What is a magnetic field? (fun mental exercise)

[the_emi_guy]

Jim,
In this case the B circles don't move. Constant current in a long straight wire creates a static magnetic field, analogous to the static E field between capacitor plates.
Motion of a charge relative to this static B field creates force.

The case of a single electron in motion is a different story. Here you will have a time & space varying set of E and H fields as the charge comes and goes.

It is the unique symmetry of the infinitely long straight wire with constant current that allows the "cat video" to nicely transform purely magnetic force in one frame of reference to purely electric force in another.

In the general case we will have time and position varying E and H fields in both frames of reference, and all of the math (Susskind, Feynman et al) shows us how to transform these fields between frames of reference.

 

[jim hardy]

Thanks. Makes sense, i figured the lone electron Coulomb had a different E-field...

I have homework to do now - those Feynman lectures, the Susskind video and the Einstein chapter anorlunda linked...

wont trouble you until have done more study..

 

[meBigGuy]

I think the key to this is that magnetic fields don't move as the wire moves along the axis of the current (even though that would explain the effects seen). The magnetic intensity remains the same all along the wire. Its not like a compass will see the longitudinal motion of the wire. Magnetic fields don't propagate. The field exists at a distance from the wire, and that field does not change as the wire moves.