Why is the parallel value one third of each resistor?

Each branch gives an extra path, so the reciprocals add to three times one value.

How is the voltage across a parallel part found?

It equals the total current times the equivalent resistance of that part.

Why are bills in kWh rather than joules?

kWh numbers stay small, so meters and bills are easy to read.

S4 Physics: Electricity and Magnetism Practice Paper 2

2026-11-01

Tutor Prep Guide · Secondary Physics

Paper two warms up with charge and Ohm law, moves to three resistors in parallel, series sharing and bills, then a mixed circuit and a kettle energy sum, ending with the power versus resistance mix-up.

Quick answer　20 C, 2 A, 9 ohms, 0.4 kWh, 2 ohms, 2 A and 12 V, 6 kWh and $7.2, 2 A and 4 V, 78.4 s, 46 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 lamp carries 0.8 A for 25 s. What charge passes through it?
FoundationA 6 V supply is applied across a 3 ohm resistor. Find the current.
FoundationA 4 ohm and a 5 ohm resistor are in series. Find the total resistance.
FoundationA 40 W lamp stays on for 10 hours. Find the energy used in kWh.
IntermediateThree 6 ohm resistors are in parallel. Find the equivalent resistance.
IntermediateA 4 ohm and a 6 ohm resistor in series are connected to 20 V. Find the current and the voltage across the 6 ohm resistor.
IntermediateAn appliance rated 2 000 W runs for 3 hours. At $1.2 per kWh, find the cost.
ChallengeA 6 ohm and a 3 ohm resistor in parallel are joined in series with 4 ohms across 12 V. Find the total current and the voltage across the parallel part.
ChallengeA 1 500 W kettle heats 0.4 kg of water from 30 C to 100 C. Find the time needed. (specific heat capacity of water 4 200 J/(kg K))
Error redoAn appliance runs on 230 V at 5 A. Find its resistance. A common mistake is answering 230 x 5; why is that wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	20 C (Q = 0.8 x 25)	Charge equals current multiplied by time.
2	2 A (6 / 3)	Use Ohm law, I = V / R.
3	9 ohms	Resistances in series add up directly.
4	0.4 kWh (40 x 10 / 1000)	Use kW and hours, then multiply to get kWh.
5	2 ohms (1/R = 3 x 1/6 = 1/2)	For n equal resistors in parallel the value is divided by n.
6	2 A; 12 V	Total resistance 10 ohms gives 2 A, then multiply by 6 ohms.
7	6 kWh, cost $7.2	First 2 kW x 3 h = 6 kWh, then multiply by the tariff.
8	2 ohms in parallel, 6 ohms total, 2 A, and 4 V across the parallel part	Reduce the parallel pair to 2 ohms, add 4 ohms, then use V = IR.
9	117 600 J, so 78.4 s	Compute E = mc dT first, then divide by power.
10	46 ohms (R = V / I); 230 x 5 gives 1 150 W, which is power not resistance	V x I gives power; resistance needs division.

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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