Electromotive force (emf) is defined as the work done by the chemical force to move unit positive charge from negative terminal to positive terminal of the battery.
- Let this chemical force be ${ F }_{ c }$.
- This chemical force does some work to move a negative charge ‘q’ from positive terminal to negative terminal against the electric force Fe. Let this work be W.
- The work done by the chemical force to move 1 coulomb of charge from the terminal to negative terminal is given by W/Q = ${ F }{ c }$d / q
Where ‘d’ is the distance between the terminals.
If we assume Fc = Fe ,
then = W/q = Fed/q
This W/q is known as emf of battery.
Therefore , emf = W/q = ${ F }{ c }$d / q
This S.I unit of emf is volt and is measured using voltmeter