Why does a power rating only mean something at the rated voltage?

Power depends on voltage, so 2 200 W holds only at 220 V.

Why is a fuse fitted in series?

The whole current passes through it, so it can melt and break the circuit when that current is too large.

Which step most often goes wrong in energy calculations?

The time unit: minutes must be divided by 60 to become hours.

S5 Physics: Circuits Practice Paper 3

2026-11-01

Tutor Prep Guide · Secondary Physics

Paper three starts with series and parallel equivalents, moves to power and energy, then voltage division, branch currents and appliance ratings, then a parallel-then-series network and a fuse rating check, ending with the kWh and minutes mix-up.

Quick answer　12 ohm gives 1 A and 3 V, 6 ohm gives 2 A, 48 W and 14 400 J, 10 V gives 0.5 A and 5 W, 40 ohm gives 0.6 A with 6 V and 18 V, 3 ohm gives 2 A with 0.5 A and 1.5 A, 2 200 W gives 10 A and 22 ohm, 12 ohm gives 2 A with 0.67 A and 1.33 A, 12.7 A blows the fuse, and the answer is 1 kWh not 60 kWh.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10 questions
Suggested time	25 minutes
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S5 Physics: Circuits – Tutor Lesson Plan
Level	S5

02　Questions (10 items)

FoundationA 12 V supply is connected to 2 ohm, 3 ohm and 7 ohm resistors in series. Find the total resistance, the current and the voltage across the 3 ohm resistor.
FoundationResistors of 8 ohm and 24 ohm are in parallel across a 12 V supply. Find the equivalent resistance and the total current.
FoundationA 3 ohm resistor carries 4 A. Find the power it dissipates and the energy used in 5 minutes in joules.
FoundationThe voltage across a 20 ohm resistor is 10 V. Find the current through it and the power it uses.
IntermediateA 24 V supply feeds 10 ohm and 30 ohm in series. Find the current and the voltage across each resistor.
IntermediateA 6 V supply is connected across a 12 ohm and a 4 ohm resistor in parallel. Find the equivalent resistance, the total current and both branch currents.
IntermediateAn electric kettle is rated 2 200 W at 220 V. Find the current and the resistance when it is working.
ChallengeA 24 V supply feeds 8 ohm in series with a 12 ohm and 6 ohm parallel pair. Find the equivalent resistance, the total current, the parallel-section voltage and both branch currents.
ChallengeA 220 V socket is protected by a 5 A fuse. A 2 000 W heater and an 800 W kettle are switched on together. Find the total current and say whether the fuse blows.
Error redoA 2 kW appliance runs for 30 minutes. A student answers “energy used = 2 x 30 = 60 kWh”. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	Total resistance 12 ohm, current 1 A, and 3 V across the 3 ohm resistor.	Series resistances add; the same current flows through each, and the three drops must total 12 V.
2	Equivalent resistance 6 ohm, total current 2 A.	Add reciprocals for parallel parts; here 24 is the common denominator.
3	Power 48 W and energy 14 400 J.	Use P = I²R for power, then multiply by time in seconds for energy.
4	Current 0.5 A and power 5 W.	Get the current from I = V/R, then the power from P = VI; write both steps.
5	Current 0.6 A, with 6 V across the 10 ohm and 18 V across the 30 ohm.	A series circuit divides voltage in the same ratio as the resistances.
6	Equivalent resistance 3 ohm, total current 2 A, branch currents 0.5 A and 1.5 A.	Branches share the same voltage; get each current from V/R and check they add to the total.
7	Current 10 A and resistance 22 ohm.	Find I from P/V, then R from V/I or from P = V squared over R.
8	Equivalent resistance 12 ohm, total current 2 A, 8 V across the parallel section and branch currents of about 0.67 A and 1.33 A.	The pair simplifies to 4 ohm, plus 8 ohm gives 12 ohm; then work back for voltages and currents.
9	Total current about 12.7 A, well above 5 A, so the fuse blows.	At 220 V a 5 A fuse allows only 1 100 W, and the two appliances together need 2 800 W.
10	The energy used is 1 kWh, which is 3.6 x 10 to the sixth joules.	Convert the time to hours first: 30 minutes is 0.5 h, and 1 kWh is 3.6 x 10 to the sixth joules.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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