Chapter 22 MCQs

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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1

A lamp has a potential difference of 12 V across it and a current of 0.50 A through it.

What is the electrical power transferred by the lamp?

A 0.042 W
B 6.0 W
C 12.5 W
D 24 W

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2

A heater has power 2.0 kW. It is used for 15 minutes.

How much energy is transferred?

A 0.030 kWh
B 0.50 kWh
C 7.5 kWh
D 30 kWh

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3

A 1.5 kW kettle is used for 4.0 minutes.

How much energy is transferred in joules?

A 360 J
B 6000 J
C 90 000 J
D 360 000 J

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4

A device transfers 18 000 J of energy in 2.0 minutes.

What is its power?

A 90 W
B 150 W
C 9000 W
D 36 000 W

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5

An electric motor has input power 500 W and useful output power 350 W.

What is its efficiency?

A 30%
B 70%
C 85%
D 143%

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6

A device is 40% efficient and has useful output power 120 W.

What is its input power?

A 48 W
B 80 W
C 300 W
D 480 W

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7

A 60 W lamp is connected to a 12 V supply.

What current flows through the lamp?

A 0.20 A
B 5.0 A
C 48 A
D 720 A

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8

A heater is connected to a 230 V supply and takes a current of 8.0 A.

What is its power?

A 28.8 W
B 238 W
C 1840 W
D 29 000 W

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9

A resistor of resistance 20 Ω has a current of 3.0 A through it.

What is the power transferred in the resistor?

A 15 W
B 60 W
C 180 W
D 1200 W

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10

A 10 Ω resistor is connected to a 20 V supply.

What is the power transferred in the resistor?

A 2.0 W
B 20 W
C 40 W
D 400 W

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For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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11

A component has resistance 5.0 Ω and transfers power at a rate of 45 W.

What is the current through the component?

A 3.0 A
B 9.0 A
C 40 A
D 225 A

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12

A resistor transfers power of 80 W when connected to a 40 V supply.

What is its resistance?

A 0.50 Ω
B 20 Ω
C 40 Ω
D 800 Ω

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13

A 12 V battery supplies a current of 2.5 A for 4.0 minutes.

How much energy is transferred by the battery?

A 30 J
B 120 J
C 7200 J
D 28 800 J

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14

An appliance transfers 0.75 kWh of energy. Electricity costs Rs 64 per kWh.

What is the cost of using the appliance?

A Rs 16
B Rs 48
C Rs 64
D Rs 85

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15

A 2.4 kW electric iron is used for 25 minutes. Electricity costs Rs 72 per kWh.

What is the cost?

A Rs 28.80
B Rs 43.20
C Rs 72.00
D Rs 172.80

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16

A 100 W lamp is used for 8.0 hours each day.

How much energy does it transfer in 5.0 days?

A 0.40 kWh
B 4.0 kWh
C 40 kWh
D 400 kWh

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17

A 1500 W heater is used for 2.0 hours. A 60 W lamp is used for 10 hours.

Which statement is correct?

A heater transfers less energy than the lamp
B heater transfers the same energy as the lamp
C heater transfers 5 times as much energy as the lamp
D heater transfers 50 times as much energy as the lamp

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18

A device has power rating 900 W when connected to 230 V.

What current does it normally take?

A 0.26 A
B 3.9 A
C 4.7 A
D 207 000 A

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19

A 230 V appliance has a normal operating current of 4.5 A.

Which fuse rating is most suitable?

A 1 A
B 3 A
C 5 A
D 13 A

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20

An appliance normally takes a current of 9.2 A.

Which fuse rating is most suitable?

A 3 A
B 5 A
C 10 A
D 30 A

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21

A fuse is placed in the live wire of an appliance.

Why?

A so it melts and disconnects the appliance if the current becomes too large
B so it makes the neutral wire dangerous
C so it increases the appliance power
D so it stops current flowing during normal operation

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22

Why should a switch be placed in the live wire rather than the neutral wire?

