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'
