(a) State Faraday's law of electromagnetic induction. (b) Draw a labelled diagram of an induction coil and explain how it works. (c) How is the effect of ed...

(a) Faraday's law of electromagnetic induction

Whenever the magnetic flux linked with a circuit changes, an e.m.f. is induced in the circuit. The magnitude of the induced e.m.f. is proportional to the rate of change of magnetic flux linkage:

\[|E|=N\left|\frac{d\Phi}{dt}\right|\]

where \(E\) is the induced e.m.f., \(N\) is the number of turns and \(\Phi\) is the magnetic flux through each turn.

(b) Labelled diagram of an induction coil

How it works: When key \(K\) is closed and the platinum contacts touch, current flows through the primary coil. The soft-iron core becomes magnetised and attracts the soft-iron armature. This separates the contacts and breaks the primary circuit. The core then loses its magnetism, so the spring returns the armature and remakes the contact. Thus, the primary current is made and broken rapidly.

At each break, the magnetic flux in the core collapses rapidly and induces an e.m.f. in the secondary coil. Since the secondary has very many turns, a very large e.m.f. is induced, sufficient to produce a spark across the spark gap.

(c) Eddy currents are minimised by making the core from a bundle of thin insulated soft-iron wires, or by laminating the soft-iron core.

(d) Reasons for connecting a capacitor across the contact points

1. It reduces sparking at the platinum contacts and therefore prevents the contacts from burning away.
2. It makes the break in the primary current more rapid, causing a faster collapse of flux and hence a larger induced e.m.f. in the secondary.

(e) Uses of an induction coil

1. In the ignition system of petrol motor vehicles to produce sparks at spark plugs.
2. For operating X-ray tubes.
3. For studying electric discharge through gases, such as in discharge tubes.