Introduction

For more than half a century after Hans Christian Oersted showed in 1820 that an electric current produces a magnetic field, physicists wondered whether the reverse effect might also be true -- could a magnetic field somehow produce an electric current? Michael Faraday in England and Joseph Henry in America, working independently around 1830-31, both found the answer: yes, but only when the magnetic field is CHANGING. A magnet held perfectly still next to a coil produces no current at all, however strong it is; it is only while the magnet is being moved, or the field is otherwise varying, that a current flows. This phenomenon -- the production of an emf (and hence a current, if the circuit is closed) by a changing magnetic environment -- is called electromagnetic induction, and it is the single physical idea behind almost every electric generator, transformer and induction motor in use today.

This chapter builds the idea up in the order WBCHSE's syllabus lists it. Section 6.2 first makes precise the quantity that has to change: magnetic flux, the 'amount' of magnetic field passing through a surface. Section 6.3 then states Faraday's two laws, which say exactly how much emf a given rate of change of flux produces. Section 6.4 looks at the different physical ways flux can actually change -- moving a conductor through a field, changing a circuit's area, or changing the field itself. Section 6.5 introduces Lenz's law, which fixes the DIRECTION of the induced current, and shows that this direction is forced on us by energy conservation. Section 6.6 looks at eddy currents, the same induction effect occurring inside the bulk of a solid conductor rather than a neat wire loop. Finally, Sections 6.7-6.8 introduce self- and mutual inductance -- the idea that a changing current induces an emf either back in its own coil or in a neighbouring one -- worked out explicitly for the solenoid, the one shape where the calculation is genuinely tractable at this level.