Practical ElectricityCopy
1. What is the primary hazard of using damaged insulation in household wiring?
A) Reduced resistance
B) Fire due to overheating
C) Magnetic field distortion
D) Reduced brightness of lamps
2. Which safety device melts when current exceeds safe levels?
A) Circuit breaker
B) Rheostat
C) Switch
D) Fuse
3. Which is the correct formula to calculate power in an electrical appliance?
A) P=EtP = frac{E}{t}
B) P=VIP = frac{V}{I}
C) P=VIP = VI
D) P=IR2P = IR^2
4. A 230 V, 3 kW appliance draws how much current?
A) 13 A
B) 15 A
C) 10 A
D) 20 A
5. What is the most appropriate fuse for a 700 W appliance on a 230 V supply?
A) 5 A
B) 10 A
C) 13 A
D) 3 A
6. What is the purpose of double insulation in an appliance?
A) Increases energy efficiency
B) Reduces circuit resistance
C) Eliminates need for earthing
D) Prevents light leakage
7. A heater rated 2 kW operates for 3 hours. How much energy is used?
A) 6 kWh
B) 600 J
C) 360 J
D) 2 kJ
8. The earth wire in a plug is connected to:
A) Neutral terminal
B) Live pin
C) Outer metal casing
D) Fuse
9. A current of 0.5 A flows for 1 hour. Total charge transferred is:
A) 0.5 C
B) 1800 C
C) 30 C
D) 3600 C
10. What is the resistance of dry human skin estimated to be?
A) 1 Ω
B) 10 Ω
C) 1,000 Ω
D) 10,000 Ω
11. Which condition most increases risk of electric shock?
A) Wearing rubber gloves
B) Dry hands
C) Barefoot on dry wood
D) Wet hands
12. Which of these is not part of a three-pin plug?
A) Earth pin
B) Neutral wire
C) Live wire
D) Battery
13. Which situation may lead to a short circuit?
A) Using an LDR
B) Touching two different wires together
C) Opening a switch
D) Using thick insulation
14. A 1500 W microwave connected to 230 V should use which fuse?
A) 5 A
B) 13 A
C) 10 A
D) 3 A
15. A child touches a 240 V wire with resistance of 8000 Ω. Current through child is:
A) 0.03 A
B) 0.24 A
C) 0.03 mA
D) 3 A
16. Which appliance is most likely to use a 3 A fuse?
A) Washing machine
B) Electric oven
C) Table lamp
D) Iron
17. Which one of these does not help prevent electrical fires?
A) Earth wire
B) Correct fuse
C) Trip switch
D) Bulb filament
18. In a ring main circuit, current reaches the sockets:
A) From only one side
B) From the live wire only
C) From both directions
D) Through the earth
19. Which material is commonly used in fuse wires?
A) Nichrome
B) Iron
C) Tinned copper
D) Graphite
20. How does a fuse operate?
A) Melts to complete the circuit
B) Closes when current is low
C) Heats up and melts to break circuit
D) Creates magnetic field to deflect current
21. A 13 A fuse is needed if current exceeds:
A) 720 W
B) 500 W
C) 100 W
D) 13 kW
22. What type of lamp emits light via fluorescent coating?
A) LED
B) Filament
C) Compact fluorescent
D) Halogen
23. Which wire in a three-pin plug is longer than others?
A) Live
B) Neutral
C) Earth
D) All same length
24. What type of appliance can function safely without earth wire?
A) Metal toaster
B) Double insulated vacuum
C) Electric oven
D) Kettle
25. Which appliance rating would be too high for a 3 A fuse?
A) 60 W
B) 120 W
C) 200 W
D) 1000 W
26. Why are circuit breakers often preferred over fuses?
A) They cannot break the circuit
B) They trip instantly and can be reset
C) They reduce resistance
D) They are disposable
27. How is overheating in extension leads best prevented?
A) Using more sockets
B) Keeping wires close
C) Avoiding excess current draw
D) Coiling wires
28. Which unit is used for electricity billing?
A) Watt
B) Ampere
C) Joule
D) Kilowatt-hour
29. Which situation might cause cables to overheat?
A) Using low power devices
B) Underloading sockets
C) Using thick copper cables
D) Drawing excess current
30. What does a red 3 A fuse protect?
A) Motors above 3 kW
B) Appliances above 1000 W
C) Appliances up to 720 W
D) Appliances below 200 W only
31. Why should fuses always be placed in the live wire?
A) So voltage stays low
B) So current flows through neutral
C) So appliance is completely isolated when blown
D) It doesn’t matter where it is
32. A device uses 1000 W for 2 hours. Energy consumed =
A) 1000 J
B) 2 kWh
C) 500 W
D) 7200 J
33. Which device converts sound into an electrical signal?
A) Speaker
B) Thermistor
C) Microphone
D) Amplifier
34. A consumer unit contains:
A) Switch, earth pin, fuse
B) Circuit breaker, isolator, meter
C) Motor, heater, processor
D) Relay, LDR, capacitor
35. Which of these is most lethal when touched in damp conditions?
A) Neutral wire
B) Earth pin
C) Live wire
D) Metal casing
1. ✅ B) Fire due to overheating
Damaged insulation can cause leakage currents → overheating → fire.
