The primary coil of a step-down transformer has 500 turns, and the secondary coil has 25 turns. An alternating voltage supply is connected to the primary coil. A device that dissipates 36 W36\text{ W}36 W at its operating potential difference of 12 V12\text{ V}12 V is connected across the secondary coil. The transformer is 100%100\%100% efficient.
Calculate the current in the primary coil.
The alternating voltage supply is replaced by a 12 V12\text{ V}12 V DC battery and an open switch in series. The switch is closed. Explain why the device is only powered briefly and then remains off.
