3.16 Internal Resistance

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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