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Ex,
EM was speaking of a static field of uneven shape being viewed by a viewer in motion around that static field. Frames of reference aside, that viewer would be seeing an apparent change in that static field. There would be induction even without an emf in that field. The only motion/change required is relative motion/change.
It is true you cannot associate a frame of reference to a field but you can to the space containing that field. This doesn't matter until the field is not some uniform shape appearing the same from all external points, as EM has suggested.
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WW, you get a good interpretation of what EM is saying.
I agree that you can associate a frame to a space containing a field, but contrarily to what EM suggests, you can't say that the field has a speed v relative to this frame, nor is at rest in this frame. You can only say that at point (x,y,z,t) of this space, the field intensity is B. B is a vector without speed.
It is a big misunderstanding to imagine a field with a speed i.e. to imagine a frame of reference in which the field would be "at rest". This misunderstanding leads to a false paradox in Faraday motor (V in F=q.VxB is relative to the observer, not to the field, so no paradox).
About the induction, a B field in "not some uniform shape" can't be the cause for an electron to move. If only a not uniform magnetic field in space could move an electron, then currents would appear in stationary circuits placed in areas near permanent magnets where there is a field gradient. This is not observed.
When an electron is at rest, in respect to its own reference frame in which it is at rest, none magnetic field can move it because V=0 so F=q.VxB=0, but only an electric field. For induction, this field is E=-dA/dt, therefore a
time variation is needed and from the viewpoint of the electron, the force it feels is F=q.E.
When an electron is not at rest (i.e. is moving at speed V relative to the frame of the observer, not of the magnetic field), then the observer see a force F=q.VxB acting onto the electron. But
the electron doesn't see the force F=q.VxB: from its own reference frame, it is still the electric field E=-dA/dt that it feels. Of course this is the two aspects of the same phenomenon of relativity, interpreted either in the reference frame of the electron (F=q.e with E=-dA/dt) or in the reference frame of an observer in which electron speed is V (F=q.VxB).
Naturally the viewpoint of the electron is the more interesting because it gives directly the causal explanation for it to move.
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Topic: Induction (Read 53957 times)
