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 –

Two 1.5V cells connected in series. EMF= 3V

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.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.