How do I tell step-up from step-down?

Compare turns: more secondary turns means step-up, fewer means step-down.

Why is power unchanged in an ideal transformer?

With no losses the input power equals the output power, so a voltage rise lowers the current.

S5 Physics Induction and Transformers, Paper 1: turns ratio and ideal power

2026-11-01

Tutor Prep Guide · Physics

Vs/Vp equals Ns/Np, and an ideal transformer keeps VpIp equal to VsIs, so a voltage rise comes with a current fall.

Focus　Work out the turns ratio first, then use equal power for currents.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 3 · Intermediate 4 · Challenge 2 · Error redo 1
Chapter	Chapter: S5 Physics: Induction and Transformers
Level	S4 to S5

02　Questions (10 items)

FoundationA transformer has 400 turns on the primary and 800 on the secondary, with 200 V across the primary. Find the secondary voltage.
FoundationA transformer has 1 000 primary turns and 100 secondary turns with 240 V input. Find the output voltage.
FoundationWhat condition is needed to induce a current in a coil?
IntermediateA transformer has 500 primary turns and 50 secondary turns with 220 V input. Find the output voltage.
IntermediateTo step 240 V down to 12 V with 60 secondary turns, how many primary turns are needed?
IntermediateAn ideal transformer takes 240 V at 0.5 A on the primary and gives 12 V. Find the secondary current.
IntermediateWhy can a transformer not change a steady direct voltage?
ChallengeA transformer has 250 primary turns at 20 V and 1 500 secondary turns carrying 0.2 A. Find the secondary voltage and the primary current.
ChallengeA transformer is 90% efficient with 200 W input and 36 V output. Find the output current.
Error redoCorrect the statement: “A transformer can raise the voltage and increase the power at the same time.”

03　Answers and one-line notes

#	Answer	One-line note
1	400 V	Vs/Vp = Ns/Np, so 200*(800/400) = 400 V.
2	24 V	This is a step-down transformer and the voltage falls by the turns ratio of ten.
3	The magnetic field through the coil must keep changing, or the coil and magnet must move relative to each other	Only a changing magnetic flux induces an e.m.f.; a steady field produces no current.
4	22 V	A turns ratio of ten to one drops the voltage tenfold.
5	1 200 turns	Np = NsVp/Vs = 60240/12 = 1200.
6	10 A	An ideal transformer keeps power constant: 240*0.5 = 120 W, and 120/12 = 10 A.
7	Direct current gives a steady field, so the flux never changes and no e.m.f. is induced	A transformer relies on changing flux from alternating current, so steady direct current gives no output.
8	120 V and 1.2 A	Vs = 201500/250 = 120 V, and VpIp = VsIs gives Ip = 1200.2/20 = 1.2 A.
9	5 A	Output power is 200*0.9 = 180 W, so 180/36 = 5 A.
10	It should read: the voltage rises while the current falls, so the power stays the same.	Energy conservation gives VpIp = VsIs, so a voltage rise comes with a current fall and never extra power.

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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