Resistors — AQA GCSE Physics
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Resistors explained
An ohmic conductor is one where, at constant temperature, the current is directly proportional to the potential difference.
Read the full explanation
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.
Your focus
- Calculate the current in an ohmic conductor at a new potential difference by scaling a measured pair of readings, without first working out its resistance.
- Determine from a graph the range of potential difference over which a component behaves ohmically.
- Explain why the line must pass through the origin to show direct proportionality.
Show all 4 objectives
- Justify why a metal wire stops behaving ohmically once it warms up.
Resistors exam tips
Marking Points
- one mark for stating that current is directly proportional to potential difference for an ohmic conductor
- one mark for adding the condition that the temperature is constant
- one mark for identifying the straight line through the origin as the evidence for proportionality
- one mark for reading the correct range of potential difference from a graph where the behaviour is ohmic
Examiner 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.
Common Mistakes
- 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