Why does a real transformer lose power?

Coil resistance, eddy currents and flux leakage all waste energy, so output is below input and efficiency is output over input.

Why does high voltage cut cable loss?

Ten times the voltage means a tenth of the current, and the I squared R loss falls by a hundred.

S5 Physics Induction and Transformers, Paper 4: efficiency and high voltage transmission

2026-11-02

Tutor Prep Guide · Primary English

Works turns ratios, ideal and real transformer currents, Faraday average e.m.f., transmission loss comparisons and generator commutation in ten graded items.

Focus　Decide step-up or step-down, then use power conservation including efficiency; in transmission always find the current before the I squared R loss.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S5 Physics: Electromagnetic Induction and Transformers
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationA transformer has 1000 primary turns at 220 V and 200 secondary turns. Find the secondary voltage.
FoundationAn ideal transformer delivers 12 V at 2 A from a 240 V supply. Find the primary current.
FoundationThe flux through a 100 turn coil falls steadily by 0.02 Wb in 0.5 s. Find the induced e.m.f.
FoundationAn a.c. generator uses slip rings and a d.c. generator uses a commutator. How do the outputs differ?
IntermediateA transformer steps 240 V up to 6000 V with 400 primary turns. How many secondary turns are needed?
IntermediateA 100 kW supply is transmitted at 400 V through cables of 0.4 ohm. Find the power lost in the cables.
IntermediateA transformer takes 500 W and delivers 10 A at 46 V. Find its efficiency.
Challenge20 MW is sent through 2 ohm cables at 20 kV and again at 200 kV. By what factor does the loss differ?
ChallengeThe flux through 200 turns falls from 0.05 Wb to 0.01 Wb in 0.02 s. Find the average induced e.m.f.
Error redoA student forgets efficiency and answers 10 A for a 600 W input at 90 per cent efficiency with a 60 V secondary. Find the correct current.

03　Answers and one-line notes

#	Answer	One-line note
1	44 V	The voltage ratio matches the turns ratio, so 220 times 200 over 1000 gives 44 V, a step-down.
2	0.1 A	Secondary power is 24 W, and an ideal transformer keeps power constant, so the primary current is 24 over 240.
3	4 V	Faraday law gives turns times flux change over time, that is 100 times 0.02 over 0.5, or 4 V.
4	Slip rings give alternating current, the commutator reverses every half turn to give direct current	Slip rings keep the same external connection so the current alternates, while a commutator swaps contacts every half turn to give direct current.
5	10000 turns	Secondary turns are 400 times 6000 over 240, that is 10000 turns.
6	25000 W, that is 25 kW	Current is 250 A, so the loss is 250 squared times 0.4, that is 25000 W.
7	92 per cent	Output power is 460 W, so the efficiency is 460 over 500, that is 92 per cent.
8	A factor of 100, 2 MW against 20 kW	Ten times the voltage means a tenth of the current and one hundredth of the I squared R loss.
9	400 V	Flux falls by 0.04 Wb, so 200 times 0.04 over 0.02 gives 400 V.
10	9 A	Only 90 per cent of the input reaches the secondary, so 540 W at 60 V gives 9 A.

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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