuse is always connected in live wire. A fuse 
wire should have 
(i) High resistance 
(ii) Low melting point. 
These days miniature circuit breakers (MCB) are used. It is AUTOMATIC breaker, when 
current flowing excess. 
?  Appliances in a house are connected in parallel. 
Page 2


8. Electricity 
? ELECTRICITY: “Is the rate of flow of electrons”. i =  
? To keep electrons move, potential difference is needed. This is done by a cell or battery. 
? Potential difference “is the amount of work done in moving a unit positive charge from one 
point to other.” 
Potential difference V = Work done (W) / charge moved (Q) 
Or W = QV but charge = it 
Hence, W = VIt or Electrical energy = VIt 
? Power is “Rate of doing work” 
P = W/t = VIt/t = VI 
Power is measured in watt or J S
-1
 
1 Watt = 1 Volt × 1 Ampere 
? S.I. unit of charge is coulomb (C). 
S.I. unit of current is Ampere (A). 
S.I. unit of P.D. is volt. 
S.I. unit of electrical energy is Joule (J) and of power is watt (W) 
1 kWh-3600000J = 3.6 x 10
6
 J 
? ELECTRIC power is generated at the GENERATING STATION at 11000 volt, or 11 kV as 
these stations are at very far off place from areas where it is to be used. The voltage 
(A.C.) is of 50 HZ frequency. 
? AT GRID SUB-STATION this alternating current (A.C) voltage is stepped up from 11 kV to 
132 kV to minise the loss of energy in transmission line wires. 
? At MAIN-SUB-STATION this voltage is stepped down from 132 kV to 33 kV and 
transmitted to city SUB-STATION. 
? At CITY SUB STATION, it is further stepped down from 33 kV to 220 V for supply to 
hourses for consumers. 
? Colour coding: Live wire — Red or Brown 
Neutral—Black or light blue 
Earth wire — Green or yellow 
? 1 kWh = 1 unit: Power Rating on an appliance 100 W – 220 V means the appliance when 
worked on a 220 V will consume 100 W electricity power 
? OVER LOADING: is the condition of Electric circuit, when it draws more current than it is 
designed for i. e. when a number of appliances are switched on at a time i.e. geyser, A.C. 
Electric motor etc. or a large number of plugs are put in the same socket. 
? EARTHING: is done in a house near the kWh meter. Earthing is a safety device which 
puts the appliance at zero potential. 
? SHORT CIRCUITING: If the insulation on the wire of cable used f in the wiring (or used 
with an appliance) breaks. The LIVE WIRE 
COMES IN CONTACT WITH THE NEUTRAL WIRE, this result in SHORT CIRCUITING 
? FUSE: “Is a device used to limit the current in an electric circuit”. The use of fuse protects 
the appliance in circuit from being damaged Fuse is always connected in live wire. A fuse 
wire should have 
(i) High resistance 
(ii) Low melting point. 
These days miniature circuit breakers (MCB) are used. It is AUTOMATIC breaker, when 
current flowing excess. 
?  Appliances in a house are connected in parallel. 
Test yourself 
A. Objective Questions 
1. Write true or false for each statement: 
(a) A fuse wire has a high melting point. 
Answer. False. 
(b) Flow of protons constitutes electric current. 
Answer. False. 
(c) Silver is an insulator of electricity. 
Answer. False. 
(d) S.I. unit and commercial unit of electrical energy are same. 
Answer. False. 
(e) Overloading of electric current in circuits can lead to short circuiting. 
Answer. True. 
(f) Our body can pass electricity through it. 
Answer. True. 
(g) All metals are insulators of electricity. 
Answer. False. 
(h) The earth wire protects us from an electric shock. 
Answer. True. 
(i) A switch should not be touched with wet hands. 
Answer. True. 
(j) AH electrical appliances in a household circuit work at the same voltage. 
Answer. True. 
(k) In a cable, the green wire is the live wire. 
Answer. False. 
(l) A fuse is connected to the live wire. 
Answer. True. 
Page 3


