CIE AS Physics (9702) exams from 2022

Revision Notes

9.2.4 Resistance in a Filament Lamp

Resistance in a Filament Lamp

  • The I-V graph for a filament lamp shows the current increasing at a proportionally slower rate than the potential difference


Filament lamp IV graph, downloadable AS & A Level Physics revision notes
I-V characteristics for a filament lamp

  • This is because:
    • As the current increases, the temperature of the filament in the lamp increases
    • Since the filament is a metal, the higher temperature causes an increase in resistance
    • Resistance opposes the current, causing the current to increase at a slower rate
  • Where the graph is a straight line, the resistance is constant
  • The resistance increases as the graph curves


Resistance and temperature

  • All solids are made up of vibrating atoms
    • The higher the temperature, the faster these atoms vibrate
  • Electric current is the flow of free electrons in a material
    • The electrons collide with the vibrating atoms which impedes their flow, hence the current decreases
  • So, if the current decreases, then the resistance will increase (V = IR)
  • Therefore, an increase in temperature causes an increase in resistance

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