Introduction

What this section does

• Oersted, Ampere and others had shown that a moving charge produces a magnetic field. This section asks the converse question — can a moving magnet produce an electric current? — and answers it with Faraday's experiments of 1831.
• It is a descriptive section. What is examinable from it is the set of experimental observations, and the single conclusion they lead to: a changing magnetic flux through a coil produces an induced emf.
• Joseph Henry (1799-1878) demonstrated in the same year that currents are indeed produced in closed circuits subjected to time-varying magnetic fields.

Faraday's experiments with a magnet and a coil

• A coil is connected to a galvanometer G to form a closed circuit, and a bar magnet is moved near it.
• When the magnet approaches the coil, a current — the induced current — is produced in it.
• When the magnet is taken away, a current is again produced, but in the opposite direction.
• If the coil, instead of the magnet, is moved towards or away from the magnet, an induced current is again produced. Only relative motion matters.
• If the polarity of the approaching or receding magnet is reversed, the direction of the induced current reverses too.
• The magnitude of the induced current depends on the relative speed of coil and magnet, and on the number of turns of the coil.
• The induced current exists only so long as there is relative motion between coil and magnet.

Faraday's experiments with two coils

• Two coils are placed with their planes facing each other. The primary coil carries a battery, rheostat and key; the ends of the secondary coil go to a galvanometer G.
• Closing or breaking the primary circuit gives a momentary deflection in G at the instant of make or break, and the two deflections are in opposite directions.
• Relative motion between the two coils, both circuits closed, induces a current in the secondary for as long as the motion lasts.
• Changing the primary current by sliding the rheostat jockey deflects the galvanometer; the induced current lasts only while the primary current is changing.

The conclusion

• So long as there is a change of magnetic flux inside a coil — produced either by a magnet or by a current-carrying coil — an induced emf is produced.
• The direction of the induced emf reverses if the flux is decreased instead of increased.
• These observations are exactly what Faraday's two laws (12.2) and Lenz's law (12.3) formalise.