Resistance of Filament Lamps (Oxford AQA IGCSE Combined Science Double Award)

Resistance of Components

• The resistance of components such as lamps, diodes, thermistors and LDRs is not constant

  • It changes with the current through the component

Determining resistance

• To find the resistance of a component, set up a circuit like the one shown below

Circuit for finding the resistance of a component

• The power supply should be set to a low voltage to avoid heating the component

  • One or two volts is typically enough

• Measurements of the potential difference and current should then be taken from the voltmeter and ammeter respectively

• Finally, these readings should be substituted into the equation for Ohm's law:

Resistance of Filament Lamps

Extension Tier only

• The resistance of a filament lamp increases as the temperature of the filament increases

• The current increases at a proportionally slower rate than the potential difference because:

  • The current causes the filament in the lamp to heat up

  • As the filament gets hot, its resistance increases due to the collisions between the electrons and metal ions in the wire

  • This opposes the current, causing it to increase at a slower rate

Current-potential difference graph for a filament bulb