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Physics required practicals

I-V characteristics

AQA 4.2 · RP4

GCSE Physics (8463) · Required practical 4 — method, variables, the marks examiners report students losing.

Board and spec code confirmed against AQA 8463 · registry checked 2026-07-17How this checking works

Investigate the current-potential difference (I-V) characteristics of circuit elements: a fixed resistor at constant temperature, a filament lamp and a diode.

Apparatus

  • Component under test: a fixed resistor, a filament lamp and a diode (with a protective resistor)
  • Ammeter connected in series with the component
  • Voltmeter connected in parallel across the component
  • Variable resistor (rheostat) or variable power supply to change the p.d.
  • Cell or power supply, switch and connecting leads

Method

  1. 1Connect the component in series with the ammeter and the variable resistor; connect the voltmeter across the component.
  2. 2Use the variable resistor to set the potential difference to a small value and record V and the current I.
  3. 3Increase the p.d. in steps, recording I at each value.
  4. 4Reverse the connections to the component to get negative p.d. and current values.
  5. 5Plot current (y-axis) against potential difference (x-axis) for each component; the diode needs a protective resistor in series to limit the current.

Variables

Independent

Potential difference across the component

Dependent

Current through the component

Control

  • The component being tested
  • Temperature (for the fixed resistor)
  • The rest of the circuit

Results & processing

  • Fixed resistor (constant temperature): a straight line through the origin — current is proportional to p.d. (ohmic).
  • Filament lamp: an S-shaped curve that gets less steep as p.d. rises, because the filament heats up and its resistance increases.
  • Diode: current flows in one direction only; its resistance is very high in the reverse direction.

Where students lose marks

Placing the meters wrongly.

Fix: Ammeter in series with the component, voltmeter in parallel across it — swapping them gives meaningless readings.

Letting the fixed resistor heat up.

Fix: Take readings quickly (or keep the current low) so the resistor stays at constant temperature, or its line will curve too.

Testing the diode without a protective resistor.

Fix: A diode has very low forward resistance, so a series protective resistor is needed to stop a large current damaging it.

Improve the method

  • Reverse the connections to record negative values and get the full characteristic.
  • Take readings quickly so a fixed resistor stays at constant temperature.
  • Repeat readings and take a mean current at each p.d.

Try it — exam-style

Medium
ORIGINAL

Describe the shape of the I-V graph for a filament lamp and explain why it has this shape.

[3 marks]

Total for this question: 3

Medium
ORIGINAL

A fixed resistor obeys Ohm's law. At 6.0 V the current through it is 0.50 A. Calculate the current when the p.d. is 3.0 V at the same temperature.

[3 marks]

Total for this question: 3

Questions are written in the style of past AQA papers — never copied from them.

Drill it properly

AQA GCSE Physics flashcards

Component characteristics and Ohm's law, on a review schedule.

Physics equations

V = I x R and the electrical equations behind this practical.

Stuck on i-v characteristics?

The three characteristic graphs and their explanations are commonly examined — I make sure you can draw and justify all three. Free intro call, then a free first lesson.

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