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Electromotive force, abbreviated emf (denoted and measured in
In electromagnetic induction, emf can be defined around a closed loop of
In the case of a two-terminal device (such as an
Devices that can provide emf include
In the case of a battery, the charge separation that gives rise to a voltage difference between the terminals is accomplished by chemical reactions at the electrodes that convert chemical potential energy into electromagnetic potential energy. A voltaic cell can be thought of as having a "charge pump" of atomic dimensions at each electrode, that is:
A source of emf can be thought of as a kind of charge pump that acts to move positive charge from a point of low potential through its interior to a point of high potential. … By chemical, mechanical or other means, the source of emf performs work dW on that charge to move it to the high potential terminal. The emf ℰ of the source is defined as the work dW done per charge dq: ℰ = dW/dq.
In the case of an electrical generator, a time-varying magnetic field inside the generator creates an electric field via