MadSci Network: Physics

Re: magnetic fields don't rotate when the magnet does?

Date: Thu Oct 1 21:06:29 1998
Posted By: Richard Bersin, Other (pls. specify below), Senior Technical Staff Member, Emergent Technologies
Area of science: Physics
ID: 906960268.Ph

Dear Dave:

I understand your confusion about the rotating magnet.  However if you will 
open up your mind and pay careful attention to what I say I think you will 
no longer be puzzled.  The problem is I think you are so used to seeing 
diagrams of magnetic fields with lines of force coming out of one pole and 
into the other that you imagine when the magnet rotates the force lines 
rotate also.  But the fact of the matter is that the magnetic field fills 
the space around the magnet, and the "lines of force" are only a 
pictorialization of the fact that the field is stronger where the lines are 
shown close together and weaker when they are farther apart;  and they show 
the direction of the magnetic field.  The only thing that means is what 
direction a tiny dipole mnagnet would line up if you put it there. 

Now imagine this magnetic field being generated by a series of currents 
flowing circularly in the magnet bar, perpendiculary to the magnet axis.  
Remember that a circulating electric current creates a magnetic field 
identical to what you get from a bar magnet.  Now if you can picture these 
current going around and creating the magnetic field you can see that the 
field is just sitting there in space and not rotating,  and so if you 
rotate the bar magnet it will make no difference-the magnetic field is not 
"stuck" to the ends of the magnet-it is a "field in space" and the magnet 
can rotate all it wants inside the field and nothing in the field will 
change assuming the axis of the magnet ia held perfectly steady.

Naturally if the currents are not perfectly circular, so that the magnetic 
field is not perfectly unifom angularly around the magnet (perhaps the 
magnet has a pimple on one side, for example)then the strength of the 
magnetic field will not be uniform going around the angular direction and 
then when you rotate the magnet the nonuniformity of course will rotate 
around with the magnet because of the angular nonumniformity of the field; 
i.e., the field strengths at the various points will change as the magnet 
rotates, but this will only mean that as the magnet rotates the attached 
metal plate will still see induced currents which will vary according to 
the strength of the magnetic field at any point  the metal surface is 
passing through.

Do you get the picture?  The magnetic field is not stuck to the magnet like 
a piece of paper!

I hope this clears the picture up for you.  If this makes you think of 
further questions please write back to me and we can discuss this further.


R. Bersin...

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