Motional Induction?

[M S La Moreaux]

If an electrically conductive permanent magnet is moved at right angles to the direction of its internal magnetic field lines, is a non-electrostatic emf induced across the magnet?

[Externet]

Cannot understand you well... if the permanent magnet is moved at a right angle, its internal magnetic field lines move with it too.

[M S La Moreaux]

That is the thing. It seems that the magnetic field is not attached to the magnet. There is evidently no such thing as translating magnetic field lines. There is obviously a field around the magnet no matter where it moves. It is evidently an illusion that the field moves with the magnet. The magnetic flux density could change at any given point over time in such a way that it appears that the field is moving when, in actuality, it is not.

[Externet]

It is evidently an illusion that the field moves with the magnet.

 

Disagreed. Would like to see the evidence of an illusion.

[granpa]

teh idea of fields having velocities is one that pops up from time to time.

the official position seems to be that fields have magnitude and direction but do not have velocity.

[M S La Moreaux]

There is a version of the homopolar motor consisting of an electrically conducting disk magnet (whose faces are the poles), mounted on an axle like a wheel, with brushes on the axle and the rim. When an electric current is passed through the disk by means of the brushes it rotates from a standing start. This would seem to indicate that the magnetic field is not attached to the disk.

[granpa]

I can see why the electrons will be deflected by the magnetic field

but if they are free to move anywhere within the conductor then

I dont see why the conductor moves.

 

http://en.wikipedia.org/w/index.php?title=Homopolar_motor&oldid=71112902

[M S La Moreaux]

They are not exactly free to move. The drift speed of the conduction electrons is something on the order of a micrometer per second. That is insignificant compared to the typical rotation speed of the disk.

[swansont]

There is a version of the homopolar motor consisting of an electrically conducting disk magnet (whose faces are the poles), mounted on an axle like a wheel, with brushes on the axle and the rim. When an electric current is passed through the disk by means of the brushes it rotates from a standing start. This would seem to indicate that the magnetic field is not attached to the disk.

 

I don't see why. There a current in the wire, and that feels a force from the magnet, whose field extends outside the disk itself. This the magnet feels a force from the wire, and since the wire is not free to move and the magnet is, the magnet rotates. Relative motion and the "cutting" of flux lines is an inherent part of a motor or generator. Here the change in flux is due to the rotation of the magnet. In other systems, it is the wire that moves while the magnet is fixed, but that's the same effect just viewed from a different reference.

[granpa]

if you rotate a conductive disk inside a uniform magnetic field you will get a radial electric current.

but for teh life of me I cant see why the opposite happens.

(why a radial current produces rotation)

 

wait. if the electrons were truly free to move independently of the conductor then they would not move in the first place when the conductor rotates and there would be no radial current.

 

I guess thats what M S La Moreaux was telling me in post 8

Edited April 12, 2012 by granpa

[swansont]

if you rotate a conductive disk inside a uniform magnetic field you will get a radial electric current.

 

The scenario under discussion is not a uniform field, though.

[M S La Moreaux]

I am not sure that the cutting of flux lines occurs in alternators, where the magnet is the moving part. Different frames seem to give different results. For example, imagine a magnet and a length of wire in the magnet's field. If the wire moves and the magnet does not, there will be a non-electrostatic emf along the wire. If the magnet moves and the wire does not, there evidently will be no emf along the wire because magnetic field lines do not move. I would be interested if a source could be found which treats this topic. I have seen a quote by Einstein which points out that the two cases are treated differently even though they would seem to be symmetrical.

 

In the case of the rotating magnet, the idea of the magnet feeling a force from the current-carrying wire seems reasonable. A magnet is supposed to contain spinning electrons. I am having trouble visualizing exactly how they would be affected by the wire's magnetic field, though.

 

The idea that there are no moving magnetic flux lines and the idea that a magnet's field is not attached to it are related to the Faraday paradox. The setup is an electrically conductive disk and a disk magnet mounted face to face on separate axles. There are brushes at the axle and the rim of the conductive disk. If the conductive disk is spun while the magnetic disk is stationary, there is a voltage between the brushes. If the conductive disk is stationary while the magnetic disk is spun, there is no voltage between the brushes. If both disks are spun together. There is a voltage between the brushes. If the voltage were the result of the cutting of a circuit wire by the field lines of the magnetic disk, then there should be a voltage when the magnetic disk alone is spun.

Edited April 13, 2012 by M S La Moreaux

[M S La Moreaux]

Someone elsewhere pointed out to me what appears to be the correct answer to my original question. There will be no voltage across the magnet because both types of induction are present and oppose each other exactly. The magnet as conductor is moving in a magnetic field (its own) and producing a motional emf. However, the magnetic field is increasing in the space in front of the moving magnet and decreasing behind it. These changes in the magnetic flux at stationary points of space are accompanied by an electric field which counteracts the motional emf.