# Why does an EMF develop diametrically in the rotating disk in a magnetic field?

please use the image as a reference

i understood that there will be an emf generated here, since the lorentz force will act on the electrons causing them to move towards the center of the disk.. as a result, a net potential difference appears between the rim and the centre.

So this voltage is the EMF generated.

But if the EMF equals the negative of the time derivative of flux, which in turn is the dot product of magnetic field and the area which in this case, seems to be constant, why does this emf develop?

(because Magnetic field is constant and the loop does not change orientation for the angle is makes with the field remains 90 degrees and that stays constant too.)

It would be great if someone would explain it in terms of flux, and not by the lorentz force method. Thanks!

• This is Example 7.4 from Griffiths. Directly below it (in the 4th ed.), he says "Example 7.4 (the Faraday disk, or Faraday dynamo) involves a motional emf that you can't calculate (at least, not directly) from the flux rule. The flux rule assumes the current flows along a well-defined path, whereas in this example the current spreads out over the whole disk. It's not even clear what the 'flux through the circuit' would mean in this context." Commented Apr 26, 2017 at 13:57
• thanks a lot! do i delete this question now though? Commented Apr 26, 2017 at 14:02
• I would leave it open. It's not a bad question, and it's possible that someone else will come up with another insightful way of answering it. Commented Apr 26, 2017 at 14:04