AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
For an ohmic conductor, semiconductor diode, and filament lamp.
Full specification wording
Current–voltage characteristics: For an ohmic conductor, semiconductor diode, and filament lamp.
Current–voltage characteristics show how current through a component varies with potential difference.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 1 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Current–voltage characteristics show how current through a component varies with potential difference. For an ohmic conductor at constant temperature, current is directly proportional to potential difference, so the I–V graph is a straight line through the origin and resistance is constant. A semiconductor diode conducts easily in forward bias above about 0.6 V, but has very high resistance in reverse bias, giving an asymmetric curve. A filament lamp's resistance increases as current increases because the filament temperature rises, so the I–V graph curves and flattens.
What earns the marks
- Describes the I–V graph for an ohmic conductor as a straight line through the origin, showing I ∝ V at constant temperature.
- States that resistance is constant for an ohmic conductor and calculates it from the gradient or from V/I.
- Describes the forward and reverse bias behaviour of a semiconductor diode, including the threshold voltage and very high reverse resistance.
- Explains that a diode conducts when forward biased above about 0.6 V and does not conduct significantly in reverse bias.
- Describes the I–V graph for a filament lamp as a curve that flattens as current increases.
- Explains that the resistance of a filament lamp increases because the filament temperature rises, increasing lattice vibrations and reducing electron drift.
- Compares the three characteristics and identifies which is ohmic and which are non-ohmic.
Common mistakes and how to put them right
Treating the filament lamp as ohmic and drawing a straight line.
Put it right by drawing a curve that flattens as current increases.
Drawing the diode as a symmetric curve.
Put it right by showing negligible current in reverse bias and a sharp increase in forward bias above the threshold.
Saying resistance decreases as temperature increases for a filament lamp.
Put it right by explaining that increased temperature increases resistance.
Confusing the gradient of an I–V graph with resistance.
Put it right by using R = V/I for a point on the graph, not the gradient.
Exam tips
- Label axes with quantity and unit, and mark the origin clearly when sketching characteristics.
- For the diode, indicate the forward threshold voltage and the reverse breakdown region if shown.
- When explaining the filament lamp, link temperature rise to increased lattice vibrations and reduced electron drift, not just 'more resistance'.
- Use the phrase 'at constant temperature' when describing ohmic behaviour.
Try the lesson
The start of the Current–voltage characteristics: For an ohmic conductor, semiconductor diode, and filament lamp. lesson, made from checked teaching notes for this specification point.
Have a go first
Which current–voltage characteristic is a straight line through the origin, showing that current is directly proportional to potential difference?
Listen
Current–voltage characteristics show how current through a component varies with potential difference. For an ohmic conductor at constant temperature, current is directly proportional to potential difference, so the I–V graph is a straight line through the origin and resistance is constant. A semiconductor diode conducts easily in forward bias above about 0.6 V, but has very high resistance in reverse bias, giving an asymmetric curve. A filament lamp's resistance increases as current increases because the filament temperature rises, so the I–V graph curves and flattens. A strong answer earns marks for these points. Describes the I–V graph for an ohmic conductor as a straight line through the origin, showing I ∝ V at constant temperature. States that resistance is constant for an ohmic conductor and calculates it from the gradient or from V/I. Describes the forward and reverse bias behaviour of a semiconductor diode, including the threshold voltage and very high reverse resistance. Explains that a diode conducts when forward biased above about 0.6 V and does not conduct significantly in reverse bias. A common slip: Treating the filament lamp as ohmic and drawing a straight line. Instead, drawing a curve that flattens as current increases. Exam tip: Label axes with quantity and unit, and mark the origin clearly when sketching characteristics.
See it
Key words
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Ohmic conductor
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Semiconductor diode
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Filament lamp
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Non-ohmic behaviour
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See every definition
- Ohmic conductor
- At constant temperature, current is directly proportional to potential difference, so the I–V graph is a straight line through the origin and resistance is constant.
- Semiconductor diode
- Conducts easily in forward bias above about 0.6 V, but has very high resistance in reverse bias, giving an asymmetric curve.
- Filament lamp
- Its resistance increases as current increases because the filament temperature rises, so the I–V graph curves and flattens.
- Non-ohmic behaviour
- Shown by components whose I–V graph is not a straight line through the origin, such as the diode and the filament lamp.
Label the diagram
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