Why is a parallel equivalent always smaller than the smallest branch?

Every extra branch adds a path, so the total resistance only falls.

Is the current the same everywhere in a series circuit?

Yes, there is only one path, so each resistor takes the same current.

Why are bills in kWh rather than joules?

kWh keeps the numbers small and easy to read on a meter.

S4 Physics: Electricity and Magnetism Practice Paper 3

2026-11-01

Tutor Prep Guide · Secondary Physics

Paper three drills charge, Ohm law, series resistance and power, then parallel reciprocals, series sharing and bills, before a mixed network and a kettle energy sum, ending with the power versus resistance mix-up.

Quick answer　20 C, 2 A, 15 ohms, 24 W, 6 ohms, 2 A and 14 V, 3 kWh and $3.6, 3 A and 12 V, 67.2 s, 12 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 0.5 A for 40 s. Find the charge that passes.
FoundationA 9 V supply drives a 4.5 ohm resistor. Find the current.
FoundationTwo resistors of 7 ohms and 8 ohms are in series. Find the total resistance.
FoundationA bulb runs on 12 V at 2 A. Find its power.
IntermediateA 10 ohm and a 15 ohm resistor are in parallel. Find the equivalent resistance.
IntermediateA 24 V supply is connected across 5 ohms and 7 ohms in series. Find the voltage across the 7 ohm resistor.
IntermediateAn appliance rated 1 200 W runs 2.5 h a day at $1.2 per kWh. Find the daily cost.
ChallengeTwo 8 ohm resistors in parallel are joined in series with 2 ohms across 18 V. Find the total current and the voltage across the parallel part.
ChallengeA 2 500 W kettle heats 0.5 kg of water from 20 C to 100 C. Find the time needed. (c = 4 200 J/(kg K))
Error redoA bulb runs on 6 V at 0.5 A. Find its resistance. A common mistake gives 3; why is that wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	20 C (Q = I t = 0.5 x 40)	Current is charge per second, so multiply by time.
2	2 A (I = V / R)	Resistance is the divisor in Ohm law.
3	15 ohms (add them in series)	A series circuit has one path, so resistances add.
4	24 W (P = V I)	Power is voltage times current.
5	6 ohms (1/10 + 1/15 = 1/6)	Parallel values add as reciprocals, so the result is below the smallest.
6	Total 12 ohms, current 2 A, so 14 V across the 7 ohm	Find the total resistance, get the current, then multiply by that resistor.
7	3 kWh used, so $3.6	Get kWh from watts and hours, then multiply by the tariff.
8	Equivalent 6 ohms, total current 3 A, 12 V across the parallel part	Parallel first: two 8 ohm give 4 ohms, plus 2 ohms makes 6.
9	168 000 J, so 67.2 s	Energy divided by power gives the time; the rise is 80 K.
10	12 ohms (6 / 0.5); 6 x 0.5 = 3 W is the power	Multiplying 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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