Electromotive force
A cell is the thing doing the pushing, and the push it can produce has a name of its own.
Electromotive force
The force that drives electrons from the negative terminal out into the external circuit is called the electromotive force.
Measuring the e.m.f. of a cell
The electromotive force of a cell is equal to the potential difference between its two terminals when no current is being drawn from it. That condition is the whole definition — measure it while it is working and you get a smaller number, for the reason below.
Why a working cell reads lower
When a current is drawn from a cell, that current also flows through the inside of the cell. There is an electrical resistance inside the cell too, and a small potential difference appears across that internal resistance. Subtract it from the electromotive force and what is left is the potential difference the cell supplies to the external circuit.
p.d. supplied = electromotive force − p.d. lost inside the cell
A cell is not a perfect source
Some of the push is spent driving the current through the cell itself before any of it reaches the circuit. That is why a torch dims as its cells age: the internal resistance rises, so more of the e.m.f. is lost inside.
Why it is also called voltage
Because the potential difference between two points in a circuit is measured in volts, it is also called the voltage. The two words mean the same thing in this chapter.