The Naked Scientists Forum

teragram

• Sr. Member
• Posts: 122
« on: 01/07/2009 16:22:48 »
Consider the version of the Faraday Disc based on a disc magnet (disc with North pole on one face and South pole on the other). The conductive disc shares an axis with the magnet, and is in close proximity to one face (pole) of the magnet.
If the conducting disc rotates and the magnet remains stationary, a voltage appears between the rim of the conducting disc and its centre.
I find it strange that if the conducting disc and the magnet both rotate at the same speed, and in the same direction, the same voltage appears between the rim of the conducting disc and its centre.
I believe however that if the conducting disc remains stationary and the magnet rotates (about the original axis) no voltage appears on the conducting disc.
This seems to suggest that the magnetic field is independent of the magnet.
Can anybody explain?

syhprum

• Neilep Level Member
• Posts: 3821
• Thanked: 19 times
« Reply #1 on: 01/07/2009 20:18:32 »
If what you say is correct the magnetic field appears not to rotate with the magnet, I wonder what happens if the magnetic field is generated by a solenoid and that is rotated about its axis.

lyner

• Guest
« Reply #2 on: 02/07/2009 09:43:36 »
For a current to flow due to the rotation, there needs to be a rotating part and a stationary part to the circuit.
When the disc is connected to a meter and the {Edit - I just read this - it's nonsense    {magnet is rotated}??? durrr] disc is rotated, there is a emf induced by the disc's motion relative to the magnet.
If magnet and disc rotate, there is an EQUAL emf induced in the stationary connecting wires but none in the disc. The value of emf is the same, because of a very valid argument involving line integrals around the circuit and the interaction with the whole of the field around the magnet, This emf is in the opposite direction.

If the magnet, alone, rotates, the total emf around the circuit adds up to zero. The emfs balance out.
There is no paradox if you consider the whole circuit.
« Last Edit: 02/07/2009 13:07:25 by sophiecentaur »

teragram

• Sr. Member
• Posts: 122
« Reply #3 on: 04/07/2009 12:00:29 »
Sophiecentaur, I think I understand, your explanation makes sense, thanks.

Syphrum, I would guess that a solenoid in place of the magnet would have the same effects.

Thanks for the replies.