# Electromotive force

….isn’t a force at all. It’s better described as ‘electrical pressure’ which we measure , like p.d, in VOLTS.

Defined as “power per unit current of a device or source of EMF” Be careful – EMF is thus the energy supplied to electric charge, POTENTIAL DIFFERENCE is energy dissipated (given up) by charge passing through resistance.

It’s written on the battery for you. These are both 1.5V cells, connected in series. Notice we connect + to –

When the cells aren’t connected to anything – in other words they’re not pushing electrons round a circuit – a voltmeter connected across the ends will give you a value for EMF.

As soon as the cells are connected into a circuit and energy is drawn from them, the voltage measured across the cells will fall because we need a little bit of energy per coulomb (a few volts) to push charge through the battery itself. We say that the battery has an INTERNAL RESISTANCE. We can’t get back or recover these LOST VOLTS across the battery. When a car battery is getting old, its internal resistance increases so it needs a lot of energy to push charge through it, the lost volts increases so it can’t deliver the required 12V to an external circuit and needs to be replaced.

Because voltage is a measure of energy per coulomb, and energy can’t just be ‘lost’, the voltages across each component add up around a circuit to equal the supply voltage.