How do the current-voltage graphs of a resistor, a lamp and a diode differ?
The current-voltage characteristics of an ohmic resistor, a filament lamp and a diode, and what each graph shows about resistance.
A CCEA GCSE Physics answer on the current-voltage characteristic graphs of an ohmic resistor, a filament lamp and a diode, and how each graph shows the way resistance changes.
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What this dot point is asking
CCEA wants you to describe and explain the current-voltage (I-V) characteristics of an ohmic resistor (at constant temperature), a filament lamp, and a diode, and to say what each graph reveals about resistance. This is also a practical-skills topic.
The answer
Measuring an I-V characteristic
To find an I-V characteristic, you vary the voltage across a component, measure the current through it (ammeter in series) and the voltage across it (voltmeter in parallel), and plot current () against voltage (), often for both directions.
The ohmic resistor
The filament lamp
The diode
Worked example: identifying a component from its graph
Examples in context
- Example 1. A heating element
- As a heater warms up, its resistance rises slightly, so the current it draws falls a little, an everyday example of resistance changing with temperature like the filament lamp.
- Example 2. Charging a phone
- A diode (or set of diodes) in a charger lets current flow only the correct way, converting alternating mains current into the one-direction (direct) current the phone battery needs.
- Example 3. A protective diode
- A diode placed in a circuit can stop current flowing if a battery is put in the wrong way round, protecting delicate components. Because the diode has a very high resistance in reverse, almost no current flows the wrong way.
When you describe an I-V characteristic in the exam, name the shape and then give the reason. For the resistor, say "straight line, constant resistance". For the lamp, say "curve, resistance rises because the filament heats up". For the diode, say "current one way only, very high resistance in reverse". Linking the shape to the resistance is what earns the explanation marks.
Try this
Q1. What shape is the I-V graph for an ohmic resistor at constant temperature? [1 mark]
- Cue. A straight line through the origin.
Q2. Why does the I-V graph of a filament lamp curve? [2 marks]
- Cue. The filament heats up as current rises, so its resistance increases.
Q3. In which direction does a diode allow current to flow? [1 mark]
- Cue. In one direction only (the forward direction).
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksDescribe the shape of the current-voltage graph for a filament lamp and explain why it has that shape.Show worked answer →
The graph is a curve (an S-shape through the origin): the current increases less and less steeply as the voltage rises, so the line curves towards the voltage axis.
As the current increases, the filament gets hotter. The hotter filament has a higher resistance, so the current does not rise in proportion to the voltage.
Markers reward: a curve (not a straight line) showing current rising less steeply at higher voltage; and the explanation that the filament heats up and its resistance increases.
CCEA style3 marksDescribe and explain the current-voltage characteristic of a diode.Show worked answer →
A diode only lets current flow in one direction (the forward direction). In the forward direction, very little current flows until a small threshold voltage is reached, then the current rises sharply.
In the reverse direction almost no current flows, because the diode has a very high resistance that way.
Markers reward: current in one direction only; little current until a threshold then a sharp rise; and very high resistance (no current) in reverse.
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Sources & how we know this
- CCEA GCSE Physics specification — CCEA (2017)