8. Electricity 
? ELECTRICITY: “Is the rate of flow of electrons”. i =  
? To keep electrons move, potential difference is needed. This is done by a cell or battery. 
? Potential difference “is the amount of work done in moving a unit positive charge from one 
point to other.” 
Potential difference V = Work done (W) / charge moved (Q) 
Or W = QV but charge = it 
Hence, W = VIt or Electrical energy = VIt 
? Power is “Rate of doing work” 
P = W/t = VIt/t = VI 
Power is measured in watt or J S
-1
 
1 Watt = 1 Volt × 1 Ampere 
? S.I. unit of charge is coulomb (C). 
S.I. unit of current is Ampere (A). 
S.I. unit of P.D. is volt. 
S.I. unit of electrical energy is Joule (J) and of power is watt (W) 
1 kWh-3600000J = 3.6 x 10
6
 J 
? ELECTRIC power is generated at the GENERATING STATION at 11000 volt, or 11 kV as 
these stations are at very far off place from areas where it is to be used. The voltage 
(A.C.) is of 50 HZ frequency. 
? AT GRID SUB-STATION this alternating current (A.C) voltage is stepped up from 11 kV to 
132 kV to minise the loss of energy in transmission line wires. 
? At MAIN-SUB-STATION this voltage is stepped down from 132 kV to 33 kV and 
transmitted to city SUB-STATION. 
? At CITY SUB STATION, it is further stepped down from 33 kV to 220 V for supply to 
hourses for consumers. 
? Colour coding: Live wire — Red or Brown 
Neutral—Black or light blue 
Earth wire — Green or yellow 
? 1 kWh = 1 unit: Power Rating on an appliance 100 W – 220 V means the appliance when 
worked on a 220 V will consume 100 W electricity power 
? OVER LOADING: is the condition of Electric circuit, when it draws more current than it is 
designed for i. e. when a number of appliances are switched on at a time i.e. geyser, A.C. 
Electric motor etc. or a large number of plugs are put in the same socket. 
? EARTHING: is done in a house near the kWh meter. Earthing is a safety device which 
puts the appliance at zero potential. 
? SHORT CIRCUITING: If the insulation on the wire of cable used f in the wiring (or used 
with an appliance) breaks. The LIVE WIRE 
COMES IN CONTACT WITH THE NEUTRAL WIRE, this result in SHORT CIRCUITING 
? FUSE: “Is a device used to limit the current in an electric circuit”. The use of fuse protects 
the appliance in circuit from being damaged Fuse is always connected in live wire. A fuse 
wire should have 
(i) High resistance 
(ii) Low melting point. 
These days miniature circuit breakers (MCB) are used. It is AUTOMATIC breaker, when 
current flowing excess. 
?  Appliances in a house are connected in parallel. 
Test yourself 
A. Objective Questions 
1. Write true or false for each statement: 
(a) A fuse wire has a high melting point. 
Answer. False. 
(b) Flow of protons constitutes electric current. 
Answer. False. 
(c) Silver is an insulator of electricity. 
Answer. False. 
(d) S.I. unit and commercial unit of electrical energy are same. 
Answer. False. 
(e) Overloading of electric current in circuits can lead to short circuiting. 
Answer. True. 
(f) Our body can pass electricity through it. 
Answer. True. 
(g) All metals are insulators of electricity. 
Answer. False. 
(h) The earth wire protects us from an electric shock. 
Answer. True. 
(i) A switch should not be touched with wet hands. 
Answer. True. 
(j) AH electrical appliances in a household circuit work at the same voltage. 
Answer. True. 
(k) In a cable, the green wire is the live wire. 
Answer. False. 
(l) A fuse is connected to the live wire. 
Answer. True. 
(m) A switch is connected to the neutral wire. 
Answer. False. 
2. Fill in the blanks 
(a) The unit in which we pay the cost of electricity is kWh. 
(b) The electrical energy consumed in a house is measured by kWh meter. 
(c) In a household electrical circuit, the appliance are connected in parallel with the 
mains. 
(d) A switch is connected to the live wire. 
(e) The red colour insulated wire in a cable is the live wire. 
(f) One kilowatt hour is equal to 3.6 x 106 joule. 
(g) A fuse wire should have low melting point. 
3. Match the following 
 
