Faraday disc: different configuration question

[girts]

I started thinking about this again because it never really left my mind.
Now we know the three different scenarios for the Faraday homopolar disc,1) The disc rotates while magnet and brushes with the circuit they connect stay stationary in the laboratory reference frame = result , current generated in the circuit
2) The disc rotates together with the magnet only the brushes with the circuit they connect stay stationary = result again is that current is generated in the circuit
3) brushes together with the disc are stationary, only the magnet under the disc rotates with respect to the lab reference frame= result is that no current is generated,
but here is the tricky part I haven't yet understood, many textbooks and sources say that even when both disc and brushes stay stationary there is still a E field in or should I say across the disc when only the magnet is rotated, but that means that if there is an E field there must be a work done on the charges in the disc or in other words there should be charge separation happening in the disc much like in the other examples where the disc is physically rotating.Yet when brushes are added no current is flowing.

So here is my question , what would happen if the disc stayed stationary in the lab frame but the magnet would rotate together with the brushes and the circuit that connects the brushes, would then there be current generated in the rotating circuit the same way current is generated when the disc rotates either with or without the magnet?I ahve come to the conclusion that in the homopolar disc case it is not about flux change with time in an enclosed loop area like it is in induction but rather is is all about relative movement of conductors in a uniform magnetic field, the weird thing about this relative movement is that I can understand why current is generated when the disc moves with velocity X and the brushes with the circuit move at some other velocity or stay stationary but I cannot understand why there would be an E field when a magnet that has a uniform field is moved around it's axis in such a way that the field strength does not change, would that imply that the B field has some other property than merely strength vs distance?
PS. I do realize that B field lines are just a man made visualization.

 

[eljose79]

I can understand your confusion and interest in the Faraday homopolar disc experiment. It is indeed a fascinating phenomenon that challenges our understanding of electromagnetism.

First, let me clarify that the concept of an E field and charge separation in the disc is not related to the disc's physical rotation. In all three scenarios, there is a uniform magnetic field passing through the disc, which creates an E field across the disc. This E field is not due to the disc's rotation, but rather due to the relative movement of the disc and the magnet.

Now, to answer your question about what would happen if the disc stayed stationary but the magnet and brushes rotated together, the result would be the same as scenario 3. No current would be generated because there is no relative movement between the disc and the magnet. The E field would still be present, but without any relative movement, there would be no force acting on the charges in the disc to create a current.

You are correct in thinking that the B field has some other property than just strength vs. distance. This property is called magnetic flux, and it is a measure of the number of magnetic field lines passing through a given area. In the case of the homopolar disc, the magnetic flux remains constant, but the relative movement of the disc and the magnet creates a changing magnetic flux, which, in turn, creates an E field and induces a current.

I hope this helps clarify your understanding of the Faraday homopolar disc experiment. It is indeed a complex phenomenon, but it ultimately demonstrates the interplay between electric and magnetic fields and how relative movement between conductors and magnetic fields can induce a current. Keep exploring and questioning, as that is the essence of scientific inquiry.