# Is electromotive force always constant?

1. Feb 20, 2010

### arymaia

is electromotive force always constant??

i've been cross-referencing different sources as to the exact definition of the electromotive force, and i keep getting the following:

emf = terminal voltage

but i know terminal voltage changes due to the voltage drop that occurs when an electrical circuit is closed. what i just need to absolutely confirm is that emf is practically constant, given the equation:

terminal V = emf - IR

where the ONLY time emf = terminal V = open-circuit voltage is when the electric circuit is open. And that open-circuit voltage is ALWAYS equal to emf and that open-circuit voltage = terminal voltage ONLY when the circuit is open.

moreover, what exactly is the role of emf once the circuit is closed? I know it's responsible for driving the current, but if this is the case, what's the role of the terminal V then if not the same thing?

Otherwise:

1) during discharge: terminal V < emf, due to the voltage drop from internal resistance
2) during recharge: terminal V > emf, due to restoration of potential difference across the terminals of the battery (i.e. voltage source)