# 12.1.2 Electromotive Force

### Electromotive Force

• When charge passes through a power supply such as a battery, it gains electrical energy
• The electromotive force (e.m.f) is the amount of chemical energy converted to electrical energy per coulomb of charge (C) when charge passes through a power supply
• e.m.f is measured in Volts (V)

Definition of e.m.f with regards to energy transfer

• e.m.f is also the potential difference across the cell when no current is flowing
• e.m.f can be measured by connecting a high-resistance voltmeter around the terminals of the cell in an open circuit

e.m.f is measured using a voltmeter connected in parallel with the cell

### EMF & Potential Difference

• The difference between potential difference and e.m.f is the type of energy transfer per unit charge

Definition of potential difference with regards to energy transfer

• When charge passes through a resistor, for example, its electrical energy is converted to heat in the resistor
• The resistor therefore has a potential difference across it
• Potential difference describes the loss of energy from charges; ie. when electrical energy is transferred to other forms of energy in a component
• e.m.f. describes the transfer of energy from the power supply to electrical charges within the circuit

#### Exam Tip

Although voltage and potential difference are the same thing, make sure not to confuse them with e.m.f, which is slightly different!

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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