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Re: [Phys-l] emf, voltage, potential difference (was induced emf...)



On 10/13/09 06:58, Edmiston, Mike wrote:

In the latter case, is John correct that voltages are not same as
potential differences when the source of the voltage is magnetic
rather than electrostatic? (Am I interpreting you correctly, John?)

Sure. It's not a tricky or subtle concept. It is a
simple, direct consequence of the Maxwell-Faraday law
of induction and the definition of "potential".

∇ × E + (∂/c∂t) cB = 0

Given a time-varying magnetic field, the voltage depends
on the path of integration, not just on the endpoints ...
so it's not a potential.


Also note that
the Hyperphysics site defines voltage as "electric potential energy
per unit charge."

That's harmless as far as it goes, but it's a bit on
the vague side. It is dimensionally correct, but there
is more to physics than dimensional analysis.

=====================

While we're on the subject, purists might argue that
much of what has been said about "voltage" would more
properly apply to "voltage differences". On the other
hand, one could argue -- by gauge invariance -- that
*all* voltages are voltage differences. I'm not gonna
worry about it either way.