Electromotive Force

Key Stage 5

Meaning

Electromotive force (emf) is the amount of work produced per unit charge by a source of electric power.

About Electromotive Force (emf)

• Emf is given by the symbol \(\varepsilon\)
• Emf is measured in volts (V).
• Emf is not actually a force, but a potential difference that drives current around a circuit.
• In a circuit, the emf is equal to the potential difference across the terminals of the power source when no current is flowing.
• The internal resistance of the power source causes the terminal voltage to be less than the emf when current is flowing.

Formulae

The electromotive force is given by the formula:

• \(\varepsilon=\frac{𝑊}{𝑄}\)

Where:

• \(\varepsilon\) is the emf
• 𝑊 is the work done to move the charge
• 𝑄 is the charge

The terminal voltage of an electrical cell or battery is related to emf by the following formula:

• \(𝑉=𝐸−𝐼𝑟\)

Where:

• 𝑉 is the terminal voltage
• 𝐼 is the current through between the terminals
• 𝑟 is the internal resistance of the power supply