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If I establish a permanent electric charge on some volume of material (as in an electret) and then move that material through space — is that motion equivalent to a current in a wire?

Also, since a current through a loop of wire generates a magnetic field, would spinning an electret generate a magnetic field as well? [Update: to be clear I mean a symmetrical spinning, i.e. either a monopolar electret or rotating a dipole charge along its own axis without flipping the field polarity.]

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Moving an electret is not equivalent to current in a wire, as you say. However, it is current. The positive and negative poles of the electret are physically separated, so a moving electret corresponds to physically separated currents in opposite directions. That makes a magnetic field.

Furthermore, the electret is surrounded by an electric field. If you move the electret, that field changes, and you get a displacement current. That induces a magnetic field.

It's analogous to a magnet. A moving magnet makes an electric field. A moving electret makes a magnetic field.

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  • $\begingroup$ Thanks, although I don't know that this addresses the spinning part of my question (which I've clarified)? You mention a displacement current when the field changes — on one hand if I simply spin a field where it is already rotationally symmetric it doesn't seem that it's really "changing", but on the other with a constant DC current through a fixed wire the electric field doesn't really "change" either so it seems there is more going on? $\endgroup$
    – natevw
    Jun 14 at 17:34
  • $\begingroup$ @natevw For the spinning case, the E field is constantly changing at any fixed point in an inertial frame. It's just like an alternator, where a spinning magnetic rotor makes a changing magnetic field which induces an electric field which drives a current in the stator. $\endgroup$
    – John Doty
    Jun 14 at 18:30
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Any motion of electric charges is an electric current. That's the very definition of an electric current.

Any electric current generates a magnetic field (see Biot and Savard law, or Maxwell-Ampère equation). If this current isn't constant in time, you might generate an electric field at the same time, since both fields are interdependent outside of the static case.

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  • $\begingroup$ Good answer. But When I searched about this electret on google and done some deep research it's showing some kind of cylindrical thing, which can be electrically polarised for infinite amount of time but we cannot call it charge though it's polarised but still that complete cylindrical thing is electricity neutral and the current is the amount of charge passes per unit time from any point but this thing is not charge so how can this be considered to fit in the definition of current? $\endgroup$ Jun 14 at 5:47
  • $\begingroup$ My understanding was that an electret can either be a dipole as you say with charges separated or that one can be fashioned to have a net excess of one charge essentially as a monopole. $\endgroup$
    – natevw
    Jun 15 at 0:52

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