AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Resistors: The resistance of components such as lamps, diodes, thermistors and LDRs is not constant; it changes with the current through the component.
Ohmic conductors keep a constant resistance when temperature is fixed, giving a straight-line current–potential difference graph through the origin.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Ohmic conductors keep a constant resistance when temperature is fixed, giving a straight-line current–potential difference graph through the origin. Many components are non-ohmic: resistance alters as current changes. In a filament lamp, larger currents heat the filament, raising resistance. A diode conducts in one direction, with high reverse resistance and a forward threshold. A thermistor's resistance falls as temperature rises, and an LDR's resistance falls as light intensity rises. To investigate, connect the component in series with an ammeter and variable resistor, and place a voltmeter in parallel across the component. Record paired readings, then calculate R = V ÷ I.
What earns the marks
- State that resistance is the ratio of potential difference to current, R = V ÷ I, and that a non-ohmic component does not keep this ratio constant as current changes.
- Explain that a filament lamp's resistance increases because a larger current transfers more energy, raising the filament temperature.
- Describe a diode as passing current in one forward direction only, with a high resistance in reverse bias and a threshold potential difference before significant conduction.
- Explain that a thermistor's resistance decreases as temperature increases, while an LDR's resistance decreases as light intensity increases.
- Describe a valid investigation: component in series with an ammeter and variable resistor, voltmeter in parallel across the component, recording paired V and I values and calculating R for each pair.
Common mistakes and how to put them right
Saying resistance changes because current 'uses up' voltage.
Put it right by explaining that current causes heating or another physical change that alters resistance.
Treating all components as ohmic and expecting a straight-line graph.
Put it right by identifying lamps, diodes, thermistors and LDRs as non-ohmic.
Confusing thermistors with LDRs.
Put it right by linking thermistors to temperature change and LDRs to light intensity change.
Placing the voltmeter in series with the component.
Put it right by connecting the voltmeter in parallel across the component being measured.
Exam tips
- When describing an investigation, explicitly state that the ammeter is in series and the voltmeter is in parallel with the component.
- For graph questions, quote the shape and what it shows about resistance rather than only reading coordinates.
- Use R = V ÷ I with consistent units, converting milliamps to amps before calculating.
Try the lesson
The start of the Resistors: The resistance of components such as lamps, diodes, thermistors and LDRs is not constant; it changes with the current through the component. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes the resistance of a filament lamp as the current through it increases?
Key words
Say what each word means before you turn the card.
Non-ohmic behaviour
Try answering aloud or in your head before turning the card.
Current–potential difference characteristics
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Resistance and energy transfer
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Sensor components in circuits
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See every definition
- Non-ohmic behaviour
- A component shows this when its resistance alters as current changes, so it does not keep the ratio of potential difference to current constant.
- Current–potential difference characteristics
- Graphs for ohmic and non-ohmic components, where the shape shows what happens to resistance rather than only reading coordinates.
- Resistance and energy transfer
- A larger current transfers more energy, raising the filament temperature, which increases the resistance of a filament lamp.
- Sensor components in circuits
- A thermistor's resistance decreases as temperature increases, while an LDR's resistance decreases as light intensity increases.
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