# Emf and terminal voltage

1. Feb 17, 2009

### jimbo71

How are a battery's emf and its terminal voltage different? Can they ever be the same value?

2. Feb 17, 2009

### CallumFR

A battery's emf is the maximum energy supplied to each coulomb of charge passing through the battery. A battery (and other sources for that matter) have internal resistance. When a current flows, some of the supply's emf is 'lost' as V = Ir where r is the internal resistance. These 'lost' volts aren't available to the terminals, therefore terminal voltage is 'lost' volts subtracted from the emf. Terminal voltage and emf have the same value when no current flows in a circuit as no volts will be 'lost'. On the other hand, when a source is short-circuited there is no external resistance (load) so all the emf is dropped across the internal resistance, and thus the entire emf becomes 'lost' volts and there is zero terminal voltage.

Last edited: Feb 17, 2009
3. Feb 18, 2009

### jimbo71

thank you this is a great explanation