• 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)