IGCSE Physics / Electrical resistance
Current-voltage graphs for components revision notes
Edexcel International GCSE Physics (4PH1), chapter 8: Electrical resistance, specification point 2.9. Free lesson.
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I-V graphs show how current changes as voltage changes for different components. Their shapes reveal whether resistance stays constant, rises as a component heats up, or depends on the direction of the current.
What an I-V graph shows
An I-V graph shows how the current through a component changes as the voltage across it changes. Voltage is on the horizontal axis, in volts, and current is on the vertical axis, in amps. A resistor or wire at constant temperature gives a straight line through the origin. This means current is proportional to voltage: if voltage doubles, current doubles. The resistance stays constant.
How to find resistance from a graph
Choose a point on the graph and use R = V ÷ I. Do not use the gradient. With current on the vertical axis and voltage on the horizontal axis, the gradient is I ÷ V, which is 1 ÷ R. Resistance is measured in ohms (Ω). For a resistor, the point at 4 V and 0.2 A gives R = 4 ÷ 0.2 = 20 Ω.
Why a filament lamp's graph flattens
A filament lamp's current rises steeply at first, then its graph flattens. Each extra volt adds less current than the previous volt, so the lamp's resistance increases. At 2 V and 0.2 A, R = 2 ÷ 0.2 = 10 Ω. At 6 V and 0.3 A, R = 6 ÷ 0.3 = 20 Ω. As voltage and current increase, the filament gets hotter. Its metal ions vibrate more, causing more collisions with electrons. This makes it harder for current to flow, so resistance rises.
How a diode behaves
A diode allows current through in one direction only. Its graph uses positive and negative voltages to show the diode connected in either direction. In the forward direction, almost no current flows at first. Past a small voltage, the current rises steeply. In reverse, the current stays at zero. This means the diode has a very high resistance in that direction.
Key graph patterns
You can identify a component by the shape of its I-V graph. Connect the shape to what is happening to its resistance. A resistor or wire at constant temperature: straight line through the origin, with constant resistance. A filament lamp: curve that flattens as resistance rises with temperature. A diode: current flows in one direction, with very high resistance in reverse.