How Does Rotational Motion Affect Magnetic Fields Between Two Magnets?

[ex3mist]

Hello,

I'd like to ask how the magnetic field that flows between two magnets would change if we begin to rotate the magnets together? (I hope the image makes my question more clear.)

Thanks

 

[berkeman]

Hello,

I'd like to ask how the magnetic field that flows between two magnets would change if we begin to rotate the magnets together? (I hope the image makes my question more clear.)

Thanks

You haven't shown the return path for the magnetic field that comes out the bottom (North end) of the bottom magnet and goes into the top (South end) of the top magnet, but that won't change the field much in between the middle faces of the magnets.

The field would rotate just as if you were rotating that piece of paper that your drawing is on. Why would it behave any differently? Am I misunderstanding your question?

 

[ex3mist]

Thanks for the replay berkeman,

I suppose you understood my question very well, I mean if the field is not influenced by the speed of the rotation, the inertia or anything else, it is o'kay for me. That's what I want. Just want to make sure that there isn't any law of physics that applies in this case. Maybe something similar to the process shown in the picture (I suppose it translates as "tongue reaction" in english but I'm not sure, that's why I'm attaching an image to show it).

Just want to make sure I'm not missing anything.

 

[sophiecentaur]

The only effect that you could expect is that, at high rotation speeds and with a large spacing, the field would lag due to the propagation of the change being limited to c.

The external field would actually end up as an EM wave, propagating outwards with a frequency equal to the rotation frequency of the magnets.

 

[ex3mist]

Well, that means that there is a change in the field. That brings couple more questions:

First, could you specify what you mean by "propagation of the change being limited to c"? Please, excuse me for that but I'm not neither a specialist, nor a native english. I couldn't undestand your point.

Second, how high is "high rotation speed"? Is there any expression for the connection between lagging and the rotation speed (spacing)?

Thanks!

 

[sophiecentaur]

I mean the nothing propagates faster than c. So the field at the centre will only change after a delay of d/c seconds, where d is the distance. This is incredibly small for 'normal' sizes and speeds. One way of looking at the speed of light is that it travels at one foot per nanosecond. We're talking incredibly small differences here ---- but you did ask!

This doesn't apply at all in practical things like motors - just work out the time delay inside a motor and what difference this would represent if the motor were rotating at 3000revs.

 

[ex3mist]

So the delay has an extremely small value!
But you compare delay with the speed of light, not the speed of the magnetic field...? Isn't it a lot "slower"? (If the word "slower" is suitable at all... :) )

 

[sophiecentaur]

The changing magnetic field IS electromagnetic and it also involves a small E field so it travels at c. Now, there would be a lot of difficulty in detecting such a small change in the direction of the field - or at least, the phase of this change, compared with the rotation of the magnets. But, given a lot of time to do the measurement, you could imagine actually detecting what would be, effectively, a 'bend' in the rotating field lines.

 

[sophiecentaur]

I'm now looking at your 'tongue reaction' diagram. This is an entirely different thing because it involves more than just free space and a rotating magnet. This is a motor with two sets of fields - one from a field winding and one due to an armature (a brush motor?).

 

[ex3mist]

Well, then it should be more correct to call it "armature reaction", I'm not sure. Whatever, thanks for the explanations, sophiecentaur!