<Li> For a circuit consisting of an electrical generator that drives current through a resistor, the emf is due solely to a time - varying magnetic field within the generator that generates an electrical voltage that in turn drives the current . (The ohmic IR drop plus the applied electrical voltage again is zero . See Kirchhoff's Law) </Li> <Li> A transformer coupling two circuits may be considered a source of emf for one of the circuits, just as if it were caused by an electrical generator; this example illustrates the origin of the term "transformer emf". </Li> <Li> A photodiode or solar cell may be considered as a source of emf, similar to a battery, resulting in an electrical voltage generated by charge separation driven by light rather than chemical reaction . </Li> <Li> Other devices that produce emf are fuel cells, thermocouples, and thermopiles . </Li>

Emf (electro-motive force) is a voltage