❌ A/D – Not direct outcomes
❌ C – Not significant here
2. ✅ D) Fuse
Fuses melt when current exceeds safe limit to protect the circuit.
❌ A – Circuit breaker trips, doesn’t melt
❌ B/C – Not safety devices
3. ✅ C) P=VIP = VI
Power = voltage × current
❌ A – Energy/time = power, but less direct
❌ B – V/I = resistance
❌ D – Incorrect
4. ✅ A) 13 A
P = VI → I = 3000 / 230 ≈ 13 A
❌ Others – Off by calculation
5. ✅ D) 3 A
700/230 ≈ 3 A → 3 A fuse is suitable
❌ Others – Would not blow in case of fault
6. ✅ C) Eliminates need for earthing
Double insulation = safety without earth wire
❌ A/B/D – Unrelated
7. ✅ A) 6 kWh
2 kW × 3 h = 6 kWh
❌ Others – Incorrect unit
8. ✅ C) Outer metal casing
Earth wire is for safety – connects to casing
❌ A/B/D – Not correct
9. ✅ B) 1800 C
Q = It = 0.5 A × 3600 s = 1800 C
❌ Others – Incorrect math
10. ✅ D) 10,000 Ω
Dry skin has very high resistance
❌ A–C – Too low
11. ✅ D) Wet hands
Water reduces resistance → more dangerous
❌ A–C – Provide some protection
12. ✅ D) Battery
A plug doesn’t contain a battery
❌ A–C – All essential plug components
13. ✅ B) Touching two different wires together
This bypasses resistance → short circuit
❌ A/C/D – Not causes
14. ✅ B) 13 A
1500/230 ≈ 6.5 A → Use 13 A for surge protection
❌ Others – May blow under load
15. ✅ B) 0.03 A
I = V/R = 240/8000 = 0.03 A
❌ Others – Orders of magnitude off
16. ✅ C) Table lamp
Low power → low current
❌ A/B/D – Need higher fuses
17. ✅ D) Bulb filament
Not a protective device
❌ A–C – All protect circuits
18. ✅ C) From both directions
Ring main = double path
❌ A/B/D – Incorrect flow paths
19. ✅ C) Tinned copper
Low melting point, conducts well
❌ A/B/D – Too resistive or unsuitable
20. ✅ C) Heats up and melts to break circuit
That’s how fuses interrupt current
❌ Others – Incorrect functions
21. ✅ D) 13 kW
13 A × 230 V = ~3000 W = 3 kW
Only D exceeds this
22. ✅ C) Compact fluorescent
Uses coating to emit visible light
❌ A/B/D – No fluorescent coating
23. ✅ C) Earth
Longest pin – makes contact first
❌ A/B/D – Equal or shorter
24. ✅ B) Double insulated vacuum
Plastic body → no need for earth
❌ A/C/D – Have metal casing
25. ✅ D) 1000 W
1000/230 ≈ 4.3 A → too high for 3 A
❌ A–C – Within limit
26. ✅ B) They trip instantly and can be reset
Major advantage of circuit breakers
❌ Others – Not true
27. ✅ C) Avoiding excess current draw
Overloading = overheating
❌ A/B/D – Worsen the issue
28. ✅ D) Kilowatt-hour
Unit of energy for billing
❌ Others – Wrong units
29. ✅ D) Drawing excess current
Too much current → heat
❌ Others – Reduce load
30. ✅ C) Appliances up to 720 W
3 A × 240 V = 720 W
❌ A/B/D – Exceed this limit
31. ✅ C) So appliance is completely isolated when blown
Live disconnection = full safety
❌ Others – Dangerous or wrong
32. ✅ B) 2 kWh
1 kW × 2 h = 2 kWh
❌ Others – Wrong units
33. ✅ C) Microphone
Input device converting sound → electricity
❌ A/B/D – Output or unrelated
34. ✅ B) Circuit breaker, isolator, meter
Standard parts of consumer unit
❌ Others – Not relevant
35. ✅ C) Live wire
Live carries full voltage – very dangerous
❌ A/B/D – Not main danger