The diagram below shows two long, vertical current-carrying wires, $P$ and $Q$, suspended in a vacuum.

Draw on this

The current in wire $P$ is directed vertically upwards, and the current in wire $Q$ is directed vertically downwards.

Explain why wire $Q$ experiences a force due to the current in wire $P$, and deduce the direction of this force.

State Faraday's law of electromagnetic induction.

Describe how a student can perform an experiment in the laboratory to show that the peak electromotive force (e.m.f.) $V_0$ induced in a small search coil placed at the centre of a long solenoid is directly proportional to the frequency $f$ of the alternating current in the solenoid, assuming the peak current in the solenoid is kept constant.