Posted on 03 January 2012 at 12:45h

Yesterday I was writing some problems on electromagnetic induction so my students would have something to do when they get back after the holiday.  There are a couple of classic HL situations, the wire moving on rails is the simplest and then there’s the rotating disc. There are some variations on the theme but the classic Faraday’s disc is a disc rotating in a uniform field. At first it might appear that there should be no EMF since there is no change of field covered by the disc however if you consider a radius then it is sweeping through the field like straight wire.

Now it gets odd, if the magnet is rotated then one would expect there to be a current but there isn’t. Curiouser and curiouser, if the magnet and disc are rotated together then there is a current, how do you explain that. It caused Faraday a headache too hence the title “Faraday’s paradox”. The simplest way to explain this is that you have to think about the whole circuit not just the disc.  Firstly the field lines are not just passing through the disc they will pass through the connecting wires too, If the magnet is rotated EMF is induced in both the disc and the connecting wire, these EMFs oppose each other so no current.

 If the disc and magnet are rotated then there will be induced EMF in the connecting wire only.

If it works as a generator then it must work as a motor.