IGCSE §2: Electricity · IGCSE
Current–Voltage Characteristics
Revision notes on Current–Voltage Characteristics for the OCR A-level Physics specification (H556). Free to read, with 4 practice questions in the app.
A current–voltage characteristic is a graph of current against p.d. for a component. Its shape is the component's fingerprint, and you are expected to recognise four.
Fixed resistor — a straight line through the origin. Doubling the p.d. doubles the current, so the resistance is constant. Ohmic, provided the temperature does not change.
Filament lamp — an S-shaped curve that flattens as the p.d. rises. As more current flows, the filament heats up, its resistance increases, and the current stops rising in proportion. The gradient falls, so the resistance rises with brightness. Not ohmic.
Diode — current flows in one direction only. In forward bias almost nothing flows until about 0.6 V, after which the current rises very steeply. In reverse bias the resistance is extremely high and the current is essentially zero. A diode is used to convert a.c. into d.c., because it removes one half of the alternating cycle.
Light-emitting diode (LED) — behaves like a diode and emits light when conducting. LEDs are used as indicators and in lighting because they are efficient and last a long time.
Two components whose resistance responds to their surroundings:
Light-dependent resistor (LDR) — its resistance decreases as light intensity increases. In the dark it has a high resistance; in bright light, a low one. Used in automatic street lights and in camera exposure circuits.
Thermistor — its resistance decreases as temperature increases (for the common negative-temperature-coefficient type). Used in thermostats, fire alarms and temperature sensors.
Note that both run the opposite way to intuition — more of the thing the sensor detects means less resistance.
Reading resistance from a characteristic. At any point, R = V / I using the coordinates of that point. It is not the gradient of the graph unless the line passes through the origin. On a filament-lamp curve, taking the gradient at a point gives the wrong answer; taking V ÷ I for that point gives the right one.
Example: a filament lamp at 2.0 V carries 0.40 A, so its resistance there is 5.0 Ω. At 8.0 V it carries 1.0 A, so the resistance has risen to 8.0 Ω — the filament is hotter and opposes the current more.
4 questions on Current–Voltage Characteristics
Reading is not revising. This topic has 4 questions in the app — multiple choice, calculations, and written answers marked against the mark scheme, point by point. Miss one and it comes back later with its options reshuffled, so you have to know it rather than remember where the answer sat.
- Every topic of Edexcel International GCSE Physics (4PH1)
- Flashcards and every equation, linked to the topic it comes from
- Full mock papers in the real paper structure
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