When a toothbrush is charging, p.d. across the primary coil is 230 V, p.d. across the secondary coil is 7.2 V. The primary coil in the charging base has 575 turns of wire on its coil. Find the number of turns on the secondary coil inside the toothbrush.

This question is on transformers. This is clear because there is a primary and secondary coil and a different voltage between the 2 coils. Transformers induce a changing voltage in a coil. The known equation for finding the number of coils is: primary voltage / secondary voltage = turns on primary / turns on secondary.

Therefore numbers can be subbed in: 230/7.2=575/turns on secondary. This is rearranged to get 18.