Why does current split in inverse ratio to resistance?

Branches share the same voltage, so the smaller resistance takes more current.

Why divide by efficiency when it is below 100%?

The supplied energy must exceed what the water absorbs, since the rest leaks away.

Where is the current largest in a series circuit?

Nowhere, as one path means the same current everywhere.

S4 Physics: Electricity and Magnetism Practice Paper 4

2026-11-01

Tutor Prep Guide · Secondary Physics

Paper four drills charge, Ohm law, three in series and V^2 over R, then a 20 and 5 ohm pair, parallel current sharing and a monthly bill, before a mixed network and an 80% efficient kettle, ending with the power versus resistance mix-up.

Quick answer　10 C, 20 V, 12 ohms, 24 W, 4 ohms, 6 A and 4 A, 180 kWh and $198, 3 A and 1 A, 210 s, 4 ohms.

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: S4 Physics: Electricity and Magnetism – Tutor Lesson Plan
Level	S4

02　Questions (10 items)

FoundationA circuit carries 0.4 A for 25 s. Find the charge that passes.
FoundationA current of 2.5 A flows through an 8 ohm resistor. Find the voltage across it.
FoundationThree 4 ohm resistors are in series. Find the total resistance.
FoundationA 6 ohm resistor is connected to a 12 V supply. Find its power.
IntermediateA 20 ohm and a 5 ohm resistor are in parallel. Find the equivalent resistance.
IntermediateA 6 ohm and a 3 ohm resistor are in parallel across 12 V. Find the total current and the current in the 3 ohm.
IntermediateAn appliance rated 2 000 W runs 3 h a day for 30 days at $1.1 per kWh. Find the energy used and the cost.
ChallengeA 4 ohm resistor is in series with a parallel pair of 6 ohms and 3 ohms across 18 V. Find the total current and the current in the 6 ohm.
ChallengeA 3 000 W kettle of 80% efficiency heats 1.5 kg of water from 20 C to 100 C. Find the time needed. (c = 4 200 J/(kg K))
Error redoA resistor takes 3 A from a 12 V supply. Find its resistance. A common mistake gives 36 ohms; why is that wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	10 C (Q = I t = 0.4 x 25)	Current is charge per second, so multiply by the time.
2	20 V (V = I R = 2.5 x 8)	Ohm law gives voltage as current times resistance.
3	12 ohms (4 x 3)	A series circuit has one path, so resistances simply add.
4	24 W (P = V^2 / R = 144 / 6)	With voltage and resistance, use V^2 / R for power.
5	4 ohms (1/20 + 1/5 = 1/4)	Parallel branches add as reciprocals, so the result is below the smallest branch.
6	Equivalent 2 ohms, total 6 A, and 4 A through the 3 ohm	Branches share the same voltage, so current splits in inverse ratio to resistance.
7	180 kWh, so $198	Get kWh from kW and hours, then multiply by the days and the tariff.
8	Equivalent 6 ohms, total 3 A, 6 V across the pair, so 1 A through the 6 ohm	Combine the parallel pair first, then share voltage and current layer by layer.
9	504 000 J useful, 630 000 J supplied, so 210 s	Get the useful energy, divide by efficiency, then by the power.
10	4 ohms (12 / 3); 36 is the 36 W power from voltage times current	Resistance needs division; multiplying gives power instead.

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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Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.