A switching the live wire disconnects the appliance from the high potential supply
B the neutral wire always has no current
C the live wire has no potential difference
D the switch works only with alternating current

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23

An appliance has a metal case. A fault causes the live wire to touch the case.

What is the purpose of the earth wire?

A to make the case positive
B to provide a low-resistance path for a large current so the fuse blows
C to reduce the resistance of the heating element
D to keep the appliance working normally during the fault

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24

A metal-cased appliance is connected without an earth wire. The live wire touches the case.

What is the main danger?

A the case may become live and give an electric shock
B the appliance becomes double insulated automatically
C the fuse always blows instantly even without current flow
D the neutral wire becomes disconnected from Earth

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25

An appliance is double insulated.

Which statement is correct?

A it must have an earth wire
B it has two layers of insulation and does not need an earth wire
C it has two fuses connected in parallel
D it works only on direct current

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For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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26

Which wire in a three-pin plug is normally at high potential relative to Earth?

A live
B neutral
C earth
D fuse wire

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27

Which wire is usually connected to the metal case of an appliance for safety?

A live
B neutral
C earth
D filament

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28

Which row correctly gives the usual wire colours in modern mains wiring?

	live	neutral	earth
A	brown	blue	green/yellow
B	blue	brown	green/yellow
C	green/yellow	brown	blue
D	brown	green/yellow	blue

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29

A student touches the live wire while standing barefoot on a wet floor.

Why is this dangerous?

A the body provides a path for current to Earth
B wet skin has infinite resistance
C current flows only if the neutral wire is touched too
D the live wire is always at 0 V

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30

Why are bathroom electrical fittings designed with extra care?

A water lowers body resistance and increases shock risk
B water makes the mains voltage become zero
C water prevents current from flowing
D water turns alternating current into direct current

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31

A circuit breaker is used instead of a fuse.

Which advantage can a circuit breaker have?

A it can be reset after it trips
B it melts permanently every time
C it increases the normal current
D it removes the need for insulation

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32

A residual current circuit breaker compares current in the live wire and neutral wire.

What does it detect?

A current leaking to Earth
B the colour of the wire insulation
C the resistance of the appliance only
D whether the appliance is hot

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33

The current in the live wire is 6.0 A and the current in the neutral wire is 5.7 A.

Why might a residual current device trip?

A 0.3 A may be leaking through another path, possibly to Earth
B the live wire has no current
C the neutral current is larger than live current
D the appliance is using no power

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34

Why is a fuse rating chosen slightly above the normal operating current?

A so it does not melt during normal operation but melts during excessive current
B so it always melts as soon as the appliance is switched on
C so it makes the appliance use less energy
D so it reduces the mains voltage

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35

An appliance has power 700 W and is connected to 230 V mains.

Which fuse should be chosen?

A 1 A
B 3 A
C 5 A
D 13 A

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36

A 230 V appliance is fitted with a 13 A fuse.

What is the maximum power it can take before the fuse is likely to blow?

A 17.7 W
B 230 W
C 2990 W
D 29 900 W

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37

A 2.0 kW heater is connected to a 230 V supply.

Which fuse is most suitable?

A 1 A
B 3 A
C 5 A
D 10 A

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38

A 230 V appliance has resistance 46 Ω when operating normally.

What fuse rating is most suitable?

A 1 A
B 3 A
C 5 A
D 13 A

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39

A short circuit occurs in an appliance.

What is meant by a short circuit?

A a low-resistance path allows a very large current to flow
B the appliance uses too little current
C current stops because resistance becomes infinite
D the live wire changes into the earth wire

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40

A fuse wire melts when current is too large because:

A electrical energy heats the wire by the current’s heating effect
B the fuse wire is cooled by the live wire
C the fuse wire loses all its charge
D the neutral wire pulls it apart magnetically

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For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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41

A mains supply is labelled 230 V, 50 Hz.

