WASSCE Physics · Paper 1 style

Transformers

One sample question with the full working. 2 more in the app, with instant feedback and repair sessions for the traps below.

A transformer has 100 primary turns and 500 secondary turns. With 20 V a.c. input, the output is

A4 V
B20 V
C100 V
D500 V

Show the answer and working

Answer: C

100 V.

Trap A Transformer errors: Vs/Vp = Ns/Np. Step-up: Ns > Np. Works only with a.c. (changing flux). Ideal: VpIp = VsIs.

Trap B Takes part of the condition as the whole answer: Check whether the option is enough on its own. Equilibrium needs zero resultant force AND zero total moment; a complete explanation names every factor in the stem.

Trap D Uses the wrong arithmetic operation: Write the formula symbolically and substitute only at the end. Check units: if the units of your answer are wrong, so is the operation.

Traps in Transformers

Transformer errors
Vs/Vp = Ns/Np. Step-up: Ns > Np. Works only with a.c. (changing flux). Ideal: VpIp = VsIs.
Takes part of the condition as the whole answer
Check whether the option is enough on its own. Equilibrium needs zero resultant force AND zero total moment; a complete explanation names every factor in the stem.
Uses the wrong arithmetic operation
Write the formula symbolically and substitute only at the end. Check units: if the units of your answer are wrong, so is the operation.
Uses everyday intuition instead of physics
Name the law that applies (Newton's first law, Archimedes' principle, kinetic theory) and reason from it. If the option only 'feels' right, test it against the law.
Chooses a true but irrelevant statement
Re-read the question word (why, which, what causes). A true fact that answers a different question is still wrong.

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