AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
The current through an ohmic conductor (at a constant temperature) is…
Full specification wording
Resistors: The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor.
An ohmic conductor is one where, at constant temperature, the current is directly proportional to the potential difference.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 2 of 13
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An ohmic conductor is one where, at constant temperature, the current is directly proportional to the potential difference. Double the potential difference and the current doubles exactly, so a graph of current against potential difference is a straight line passing through the origin. The line through the origin is the part that proves proportionality; a straight line that misses the origin shows a linear relationship but not a proportional one. A fixed resistor and a length of metal wire behave this way provided they do not warm up. Even a filament lamp is ohmic over a small range near the origin, typically from 0 V to about 2 V, where the current is still too small to heat the filament noticeably.
What earns the marks
- Stating that current is directly proportional to potential difference for an ohmic conductor.
- Adding the condition that the temperature is constant.
- Identifying the straight line through the origin as the evidence for proportionality.
- Reading the correct range of potential difference from a graph where the behaviour is ohmic.
Common mistakes and how to put them right
Describing the relationship as linear when the question requires directly proportional.
Omitting the constant temperature condition, which is where the mark usually sits.
Accepting a straight line that does not pass through the origin as proof of proportionality.
Reading the ohmic range of a lamp graph from the flattest section rather than from the origin.
Exam tips
- Give both halves of the definition: directly proportional and at constant temperature.
- Support a range answer from a graph with the reason, such as the line is straight and passes through the origin.
- Ranges are usually credited generously, for instance any upper limit between 1 V and 2 V, provided the reasoning is right.
Try the lesson
The start of the Resistors: The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor. lesson, made from checked teaching notes for this specification point.
Have a go first
For an ohmic conductor at constant temperature, what is the relationship between current and potential difference?
Listen
An ohmic conductor is one where, at constant temperature, the current is directly proportional to the potential difference. Double the potential difference and the current doubles exactly, so a graph of current against potential difference is a straight line passing through the origin. The line through the origin is the part that proves proportionality; a straight line that misses the origin shows a linear relationship but not a proportional one. A fixed resistor and a length of metal wire behave this way provided they do not warm up. Even a filament lamp is ohmic over a small range near the origin, typically from 0 V to about 2 V, where the current is still too small to heat the filament noticeably. A strong answer earns marks for these points. stating that current is directly proportional to potential difference for an ohmic conductor. adding the condition that the temperature is constant. identifying the straight line through the origin as the evidence for proportionality. reading the correct range of potential difference from a graph where the behaviour is ohmic. Exam tip: Give both halves of the definition: directly proportional and at constant temperature.
See it
Key words
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ohmic conductor
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direct proportionality
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constant temperature
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straight line through the origin
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See every definition
- ohmic conductor
- A conductor where, at constant temperature, the current is directly proportional to the potential difference.
- direct proportionality
- The relationship where doubling the potential difference doubles the current exactly, shown by a straight line passing through the origin.
- constant temperature
- The condition that must hold for an ohmic conductor, so that current is directly proportional to potential difference.
- straight line through the origin
- The evidence for proportionality on a graph of current against potential difference; a straight line that misses the origin shows a linear relationship but not a proportional one.
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