I–V Characteristics
Build confidence in I–V Characteristics for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A fixed resistor gives a straight line, a filament lamp gives a curve because it heats up, and a diode conducts in one direction only.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
An I–V characteristic graph shows how the current through a component varies with the potential difference across it. This is a required practical.
Three shapes must be recognised. A fixed resistor at constant temperature gives a straight line through the origin, because its resistance is constant. A filament lamp gives an S-shaped curve that flattens, because the filament heats up and its resistance increases.
A diode gives almost no current in the reverse direction and rising current in the forward direction, because it has very high resistance one way. To obtain the reverse readings, the component is disconnected and reconnected the other way round.
Revision notes
The fixed resistor
A straight line through the origin, because the resistance is constant.
Current is directly proportional to potential difference, so the resistor is an ohmic conductor. This holds only while the temperature stays constant.
The filament lamp
An S-shaped curve that becomes less steep at higher potential differences.
As the current increases, the filament gets hotter. The metal ions vibrate more, making it harder for electrons to pass, so the resistance increases and the graph flattens.
The diode
Almost no current flows in the reverse direction, because the resistance is very high that way.
In the forward direction the current rises sharply once the potential difference is large enough. To measure the reverse characteristic, disconnect the diode and reconnect it the other way round.
Key points
- A fixed resistor gives a straight line through the origin.
- Its resistance is constant, so it is ohmic.
- A filament lamp gives an S-shaped curve.
- The lamp's resistance increases as it heats up.
- A diode conducts in one direction only.
- Reverse the component to obtain reverse readings.
Worked examples
Example 1
Explain the shape of the I–V graph for a filament lamp. [3 marks]
Model answer
As the current increases, the filament becomes hotter (1 mark). The metal ions vibrate more, making it harder for the electrons to pass through (1 mark), so the resistance increases and the graph becomes less steep (1 mark).
Example 2
Explain why a fixed resistor at constant temperature gives a straight line through the origin. [2 marks]
Model answer
Its resistance stays constant (1 mark), so the current is directly proportional to the potential difference across it (1 mark).
Example 3
Describe the I–V characteristic of a diode. [2 marks]
Model answer
In the forward direction the current rises once the potential difference is large enough (1 mark). In the reverse direction almost no current flows, because the resistance is very high (1 mark).
Common mistakes
Saying the filament lamp is ohmic.
Its resistance changes with temperature, so it is not.
Not explaining the mechanism for the lamp.
The marks come from ions vibrating more and impeding electron flow.
Saying a diode has zero resistance forwards.
It has low resistance forwards and very high resistance in reverse.
Forgetting to reverse the component for negative readings.
The component must be disconnected and reconnected the other way round.
Exam tips
- Learn all three graph shapes and be able to sketch them.
- Explain the lamp through ion vibration and increasing resistance.
- Say directly proportional for the fixed resistor.
- Mention reversing the component for the diode practical.
Key terms
- I–V characteristic
- A graph of current against potential difference.
- Ohmic conductor
- A component with constant resistance.
- Filament
- The thin wire in a lamp that heats and glows.
- Diode
- A component allowing current in one direction only.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.