What does 50 Hz mean?

A current changes direction 50 times every minute
B current completes 50 cycles every second
C voltage is always exactly +50 V
D resistance of the supply is 50 Ω

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42

A mains alternating voltage has frequency 50 Hz.

What is its period?

A 0.0020 s
B 0.020 s
C 0.20 s
D 50 s

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43

Which statement about alternating current is correct?

A it flows in one direction only
B it repeatedly reverses direction
C it has zero energy transfer
D it can only flow through insulators

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44

Which statement about direct current is correct?

A it repeatedly reverses direction
B it flows in one direction only
C it exists only in mains sockets
D it has no charge movement

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45

A cell supplies direct current, but mains electricity is alternating current.

Which graph best represents mains potential difference against time?

A horizontal straight line above zero
B sine-like graph alternating positive and negative
C straight line through origin with constant positive gradient
D horizontal straight line at zero

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46

A 230 V mains supply is described as an r.m.s. value.

What does this mean for a heater connected to the supply?

A it has the same heating effect as 230 V direct current
B its peak voltage is exactly 230 V
C its voltage never changes
D it transfers no energy because it alternates

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47

A 60 W lamp and a 100 W lamp are both connected correctly to the same 230 V mains supply.

Which lamp has the smaller resistance when operating?

A 60 W lamp
B 100 W lamp
C both have equal resistance
D cannot be determined because voltage is the same

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48

A 100 W lamp is connected to a 230 V supply.

What is its operating resistance?

A 0.43 Ω
B 2.3 Ω
C 529 Ω
D 52 900 Ω

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49

A 60 W lamp is used for 30 minutes. Electricity costs Rs 80 per kWh.

What is the cost?

A Rs 0.24
B Rs 2.40
C Rs 24
D Rs 240

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50

A student says, “A fuse protects the user by stopping them from touching live parts.”

Which correction is best?

A a fuse protects mainly against excessive current; insulation and earthing help protect the user from live parts
B a fuse prevents all electric shocks in every situation
C a fuse only works in the neutral wire
D a fuse reduces the mains voltage to a safe value

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For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

Chapter 22 Answer Key

Q	Ans	Q	Ans	Q	Ans	Q	Ans	Q	Ans
1	B	11	A	21	A	31	A	41	B
2	B	12	B	22	A	32	A	42	B
3	D	13	C	23	B	33	A	43	B
4	B	14	B	24	A	34	A	44	B
5	B	15	C	25	B	35	C	45	B
6	C	16	B	26	A	36	C	46	A
7	B	17	C	27	C	37	D	47	B
8	C	18	B	28	A	38	C	48	C
9	C	19	C	29	A	39	A	49	B
10	C	20	C	30	A	40	A	50	A

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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Detailed Explanations

1. B

• Power = voltage × current

• P = VI

• P = 12 × 0.50

• P = 6.0 W

2. B

• Energy in kWh = power in kW × time in hours

• Time = 15 minutes = 15/60 h = 0.25 h

• Energy = 2.0 × 0.25

• Energy = 0.50 kWh

3. D

• Power = 1.5 kW = 1500 W

• Time = 4.0 minutes = 240 s

• Energy = power × time

• E = 1500 × 240

• E = 360 000 J

4. B

• Power = energy / time

• Time = 2.0 minutes = 120 s

• P = 18 000 / 120

• P = 150 W

5. B

• Efficiency = useful output power / input power × 100

• Efficiency = 350 / 500 × 100

• Efficiency = 70%

6. C

• Efficiency = useful output power / input power

• 0.40 = 120 / input power

• Input power = 120 / 0.40

• Input power = 300 W

7. B

• Power = voltage × current

• I = P / V

• I = 60 / 12

• I = 5.0 A

8. C

• Power = voltage × current

• P = 230 × 8.0

• P = 1840 W

9. C

• Power in a resistor:

