Resistance in a Filament Lamp (CIE A Level Physics)

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

 Filament Lamp Graph

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 temperature increases, the resistance increases
• And as the resistance increases, the current decreases