 
 
4. Select the correct alternative 
Page 4


8. Electricity 
? ELECTRICITY: “Is the rate of flow of electrons”. i =  
? To keep electrons move, potential difference is needed. This is done by a cell or battery. 
? Potential difference “is the amount of work done in moving a unit positive charge from one 
point to other.” 
Potential difference V = Work done (W) / charge moved (Q) 
Or W = QV but charge = it 
Hence, W = VIt or Electrical energy = VIt 
? Power is “Rate of doing work” 
P = W/t = VIt/t = VI 
Power is measured in watt or J S
-1
 
1 Watt = 1 Volt × 1 Ampere 
? S.I. unit of charge is coulomb (C). 
S.I. unit of current is Ampere (A). 
S.I. unit of P.D. is volt. 
S.I. unit of electrical energy is Joule (J) and of power is watt (W) 
1 kWh-3600000J = 3.6 x 10
6
 J 
? ELECTRIC power is generated at the GENERATING STATION at 11000 volt, or 11 kV as 
these stations are at very far off place from areas where it is to be used. The voltage 
(A.C.) is of 50 HZ frequency. 
? AT GRID SUB-STATION this alternating current (A.C) voltage is stepped up from 11 kV to 
132 kV to minise the loss of energy in transmission line wires. 
? At MAIN-SUB-STATION this voltage is stepped down from 132 kV to 33 kV and 
transmitted to city SUB-STATION. 
? At CITY SUB STATION, it is further stepped down from 33 kV to 220 V for supply to 
hourses for consumers. 
? Colour coding: Live wire — Red or Brown 
Neutral—Black or light blue 
Earth wire — Green or yellow 
? 1 kWh = 1 unit: Power Rating on an appliance 100 W – 220 V means the appliance when 
worked on a 220 V will consume 100 W electricity power 
? OVER LOADING: is the condition of Electric circuit, when it draws more current than it is 
designed for i. e. when a number of appliances are switched on at a time i.e. geyser, A.C. 
Electric motor etc. or a large number of plugs are put in the same socket. 
? EARTHING: is done in a house near the kWh meter. Earthing is a safety device which 
puts the appliance at zero potential. 
? SHORT CIRCUITING: If the insulation on the wire of cable used f in the wiring (or used 
with an appliance) breaks. The LIVE WIRE 
COMES IN CONTACT WITH THE NEUTRAL WIRE, this result in SHORT CIRCUITING 
? FUSE: “Is a device used to limit the current in an electric circuit”. The use of fuse protects 
the appliance in circuit from being damaged Fuse is always connected in live wire. A fuse 
wire should have 
(i) High resistance 
(ii) Low melting point. 
These days miniature circuit breakers (MCB) are used. It is AUTOMATIC breaker, when 
current flowing excess. 
?  Appliances in a house are connected in parallel. 
Test yourself 
A. Objective Questions 
1. Write true or false for each statement: 
(a) A fuse wire has a high melting point. 
Answer. False. 
(b) Flow of protons constitutes electric current. 
Answer. False. 
(c) Silver is an insulator of electricity. 
Answer. False. 
(d) S.I. unit and commercial unit of electrical energy are same. 
Answer. False. 
(e) Overloading of electric current in circuits can lead to short circuiting. 
Answer. True. 
(f) Our body can pass electricity through it. 
Answer. True. 
(g) All metals are insulators of electricity. 
Answer. False. 
(h) The earth wire protects us from an electric shock. 
Answer. True. 
(i) A switch should not be touched with wet hands. 
Answer. True. 
(j) AH electrical appliances in a household circuit work at the same voltage. 
Answer. True. 
(k) In a cable, the green wire is the live wire. 
Answer. False. 
(l) A fuse is connected to the live wire. 
Answer. True. 
(m) A switch is connected to the neutral wire. 
Answer. False. 
2. Fill in the blanks 
(a) The unit in which we pay the cost of electricity is kWh. 
(b) The electrical energy consumed in a house is measured by kWh meter. 
(c) In a household electrical circuit, the appliance are connected in parallel with the 
mains. 
(d) A switch is connected to the live wire. 
(e) The red colour insulated wire in a cable is the live wire. 
(f) One kilowatt hour is equal to 3.6 x 106 joule. 
(g) A fuse wire should have low melting point. 
3. Match the following 
 