  • P = I²R

• P = 3.0² × 20

• P = 9 × 20

• P = 180 W

10. C

• Power:

  • P = V² / R

• P = 20² / 10

• P = 400 / 10

• P = 40 W

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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11. A

• Power:

  • P = I²R

• 45 = I² × 5.0

• I² = 45 / 5.0 = 9

• I = 3.0 A

12. B

• Power:

  • P = V² / R

• Rearrange:

  • R = V² / P

• R = 40² / 80

• R = 1600 / 80

• R = 20 Ω

13. C

• Energy = voltage × current × time

• Time = 4.0 minutes = 240 s

• E = VIt

• E = 12 × 2.5 × 240

• E = 7200 J

14. B

• Cost = energy used × cost per kWh

• Cost = 0.75 × 64

• Cost = Rs 48

15. C

• Energy in kWh = power in kW × time in hours

• Time = 25 minutes = 25/60 h

• Energy = 2.4 × 25/60

• Energy = 1.0 kWh

• Cost = 1.0 × 72

• Cost = Rs 72.00

16. B

• Lamp power = 100 W = 0.100 kW

• Total time = 8.0 h/day × 5.0 days = 40 h

• Energy = 0.100 × 40

• Energy = 4.0 kWh

17. C

• Heater:

  • energy = 1.5 kW × 2.0 h = 3.0 kWh

• Lamp:

  • energy = 0.060 kW × 10 h = 0.60 kWh

• Ratio = 3.0 / 0.60 = 5

• Heater transfers 5 times as much energy.

18. B

• Current = power / voltage

• I = 900 / 230

• I = 3.91 A

• Closest answer = 3.9 A

19. C

• Normal current = 4.5 A

• Fuse should be slightly above normal operating current.

• 5 A is the most suitable.

• 3 A would blow during normal use.

• 13 A is unnecessarily high.

20. C

• Normal current = 9.2 A

• Fuse should be just above normal current.

• 10 A is most suitable.

• 5 A would blow during normal use.

• 30 A is too high for protection.

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21. A

• A fuse is placed in the live wire.

• If current becomes too large, the fuse wire melts.

• This breaks the circuit and disconnects the appliance from the live supply.

22. A

• The switch should be in the live wire.

• When switched off, the appliance is disconnected from the high-potential live supply.

• If the switch were in the neutral wire, parts of the appliance could still remain live.

23. B

• If the live wire touches the metal case, the case could become live.

• The earth wire provides a low-resistance path to Earth.

• A large current flows through the earth wire.

• This blows the fuse and disconnects the appliance.

24. A

• Without an earth wire, a live metal case may remain dangerous.

• A person touching the case could provide a path to Earth.

• This can cause an electric shock.

25. B

• A double-insulated appliance has two layers of insulation.

• The user cannot touch live metal parts.

• It does not need an earth wire.

• These appliances often have plastic cases.

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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26. A

• The live wire is normally at high potential relative to Earth.

• In many mains systems, it is around 230 V r.m.s. relative to Earth/neutral.

• This is why the live wire is dangerous.

27. C

• The earth wire is connected to the metal case of an appliance.

• It provides a safe path for fault current.

• This helps make the fuse blow if the case becomes live.

28. A

• Modern mains wire colours:

  • live = brown

  • neutral = blue

  • earth = green/yellow

29. A

• Wet skin has lower resistance than dry skin.

• Standing barefoot on a wet floor gives current a path through the body to Earth.

• This is extremely dangerous.

30. A

• Water lowers the body’s resistance.

• Lower resistance means larger current for the same voltage.

• This increases the risk of electric shock in bathrooms.

31. A

• A circuit breaker can trip when current is too large.

• Unlike a fuse, many circuit breakers can be reset.

• A fuse must usually be replaced after it melts.

32. A

• A residual current device compares current in live and neutral wires.

• If some current leaks to Earth, live and neutral currents are not equal.

