Does a transformer amplify power?

No, an ideal transformer keeps power constant, so a step-up in voltage brings a matching fall in current.

Why transmit at high voltage?

For the same power a higher voltage means a smaller current, and the I squared R loss falls with the square of the current.

S5 Physics Induction and Transformers, Paper 3: Faraday law and transmission

2026-11-02

Tutor Prep Guide · Primary English

Covers transformer turns and current, induced e.m.f. from flux change, transmission current and I squared R loss, and efficiency in ten graded items.

Focus　Decide step-up or step-down first, then use the turns ratio; in transmission questions always find the current from P equals VI 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 and 50 secondary turns with 220 V input. Find the output voltage.
FoundationA transformer has 200 primary turns and 1000 secondary turns with 12 V input. Find the output voltage.
FoundationAn ideal transformer takes 220 V at 0.5 A and gives 11 V. Find the output current.
FoundationThe flux through a 500 turn coil falls from 0.004 Wb to zero in 0.02 s. Find the induced e.m.f.
IntermediateA power station sends 100 kW. Find the current at 11 kV and at 1 kV.
IntermediateA transmission line has a total resistance of 2 ohm. Find the loss at 100 A and at 9.1 A.
IntermediateA transformer is 90 per cent efficient with 500 W input and 22 V output. Find the output power and current.
ChallengeThe flux through a 200 turn coil rises from 0.015 Wb to 0.055 Wb in 0.1 s. Find the average induced e.m.f.
ChallengeThe same 100 kW is sent through a 2 ohm line at 5 kV and at 50 kV. Find both losses and the ratio.
Error redoA student thinks a step-up transformer multiplies power and claims 22000 W from a 220 V, 10 A input. What is the correct secondary power?

03　Answers and one-line notes

#	Answer	One-line note
1	11 V	The turns ratio sets the voltage ratio, so 220 times 50 over 1000 gives 11 V.
2	60 V	This is a step-up transformer, so 12 times 1000 over 200 gives 60 V.
3	10 A	An ideal transformer keeps power constant, so 110 watts at 11 volts gives 10 A.
4	100 V	Faraday law gives 500 times 0.004 over 0.02, that is 100 V.
5	9.1 A and 100 A	Current is power over voltage, giving about 9.1 A at 11 kV and 100 A at 1 kV.
6	20000 W and about 165 W	Loss is I squared R, so 100 squared times 2 is 20000 watts, while 9.1 squared times 2 is only about 165 watts.
7	450 W and 20.5 A	Output power is 450 watts, so the current is 450 over 22, about 20.5 A.
8	80 V	The flux changes by 0.040 webers, so 200 times 0.04 over 0.1 gives 80 V.
9	800 W and 8 W, a factor of 100	At 5 kV the current is 20 A and the loss 800 watts; at 50 kV the current is 2 A and the loss 8 watts, a hundredfold saving.
10	2200 W	An ideal transformer changes voltage and current but not power, so the secondary power is still 2200 watts.

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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