The electromotive force (EMF) of a battery is a measure of work done (joule) per unit of charge (coulomb) moving the charge carriers through the battery to the terminal with highest potential (+ terminal for conventional current). EMF is measured in volts (JC-1).
In an open circuit, the EMF of a battery is the potential difference of the two terminals: EMF = V+-. When connected to a closed circuit, the potential difference at the terminals is lowered as there is internal resistance that the charge carriers experience in the circuit. So EMF = V+- + IRinternal, where I is the current.
Fischer-Cripps. A.C., The Electronics Companion. Institute of Physics, 2005.
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