• The device trips quickly.

33. A

• Live current = 6.0 A

• Neutral current = 5.7 A

• Difference = 0.3 A

• This suggests 0.3 A may be leaking through another path, possibly to Earth.

• An RCD may trip.

34. A

• Fuse rating should be slightly above normal operating current.

• It should not melt during normal use.

• It should melt when current becomes dangerously large.

35. C

• Current = power / voltage

• I = 700 / 230

• I ≈ 3.0 A

• A 3 A fuse is too close to the normal current.

• A 5 A fuse is most suitable.

36. C

• Maximum power before a 13 A fuse is likely to blow:

  • P = VI

  • P = 230 × 13

  • P = 2990 W

37. D

• Power = 2.0 kW = 2000 W

• Current = P / V

• I = 2000 / 230

• I ≈ 8.7 A

• The most suitable fuse is 10 A.

38. C

• Current = voltage / resistance

• I = 230 / 46

• I = 5.0 A

• The most suitable listed fuse is 5 A.

39. A

• A short circuit is a low-resistance path.

• It allows a very large current to flow.

• This can overheat wires and cause fire if protection fails.

40. A

• A fuse wire has resistance.

• When current is too large, electrical heating becomes large.

• The wire melts and breaks the circuit.

• Heating effect:

  • P = I²R

• Large current is the menace here.

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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41. B

• Frequency = number of cycles per second.

• 50 Hz means the alternating voltage/current completes 50 cycles every second.

42. B

• Period = 1 / frequency

• T = 1 / 50

• T = 0.020 s

43. B

• Alternating current repeatedly reverses direction.

• Mains electricity is alternating current.

• It can still transfer energy even though the direction changes.

44. B

• Direct current flows in one direction only.

• Cells and batteries usually provide direct current.

45. B

• Mains potential difference alternates.

• A graph of mains voltage against time is sine-like.

• It goes positive and negative repeatedly.

46. A

• 230 V r.m.s. means the alternating supply has the same heating effect as 230 V direct current.

• The peak voltage is higher than 230 V.

• This is a classic r.m.s. trap.

47. B

• For appliances on the same voltage:

  • P = V² / R

• Larger power means smaller resistance.

• Therefore the 100 W lamp has smaller operating resistance than the 60 W lamp.

48. C

• Power:

  • P = V² / R

• Rearrange:

  • R = V² / P

• R = 230² / 100

• R = 52 900 / 100

• R = 529 Ω

49. B

• Power = 60 W = 0.060 kW

• Time = 30 minutes = 0.50 h

• Energy = 0.060 × 0.50

• Energy = 0.030 kWh

• Cost = 0.030 × 80

• Cost = Rs 2.40

50. A

• A fuse mainly protects against excessive current.

• It melts and disconnects the circuit when current is too large.

• Insulation helps stop users touching live parts.

• Earthing helps protect users if a metal case becomes live.

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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Common Traps From This Chapter

Trap	Correct Rule
Electrical power	P = VI
Resistor power	P = I²R or P = V²/R
Energy in joules	E = Pt
Energy in kWh	kW × hours
Cost	kWh × cost per kWh
Efficiency	useful output / total input × 100
Fuse rating	slightly above normal current
Fuse wire	placed in live wire
Switch	placed in live wire
Earth wire	connected to metal case
Live fault to case	earth wire causes large current, fuse blows
Double insulation	no earth wire needed
Modern wire colours	live brown, neutral blue, earth green/yellow
Wet skin/floor	lower resistance, higher shock risk
Circuit breaker	can be reset
RCD/RCCB	detects live-neutral current difference
Short circuit	low resistance path, very large current
AC	repeatedly reverses direction
DC	one direction only
50 Hz	50 cycles per second
Period of 50 Hz	0.020 s
r.m.s. voltage	same heating effect as DC value
Higher power at same voltage	lower resistance
Fuse limitation	does not prevent all shocks