These eleven questions cover ratings, energy in joules and kWh, fuses, switches, earthing and circuit faults from electrical energy and safe interpretation. All data, including the electricity price, is invented for practice and is not taken from any exam paper. They go from easier to harder.
Write your answer with its unit on paper first, then open the answer.
Use a supply of 230 V where a mains voltage is needed. Fuses available are 3 A, 5 A and 13 A. Check the Cambridge page for your exam year for exactly what your syllabus expects.
Questions
1. A lamp is marked 230 V, 46 W. Find the current it takes.
Show answer
I = P ÷ V = 46 ÷ 230 = 0.2 A.
2. A device draws 4.0 A from a 230 V supply. Find its power.
Show answer
P = I × V = 4.0 × 230 = 920 W.
3. A kettle is marked 230 V, 2300 W. How much energy does it transfer in 4 minutes?
Show answer
t = 4 × 60 = 240 s. E = 2300 × 240 = 552 000 J (552 kJ).
4. An appliance is marked 230 V, 1380 W. Which fuse from 3 A, 5 A and 13 A should be used? Show why.
Show answer
I = 1380 ÷ 230 = 6 A. The 3 A and 5 A fuses are below this and would melt in normal use. The smallest fuse above 6 A is 13 A.
5. A 3 kW heater runs for 90 minutes. Find the energy in kWh and the cost at RM0.40 per kWh (invented price).
Show answer
t = 90 ÷ 60 = 1.5 h. E = 3 × 1.5 = 4.5 kWh. Cost = 4.5 × 0.40 = RM1.80.
6. An energy meter records 7.2 MJ of electrical energy. Convert this to kWh.
Show answer
7.2 MJ = 7 200 000 J. 1 kWh = 3 600 000 J. 7 200 000 ÷ 3 600 000 = 2 kWh.
7. State one difference in purpose between a fuse and a switch, and say in which wire the fuse and the switch are placed in a mains appliance.
Show answer
A switch lets the user turn the appliance on and off. A fuse automatically breaks the circuit if the current becomes too large. Both are placed in the live wire, so that when either is open the appliance is cut off from the live supply.
8. A toaster has a metal case and a three-core cable. Inside, the live wire comes loose and touches the case. Explain why a user who then touches the case is protected.
Show answer
The earth wire connects the metal case to earth and has very low resistance. When the live wire touches the case, a large current flows through the case and earth wire to earth. This large current melts the fuse in the live wire. The appliance is then disconnected from the live supply, so the case is no longer live.
9. A 9 V battery is in series with a 3 Ω and a 6 Ω resistor. (a) Find the normal current and the potential difference across the 6 Ω resistor. (b) A wire is connected across the 6 Ω resistor. Find the new current and explain the change.
Show answer
(a) R = 3 + 6 = 9 Ω. I = 9 ÷ 9 = 1 A. Across the 6 Ω resistor: 1 × 6 = 6 V.
(b) The wire is a short circuit across the 6 Ω resistor, so only the 3 Ω remains. I = 9 ÷ 3 = 3 A. The total resistance has fallen from 9 Ω to 3 Ω and the supply voltage is unchanged, so the current rises. The potential difference across the 6 Ω resistor is now 0 V.
10. A phone charger supplies 5.0 V at 2.0 A for 30 minutes. Find the energy transferred in joules, and in kWh.
Show answer
P = 5.0 × 2.0 = 10 W. t = 30 × 60 = 1800 s. E = 10 × 1800 = 18 000 J. In kWh: 18 000 ÷ 3 600 000 = 0.005 kWh. Check the other way: 0.010 kW × 0.5 h = 0.005 kWh.
11. A student writes: “A 60 W lamp on for 10 hours uses 60 × 10 = 600 kWh.” Find the error and give the correct energy and the cost at RM0.40 per kWh (invented price).
Show answer
The student used watts with hours, so the number is 1000 times too big. Convert: 60 W = 0.06 kW. E = 0.06 × 10 = 0.6 kWh. Cost = 0.6 × 0.40 = RM0.24. A plausibility check would have flagged 600 kWh: a small lamp cannot use that much in one evening.
If you got these wrong
| What went wrong | Questions | Go back to |
|---|---|---|
| Chose the wrong formula, or the answer was not in joules | 1, 2, 3, 10 | Calculate electrical energy from stated ratings |
| Fuse value or fuse and switch purpose | 4, 7 | Compare a fuse purpose with a switch purpose |
| Earthing explanation missing a step | 8 | Interpret earthing on a supplied diagram |
| Fault readings or explanation | 9 | Explain a circuit fault without giving mains-work instructions |
| Units: kW, hours, kWh, joules | 5, 6, 11 | Check a unit conversion in an energy-use calculation |
Record the kind of slip in the mistake log and retest queue, then retry a similar question a few days later. The Physics learning guide shows where this module sits in the wider course.
Students who find the same slip returning often gain from online one-to-one Physics tuition, where a teacher can set new questions on the exact skill.