AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Students should be able to explain that, for some resistors, the value of…
Full specification wording
Resistors: Students should be able to explain that, for some resistors, the value of R remains constant but that in others it can change as the current changes.
Resistance is defined as R = V ÷ I, and you can work it out for any component at any moment, but whether that value stays put is another matter.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 1 of 13
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Resistance is defined as R = V ÷ I, and you can work it out for any component at any moment, but whether that value stays put is another matter. For a fixed resistor at constant temperature it does: raise the potential difference, the current rises in step, and the ratio is unchanged. For a filament lamp it does not, because the current itself heats the filament and a hotter filament resists more. A diode is more extreme still, passing current easily one way and almost none the other. Thermistors and light dependent resistors change with their surroundings rather than with the current. So the first question about any component is whether R is a constant or a value that has to be recalculated at each point.
What earns the marks
- Stating that resistance is calculated as potential difference divided by current.
- Identifying a component whose resistance stays constant at constant temperature.
- Identifying a component whose resistance changes, with the reason for the change.
- Supporting the classification with readings or with the shape of a graph.
Common mistakes and how to put them right
Assuming every component obeys a fixed resistance, so using one value across a whole graph.
Quoting the gradient of a current against potential difference graph as the resistance rather than as one divided by the resistance.
Saying a lamp has no resistance at low potential difference because the current is small.
Treating the change in a thermistor as caused by the current when it is caused by temperature.
Exam tips
- Recalculate R = V ÷ I at each point on a curved graph rather than using one value throughout.
- Name the reason for a change: heating of a filament, temperature of a thermistor, light on an LDR.
- For any component, state the condition, such as at constant temperature, before claiming the resistance is fixed.
Try the lesson
The start of the Resistors: Students should be able to explain that, for some resistors, the value of R remains constant but that in others it can change as the current changes. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes the resistance of a fixed resistor as the current through it changes?
Listen
Resistance is defined as R = V ÷ I, and you can work it out for any component at any moment, but whether that value stays put is another matter. For a fixed resistor at constant temperature it does: raise the potential difference, the current rises in step, and the ratio is unchanged. For a filament lamp it does not, because the current itself heats the filament and a hotter filament resists more. A diode is more extreme still, passing current easily one way and almost none the other. Thermistors and light dependent resistors change with their surroundings rather than with the current. So the first question about any component is whether R is a constant or a value that has to be recalculated at each point. A strong answer earns marks for these points. stating that resistance is calculated as potential difference divided by current. identifying a component whose resistance stays constant at constant temperature. identifying a component whose resistance changes, with the reason for the change. supporting the classification with readings or with the shape of a graph. Exam tip: Recalculate R = V ÷ I at each point on a curved graph rather than using one value throughout.
Key words
Say what each word means before you turn the card.
R = V ÷ I
Try answering aloud or in your head before turning the card.
constant and changing resistance
Try answering aloud or in your head before turning the card.
effect of temperature
Try answering aloud or in your head before turning the card.
reading graphs
Try answering aloud or in your head before turning the card.
See every definition
- R = V ÷ I
- Resistance is calculated as potential difference divided by current, and can be worked out for any component at any moment.
- constant and changing resistance
- For some resistors the value of R remains constant, but in others it can change as the current changes.
- effect of temperature
- In a filament lamp the current itself heats the filament, and a hotter filament resists more.
- reading graphs
- On a curved graph, R = V ÷ I is recalculated at each point rather than using one value throughout.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Resistors: Students should be able to explain that, for some resistors, the value of R remains constant but that in others it can change as the current changes.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge