The electromotive force (EMF) is the work per unit charge around a wire loop caused by a time-varying magnetic flux threading the loop. It is due to a force moving the charges around the loop. This is true whether the change in flux is due to the wire loop being stationary and the field changing in time, or the loop moving through a spatially varying field. In the first case, we say that the time-varying magnetic field induces an electric field that provides the force; in the second, we say that the force is due to the magnetic field acting on the charges in the moving loop. The theory of relativity states that both viewpoints must be equivalent, but it is sometimes difficult to harmonize them.

1.
David
Griffiths
,
Introduction to Electrodynamics
, 4th ed. (
Pearson
,
Boston
,
2013
), pp.
312
314
. Equation (1) of this paper is Eq. 7.17 of the textbook.
2.
For example:
C. J.
Foot
, Atomic Physics (
Oxford University Press
,
Oxford
,
2005
), pp.
190
194
.
3.
Ref. 1, p. 315.
4.
For example, see:
H. F.
Meiners
(ed.), Physics Demonstration Experiments, Vol. II (
The Ronald Press Co.
,
New York
,
1970
), pp.
932
951
. This is a long out-of-print book with a number of very nice demos.
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