 
 
4. Select the correct alternative 
(a) All wires used in electric circuits should be covered with 
1.  colouring material 
2.  conducting material 
3.  an insulating material 
4.  none of the above 
(b) Electric work done per unit time is 
1.  electrical energy 
2.  electric current 
3.  electric voltage 
4.  electrical power 
(c) One kilowatt ¡s equal to 
1.  100 watt 
2.  1000 watt 
3.  10 watt 
4.  none of these 
(d) Fuse wire is an alloy of 
1.  tin-lead 
2.  copper-lead 
3.  tin-copper 
4.  lead-silver 
(e) A fuse wire should have 
1.  a low melting point 
2.  high melting point 
3.  very high melting point 
4.  none of the above 
(f) When switch of an electric appliance is put off, it disconnects 
1.  the live wire 
2.  the neutral wire 
3.  the earth wire 
4.  the live and the neutral wire 
(g) The purpose of an electric meter in a house is 
1.  to give the cost of electricity directly 
2.  to give the consumption of electrical energy 
Page 5


8. Electricity 
? ELECTRICITY: “Is the rate of flow of electrons”. i =  
? To keep electrons move, potential difference is needed. This is done by a cell or battery. 
? Potential difference “is the amount of work done in moving a unit positive charge from one 
point to other.” 
Potential difference V = Work done (W) / charge moved (Q) 
Or W = QV but charge = it 
Hence, W = VIt or Electrical energy = VIt 
? Power is “Rate of doing work” 
P = W/t = VIt/t = VI 
Power is measured in watt or J S
-1
 
1 Watt = 1 Volt × 1 Ampere 
? S.I. unit of charge is coulomb (C). 
S.I. unit of current is Ampere (A). 
S.I. unit of P.D. is volt. 
S.I. unit of electrical energy is Joule (J) and of power is watt (W) 
1 kWh-3600000J = 3.6 x 10
6
 J 
? ELECTRIC power is generated at the GENERATING STATION at 11000 volt, or 11 kV as 
these stations are at very far off place from areas where it is to be used. The voltage 
(A.C.) is of 50 HZ frequency. 
? AT GRID SUB-STATION this alternating current (A.C) voltage is stepped up from 11 kV to 
132 kV to minise the loss of energy in transmission line wires. 
? At MAIN-SUB-STATION this voltage is stepped down from 132 kV to 33 kV and 
transmitted to city SUB-STATION. 
? At CITY SUB STATION, it is further stepped down from 33 kV to 220 V for supply to 
hourses for consumers. 
? Colour coding: Live wire — Red or Brown 
Neutral—Black or light blue 
Earth wire — Green or yellow 
? 1 kWh = 1 unit: Power Rating on an appliance 100 W – 220 V means the appliance when 
worked on a 220 V will consume 100 W electricity power 
? OVER LOADING: is the condition of Electric circuit, when it draws more current than it is 
designed for i. e. when a number of appliances are switched on at a time i.e. geyser, A.C. 
Electric motor etc. or a large number of plugs are put in the same socket. 
? EARTHING: is done in a house near the kWh meter. Earthing is a safety device which 
puts the appliance at zero potential. 
? SHORT CIRCUITING: If the insulation on the wire of cable used f in the wiring (or used 
with an appliance) breaks. The LIVE WIRE 
COMES IN CONTACT WITH THE NEUTRAL WIRE, this result in SHORT CIRCUITING 
? FUSE: “Is a device used to limit the current in an electric circuit”. The use of fuse protects 
the appliance in circuit from being damaged Fuse is always connected in live wire. A fuse 
wire should have 
(i) High resistance 
(ii) Low melting point. 
These days miniature circuit breakers (MCB) are used. It is AUTOMATIC breaker, when 
current flowing excess. 
?  Appliances in a house are connected in parallel. 
Test yourself 
A. Objective Questions 
1. Write true or false for each statement: 
(a) A fuse wire has a high melting point. 
Answer. False. 
(b) Flow of protons constitutes electric current. 
Answer. False. 
(c) Silver is an insulator of electricity. 
Answer. False. 
(d) S.I. unit and commercial unit of electrical energy are same. 
Answer. False. 
(e) Overloading of electric current in circuits can lead to short circuiting. 
Answer. True. 
(f) Our body can pass electricity through it. 
Answer. True. 
(g) All metals are insulators of electricity. 
Answer. False. 
(h) The earth wire protects us from an electric shock. 
Answer. True. 
(i) A switch should not be touched with wet hands. 
Answer. True. 
(j) AH electrical appliances in a household circuit work at the same voltage. 
Answer. True. 
(k) In a cable, the green wire is the live wire. 
Answer. False. 
(l) A fuse is connected to the live wire. 
Answer. True. 
(m) A switch is connected to the neutral wire. 
Answer. False. 
2. Fill in the blanks 
(a) The unit in which we pay the cost of electricity is kWh. 
(b) The electrical energy consumed in a house is measured by kWh meter. 
(c) In a household electrical circuit, the appliance are connected in parallel with the 
mains. 
(d) A switch is connected to the live wire. 
(e) The red colour insulated wire in a cable is the live wire. 
(f) One kilowatt hour is equal to 3.6 x 106 joule. 
(g) A fuse wire should have low melting point. 
3. Match the following 
 
