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