EMF and Internal Resistance

Every real battery or power supply has some resistance of its own. This internal resistance affects the voltage available to the external circuit and is a key concept for A-Level Physics.

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Electromotive Force (EMF)

The electromotive force (EMF), symbol ε (epsilon), of a source is the total energy transferred per unit charge as charge passes through the source. It is measured in volts.

ε = W / Q

Despite its name, EMF is not a force — it is an energy per unit charge, just like potential difference. The key distinction is:

• EMF is the energy given to each coulomb of charge by the source (e.g., a battery converts chemical energy into electrical energy).
• Potential difference is the energy transferred by each coulomb as it passes through a component.

EMF represents the total "budget" of energy that the battery provides to each coulomb. But not all of this energy reaches the external circuit — some is wasted inside the battery itself.

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Internal Resistance