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

Revision Note

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

A circuit used to determine the resistance of a component consists of a power supply, a component and an ammeter all connected in series. A voltmeter is then connected in parallel with the component.
A circuit to determine 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:

R space equals space V over I

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

The I-V graph of a filament lamp, for IGCSE & GCSE Physics revision notes
The I-V graph of a filament lamp shows a curve with a decreasing positive gradient as the current moves away from zero and the potential difference increases

Examiner Tips and Tricks

In your exam, you need to be able to explain the change in resistance in terms of ions and electrons.

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