OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Use graphs and relate the curves produced to the function and properties…
Full specification wording
Simple circuits: use graphs and relate the curves produced to the function and properties of circuit elements (to include: components such as wire of varying resistance, filament lamps, diodes, NTC thermistors and LDRs)
Learners plot or read current–voltage graphs and use the shape of each curve to identify a component and explain how its resistance behaves.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 10 of 14
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Learners plot or read current–voltage graphs and use the shape of each curve to identify a component and explain how its resistance behaves. A metallic wire at constant temperature gives a straight line through the origin, so resistance stays constant. A filament lamp curves because heating raises resistance, so the current rises less steeply at higher voltages. A diode conducts in one direction only, giving almost no current in reverse and a steep rise once forward voltage is reached. An NTC thermistor drops in resistance as temperature rises, and an LDR drops in resistance as light intensity rises. The gradient of a current–voltage graph is not resistance itself; resistance is found from voltage divided by current at a chosen point.
What earns the marks
- Identifies a straight line through the origin on a current–voltage graph as a component of constant resistance such as a fixed wire.
- Explains that the filament lamp curve flattens because heating increases resistance as current rises.
- Recognises the diode from its one-direction conduction, with negligible current in reverse bias.
- States that an NTC thermistor decreases in resistance as its temperature increases.
- States that an LDR decreases in resistance as light intensity increases.
- Calculates resistance at a point by dividing voltage by current rather than reading the gradient.
Common mistakes and how to put them right
Treats the gradient of a current–voltage graph as the resistance.
Put it right: Divide voltage by current at the point instead.
Assumes every component obeys Ohm's law and gives a straight line.
Put it right: Match each curve to its named component.
Claims a filament lamp's resistance falls as it heats.
Put it right: State that it rises with temperature.
Describes a diode as conducting equally in both directions.
Put it right: Note that it conducts in one direction only.
Exam tips
- Name the component first, then justify the shape using the property that changes, such as temperature or light intensity.
- Quote the axis quantities before interpreting a gradient, because current–voltage and voltage–current graphs differ.
- Use the phrase 'resistance increases' or 'resistance decreases' explicitly rather than saying the line 'bends'.
- Check reverse-bias behaviour for a diode, where current is close to zero.
Try the lesson
The start of the Simple circuits: use graphs and relate the curves produced to the function and properties of circuit elements (to include: components such as wire of varying resistance, filament lamps, diodes, NTC thermistors and LDRs) lesson, made from checked teaching notes for this specification point.
Have a go first
A diode is connected so that it is forward biased and its current–potential difference graph is plotted. Which description matches the graph of a diode?
Listen
Learners plot or read current–voltage graphs and use the shape of each curve to identify a component and explain how its resistance behaves. A metallic wire at constant temperature gives a straight line through the origin, so resistance stays constant. A filament lamp curves because heating raises resistance, so the current rises less steeply at higher voltages. A diode conducts in one direction only, giving almost no current in reverse and a steep rise once forward voltage is reached. An NTC thermistor drops in resistance as temperature rises, and an LDR drops in resistance as light intensity rises. The gradient of a current–voltage graph is not resistance itself; resistance is found from voltage divided by current at a chosen point. A strong answer earns marks for these points. Identifies a straight line through the origin on a current–voltage graph as a component of constant resistance such as a fixed wire. Explains that the filament lamp curve flattens because heating increases resistance as current rises. Recognises the diode from its one-direction conduction, with negligible current in reverse bias. States that an NTC thermistor decreases in resistance as its temperature increases. Exam tip: Name the component first, then justify the shape using the property that changes, such as temperature or light intensity.
See it
Key words
Say what each word means before you turn the card.
Current–voltage characteristics
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Ohmic and non-ohmic behaviour
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Temperature dependence of resistance
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Light-dependent and thermistor sensing
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See every definition
- Current–voltage characteristics
- Graphs of current against voltage whose shape is used to identify a component and explain how its resistance behaves.
- Ohmic and non-ohmic behaviour
- A straight line through the origin shows constant resistance, while a curve shows resistance that changes as current rises.
- Temperature dependence of resistance
- Heating raises the resistance of a filament lamp, so the current rises less steeply at higher voltages.
- Light-dependent and thermistor sensing
- An NTC thermistor drops in resistance as temperature rises, and an LDR drops in resistance as light intensity rises.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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