Edexcel A Level Physics

Revision Notes

3.16 Internal Resistance

Test Yourself

Internal Resistance

  • All power supplies have some resistance between their terminals
    • This is called internal resistance (r)

  • Internal resistance causes some electrical energy to be transformed to heat energy in the power supply itself
    • This is why the cell becomes warm after a period of time

  • Therefore, the internal resistance causes energy loss in a power supply
  • A cell can be thought of as a source of e.m.f. with an internal resistance connected in series
  • The amount of voltage lost is known as the 'lost volts' 
    • A higher internal resistance will result in a higher value for lost volts


Circuit showing the e.m.f and internal resistance of a power supply

  • Where:
    • R = resistance of the circuit (the ‘load resistor’)
    • r = internal resistance
    • ε = e.m.f.
    • Vr = 'lost volts'
    • VR = voltage across the load (sometimes also called VT, the terminal voltage)

Exam Tip

The internal resistance concept catches many students out. Make sure you fully understand the circuit diagram;

  • Internal resistance of the cell can be treated as though it were a separate resistor - although it isn't!
  • The load  resistance is treated as another resistor in series
  • Potential difference is measured across the load resistor
  • The lost volts are calculated as though they were the potential difference across the 'internal resistor'

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