Internal Resistance (Edexcel International AS Physics): Revision Note
Exam code: XPH11
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)
Examiner Tips and Tricks
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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