Why does high-voltage transmission reduce loss?

At the same power a higher voltage means a lower current, and the loss goes as I squared R.

Where do transformer losses go?

Into heat in the coil resistance and into repeated magnetising of the iron core.

S5 Physics Induction and Transformers, Paper 2: efficiency, transmission loss and induced current direction

2026-11-02

An ideal transformer keeps power constant, and raising the transmission voltage by n cuts the I squared R loss by n squared.

Focus　Get the power from VI first, then use I squared R for line loss and multiply efficiency into the output.

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 200 primary turns and 1 000 secondary turns with 12 V input. Find the output voltage.
FoundationA transformer has 800 primary turns and 40 secondary turns with 200 V input. Find the output voltage.
FoundationWhich law gives the direction of an induced current?
IntermediateAn ideal transformer takes 60 V at 0.4 A on the primary and gives 240 V. Find the secondary current.
IntermediateA transmission line carries 20 A through a total resistance of 5 ohm. Find the power loss.
IntermediateA transformer is 80 percent efficient with 500 W input. Find the output power.
IntermediateA transformer has 100 secondary turns giving 25 V and 400 primary turns. Find the primary voltage.
ChallengeThe transmission voltage is raised from 11 kV to 220 kV at the same power. By what factor does the line loss change?
ChallengeA 95 percent efficient transformer has 2 000 W input and 190 V output. Find the output current.
Error redoCorrect the claim: a transformer changes 50 Hz alternating current into 100 Hz.

03　Answers and one-line notes

#	Answer	One-line note
1	60 V	Vs = 12*(1000/200) = 60 V, so this is a step-up transformer.
2	10 V	A turns ratio of twenty to one drops the voltage twentyfold to 10 V.
3	Lenz law	The induced current always produces a field that opposes the change in flux.
4	0.1 A	Power is constant: 60*0.4 = 24 W, so 24/240 = 0.1 A.
5	2 000 W	P = I squared times R, so 20205 = 2 000 W.
6	400 W	500*0.8 = 400 W, and the other 100 W becomes heat.
7	100 V	Vp = 25*(400/100) = 100 V.
8	One four-hundredth	A twentyfold voltage rise cuts the current twentyfold, so the I squared R loss falls 400 times.
9	10 A	Output is 2000*0.95 = 1 900 W, so 1900/190 = 10 A.
10	It should read: the frequency is unchanged; a transformer only changes voltage and current.	The flux in the core reverses at the supply frequency, so the output frequency is unchanged.

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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