OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(i) I – V characteristics of resistor, filament lamp, thermistor, diode…
Full specification wording
Resistance: (i) I – V characteristics of resistor, filament lamp, thermistor, diode and light-emitting diode (LED)
An I–V characteristic is a graph of current through a component against potential difference across it.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 3 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An I–V characteristic is a graph of current through a component against potential difference across it. For a fixed resistor at constant temperature, current is directly proportional to p.d., so the line is straight through the origin. A filament lamp heats as current rises, so its resistance increases and the graph curves, with a smaller gradient at higher currents. A thermistor's resistance falls as temperature rises, so its I–V curve is non-linear and depends on heating. A diode conducts only above a forward threshold, roughly 0.6 V for silicon, and blocks reverse current; an LED is a diode that emits light when forward biased. You identify a component from the shape, symmetry and threshold of its graph.
What earns the marks
- Fixed resistor: straight line through the origin, because current is proportional to p.d. when temperature is constant.
- Filament lamp: curve through the origin whose gradient decreases as current increases, because the filament temperature and resistance rise.
- Thermistor: non-linear curve; resistance decreases as temperature increases, so the graph is not a straight line.
- Diode: negligible current in reverse bias and a forward threshold near 0.6 V for silicon before current rises steeply.
- LED: forward-biased diode characteristic that emits light; it also has a threshold and blocks reverse current.
Common mistakes and how to put them right
Thinking a filament lamp obeys Ohm's law at all currents.
Put it right: It is non-ohmic because its resistance changes with temperature.
Drawing a diode as a straight line through the origin.
Put it right: A diode has a forward threshold and blocks reverse current.
Confusing thermistor and filament lamp curves.
Put it right: A thermistor's resistance falls with temperature, while a filament lamp's resistance rises with temperature.
Exam tips
- Label axes with current I and potential difference V, and state the component before describing the shape.
- Use the gradient or the ratio V/I to explain resistance changes rather than just saying 'the graph curves'.
- For diode and LED questions, quote the forward threshold and the reverse-bias behaviour.
Try the lesson
The start of the Resistance: (i) I – V characteristics of resistor, filament lamp, thermistor, diode and light-emitting diode (LED) lesson, made from checked teaching notes for this specification point.
Listen
An I–V characteristic is a graph of current through a component against potential difference across it. For a fixed resistor at constant temperature, current is directly proportional to p.d., so the line is straight through the origin. A filament lamp heats as current rises, so its resistance increases and the graph curves, with a smaller gradient at higher currents. A thermistor's resistance falls as temperature rises, so its I–V curve is non-linear and depends on heating. A diode conducts only above a forward threshold, roughly 0.6 V for silicon, and blocks reverse current; an LED is a diode that emits light when forward biased. You identify a component from the shape, symmetry and threshold of its graph. A strong answer earns marks for these points. Fixed resistor: straight line through the origin, because current is proportional to p.d. when temperature is constant. Filament lamp: curve through the origin whose gradient decreases as current increases, because the filament temperature and resistance rise. Thermistor: non-linear curve; resistance decreases as temperature increases, so the graph is not a straight line. Diode: negligible current in reverse bias and a forward threshold near 0.6 V for silicon before current rises steeply. A common slip: Thinking a filament lamp obeys Ohm's law at all currents. Instead, it is non-ohmic because its resistance changes with temperature. Exam tip: Label axes with current I and potential difference V, and state the component before describing the shape.
Key words
Say what each word means before you turn the card.
Ohmic and non-ohmic behaviour
Try answering aloud or in your head before turning the card.
Temperature dependence of resistance
Try answering aloud or in your head before turning the card.
Forward and reverse bias
Try answering aloud or in your head before turning the card.
Graph interpretation
Try answering aloud or in your head before turning the card.
See every definition
- Ohmic and non-ohmic behaviour
- A fixed resistor at constant temperature has current directly proportional to p.d., giving a straight line through the origin, while a filament lamp is non-ohmic because its resistance changes with temperature.
- Temperature dependence of resistance
- A filament lamp's resistance rises as it heats with increasing current, whereas a thermistor's resistance falls as temperature rises.
- Forward and reverse bias
- A diode conducts only above a forward threshold and blocks reverse current, and an LED is a diode that emits light when forward biased.
- Graph interpretation
- A component is identified from the shape, symmetry and threshold of its graph, using the gradient or the ratio V/I to explain resistance changes.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Resistance: (i) I – V characteristics of resistor, filament lamp, thermistor, diode and light-emitting diode (LED)
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge