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Current–voltage characteristics practical

Connect an ammeter and voltmeter correctly, interpret resistor and lamp data, and evaluate heating.

Practise the calculations · Print a classroom set

Apparatus and measurements

Use a low-voltage supply, switch, variable resistor and ammeter in series with the component. Connect a voltmeter in parallel across the component. A diode needs a suitable series current-limiting resistor.

AVSupplyVariable resistorSwitchTest component
Low-voltage supply, switch, ammeter and variable resistor in series with the test component. Voltmeter in parallel across the measured component or wire section. Schematic connections.
Larger apparatus diagram
AVSupplyVariable resistorSwitchTest component

Method and controls

  1. Begin with a small current. Change the variable resistance or supply setting and record the component current and potential difference together.
  2. For negative readings, switch off and reverse the supply polarity, then repeat. Keep the same instrument sign convention. Never reconnect a circuit while the supply is on.
  3. Plot current vertically against potential difference horizontally. A constant-temperature resistor gives a straight line through the origin; a filament lamp’s changing temperature produces a curve.

Results to analyse

Original synthetic data for practice.

Current–voltage characteristics practical: sample readings
Potential difference / VResistor current / ALamp current / A
000
10.100.15
20.200.24
30.300.30

Questions

  1. Find the resistor resistance at 2 V. What is the gradient of its current–voltage graph?
  2. Find the lamp resistance at 1 V and 3 V. Explain its change.
  3. Why is an ammeter connected in parallel across the supply a poor connection?
Show worked answers
  1. R = V/I = 2/0.20 = 10 Ω. The I–V gradient is 0.10 A V⁻¹ = 1/R, not R.
  2. At 1 V, R = 1/0.15 = 6.67 Ω; at 3 V, R = 3/0.30 = 10.0 Ω. The hotter filament has greater resistance.
  3. An ammeter has low resistance. Across the supply it can draw a large current and damage the meter or circuit. It belongs in series.

Evaluation

Switch off between readings when temperature should stay constant. A filament lamp deliberately changes temperature during its characteristic, so do not describe all component heating as an unwanted effect.

Use suitable instrument ranges and a current-limited school supply. A diode’s reverse current is very small over its normal range; avoid destructive reverse voltages.

Specification links

AQA 8463: Required practical 4; 4.2.1.4. OCR Gateway J249: P3.2; PAG P6 (selected). Selected skills only; no practical-completion assessment.

AQA practical requirements · OCR Gateway specification

Continue with calculation practice · Graph skills · All six practical guides