OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Use graphs to explore whether circuit elements are linear or non-linear
Full specification wording
Simple circuits: use graphs to explore whether circuit elements are linear or non-linear
A linear element gives a straight line through the origin on a potential difference–current graph, so its resistance stays constant as current changes; a fixed resistor at constant temperature…
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 9 of 14
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A linear element gives a straight line through the origin on a potential difference–current graph, so its resistance stays constant as current changes; a fixed resistor at constant temperature behaves this way. A non-linear element gives a curve, so resistance changes with current. A filament lamp curves because the metal heats and its resistance rises. A diode conducts in forward bias and blocks in reverse, so its graph is not a straight line through the origin. An NTC thermistor and an LDR are non-linear with respect to temperature and light intensity. To judge linearity, plot the points, draw the line of best fit and check whether it is straight and passes through the origin. Read the gradient as resistance only where the line is straight.
What earns the marks
- Identifies a straight line through the origin on a potential difference–current graph as showing a linear element with constant resistance.
- Recognises a curve on the graph as showing a non-linear element whose resistance changes with current.
- Explains that a filament lamp is non-linear because heating raises its resistance as current increases.
- Explains that a diode is non-linear because it conducts in forward bias and blocks in reverse bias.
- Uses the line of best fit through plotted points to decide whether the relationship is linear.
Common mistakes and how to put them right
Calls any straight line linear even when it misses the origin.
Put it right: Check that the line passes through the origin.
Assumes a curved graph means the component is faulty.
Put it right: Recognise the curve as evidence of changing resistance.
Reads resistance from the gradient of a curved section.
Put it right: Use the gradient only where the graph is straight.
Exam tips
- Check two features of a linear graph: straight and through the origin.
- Link each shape of graph to the physical behaviour of the named component.
- State the direction of resistance change when explaining a curve.
Try the lesson
The start of the Simple circuits: use graphs to explore whether circuit elements are linear or non-linear lesson, made from checked teaching notes for this specification point.
Have a go first
A student plots a current–potential difference graph for a fixed resistor. The plotted points lie on a straight line that passes through the origin. What does this show about the resistor?
Listen
A linear element gives a straight line through the origin on a potential difference–current graph, so its resistance stays constant as current changes; a fixed resistor at constant temperature behaves this way. A non-linear element gives a curve, so resistance changes with current. A filament lamp curves because the metal heats and its resistance rises. A diode conducts in forward bias and blocks in reverse, so its graph is not a straight line through the origin. An NTC thermistor and an LDR are non-linear with respect to temperature and light intensity. To judge linearity, plot the points, draw the line of best fit and check whether it is straight and passes through the origin. Read the gradient as resistance only where the line is straight. A strong answer earns marks for these points. Identifies a straight line through the origin on a potential difference–current graph as showing a linear element with constant resistance. Recognises a curve on the graph as showing a non-linear element whose resistance changes with current. Explains that a filament lamp is non-linear because heating raises its resistance as current increases. Explains that a diode is non-linear because it conducts in forward bias and blocks in reverse bias. Exam tip: Check two features of a linear graph: straight and through the origin.
Key words
Say what each word means before you turn the card.
Linear element
Try answering aloud or in your head before turning the card.
Non-linear element
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Filament lamp
Try answering aloud or in your head before turning the card.
Diode
Try answering aloud or in your head before turning the card.
See every definition
- Linear element
- Gives a straight line through the origin on a potential difference–current graph, so its resistance stays constant as current changes.
- Non-linear element
- Gives a curve on the graph, so its resistance changes with current.
- Filament lamp
- A non-linear element that curves because the metal heats and its resistance rises.
- Diode
- Conducts in forward bias and blocks in reverse, so its graph is not a straight line through the origin.
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