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GCSE Level Questions: Transformers

Q9. An electric toothbrush is charged by standing it on a separate charging base that is plugged into the mains electric supply.

The diagram shows the inside of the electric toothbrush and the charging base.

(a) An alternating potential difference (p.d.) across the coil in the charging base creates an alternating current in the coil inside the toothbrush. Explain how.

An alternating current flows through the primary coil (in the charging base). This causes a changing/ alternating magnetic field around the (iron) bar. The field around the bar then induces an (alternating) p.d. across the secondary coil (in the toothbrush).

[3 marks]

(b) When the toothbrush is being charged, the p.d. across the primary coil in the charging base is 230 V. The charging p.d. across the secondary coil in the toothbrush is 7.2 V. The primary coil in the charging base has 575 turns of wire on its coil.

Calculate the number of turns on the secondary coil inside the toothbrush.

Vp/Np = Vs/Ns

230/575 = 7.2/Ns

Ns = 7.2 x 575/230

Ns = 18

[2 marks]

(Total 5 marks)