(a) Define electromotive force.

(b) State:

(i) the principle of operation of a potentiometer,

(ii) two advantages that a potentiometer has over a voltmeter in measuring potential difference.

(c)(i) Sketch and label a diagram of a gold-leaf electroscope.

(ii) Give one use of a gold-leaf electroscope.

(d)(i) Explain the action of a magnetic relay.

(ii) List two factors which determine the magnitude of an induced emf in a coil.

(iii) A current of 5 A passes through a straight wire in a uniform magnetic field of flux density 2.0 x10\(^{-3}\) T. Calculate the force per unit length exerted on the wire when it is inclined at 30° to the field.