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Why do they vibrate and simply not get pulled closer or further from the source of emf?

This may be related to the shape, but if I take a small N45 disc magnet and put it near an electrical appliance, the magnet will vibrate. The intensity of the vibrations varies depending upon where you move the magnet as well.

Is there an optimal magnet shape to generate the strongest vibrations?

I assume there is at least an optimal mass/magnetic strength ratio for the strongest vibrations?

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  • $\begingroup$ That's more of an engineering question... see loudspeaker design. $\endgroup$ – CuriousOne May 25 '15 at 2:22
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I can't answer your last two questions, but I'll have a stab at the first.

Imagine if I had a device which rapidly changed charge from positive charge to negative charge. What would happen if I brought a charged particle (of either sign) near this device?

Suppose my charged particle is positive. While my device had a negative charge, the charged particle would be attracted to it. While my device had a positive charge, the charged particle would be repelled from it. Therefore, an alternating charge makes a permanent charge vibrate.

Repeat the same argument with a permanent magnet. Just like a constant current creates a constant magnetic field, an alternating current creates an alternating magnetic field. The alternating magnetic field creates an alternating torque on the permanent magnetic dipole causing it to vibrate.

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