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Voltage or the terminal potential difference of a battery or a generator when it delivers a current I is related to its electromotive force ε and its internal resistance r as follows:

1. When delievering currrent (on discharge)

Terminal voltage = emf - voltage drop in internal resistance or V = ε - Ir

2. When receiving current (on charge)

Terminal voltage = emf + voltage drop in internal resistance or V = ε + Ir

3. When no current exists:

Terminal voltage = emf of battery or generator