 
 
4. Select the correct alternative 
(a) All wires used in electric circuits should be covered with 
1.  colouring material 
2.  conducting material 
3.  an insulating material 
4.  none of the above 
(b) Electric work done per unit time is 
1.  electrical energy 
2.  electric current 
3.  electric voltage 
4.  electrical power 
(c) One kilowatt ¡s equal to 
1.  100 watt 
2.  1000 watt 
3.  10 watt 
4.  none of these 
(d) Fuse wire is an alloy of 
1.  tin-lead 
2.  copper-lead 
3.  tin-copper 
4.  lead-silver 
(e) A fuse wire should have 
1.  a low melting point 
2.  high melting point 
3.  very high melting point 
4.  none of the above 
(f) When switch of an electric appliance is put off, it disconnects 
1.  the live wire 
2.  the neutral wire 
3.  the earth wire 
4.  the live and the neutral wire 
(g) The purpose of an electric meter in a house is 
1.  to give the cost of electricity directly 
2.  to give the consumption of electrical energy 
3.  to safeguard the circuit from short circuiting 
4.  to put on or off the mains. 
(h) If out of the two lighted bulbs in a room, one bulb suddenly fuses, then 
1.  other bulb will glow more 
2.  other bulb will glow less 
3.  other bulb will also fuse 
4.  other bulb will remain lighted unaffected. 
B. Short/Long Answer Questions 
Question 1. 
From where does the electricity come to our home ? 
Answer: 
In our homes electricity comes from the CITY SUBSTATION. 
Question 2. 
What is an electric meter ? Where is it fixed in our house ? 
Answer: 
ELECTRIC METER : Is ENERGY meter or kWh meter and measures the amount of 
electric energy consumed in kWh for which electricity bill is paid. 
It is connected through a cable after company fuse on the front or outside wall of the 
house. 
Question 3. 
State the purpose of kWh meter. 
Answer: 
Purpose of kWh meter is to measure the electric energy consumed in kWh for which the 
electricity bill is to be paid. i.e. units of energy consumed. 
Question 4. 
For which unit do we pay our electricity bill ? 
Answer: 
UNIT — is boad of trade unit i. e. kWh = 1000 W used in 1 hour. 
Question 5. 
How can you check just by seeing the meter whether the electricity is in use or not ? 
Answer: 
If the electricity is in use, the counter fixed on the armature ofthe meter rotates and 
reads the number of rotations 
(A red mark is seen rotating).