Interactive practical

Current–potential difference characteristics

Put a resistor, a filament lamp and a diode in turn into a circuit with a reversing switch, read the ammeter and voltmeter in both directions, and compare the three I–V graphs.

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GCSE Physics

Investigating current–potential difference characteristics

AQA 8464 — matched to your course.

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Practical focuses1

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Available question marks4

Practical focus

Measure current–potential difference characteristics and compare electrical behaviour in simple series and parallel circuits.

Practical method

Aim

Investigate how current varies with potential difference for a fixed resistor, a filament lamp and a diode.

Variables

Independent variable: potential difference across the component
Dependent variable: current through the component
Control variables: the same component; temperature of a fixed resistor (small currents, switch off between readings); the same meters.

Apparatus

Low-voltage d.c. supply
Variable resistor
Ammeter
Voltmeter
Fixed resistor, filament lamp and diode (with protective resistor)
Connecting leads

Method

Set up the component in series with an ammeter and variable resistor; connect the voltmeter in parallel with the component.
Adjust the variable resistor to take readings of p.d. and current at about eight values from 0 V to the maximum safe value.
Reverse the supply connections and repeat to get negative values.
For the fixed resistor, switch off between readings so it does not heat up.
Repeat each reading and calculate mean currents.
Plot current (y-axis) against potential difference (x-axis) for each component.

Sample results

Readings for a filament lamp (positive direction).

p.d. / V	Current / A	Resistance / Ω
1.0	0.10	10
2.0	0.16	13
4.0	0.24	17
6.0	0.29	21
8.0	0.33	24
10.0	0.36	28

Analysis

A fixed resistor gives a straight line through the origin, so current is proportional to p.d. and R = \(\dfrac{V}{I}\) is constant (for example 5.0 V ÷ 0.50 A = 10 Ω). The filament lamp's graph curves towards the p.d. axis: as the filament heats, its resistance rises from 10 Ω to 28 Ω. A diode lets current flow in one direction only, and only once the forward p.d. exceeds about 0.6 V.

Errors and improvements

Problem	Effect	Improvement
Resistor heats up during readings	Resistance increases, graph curves	Use small currents and switch off between readings
Too few readings near the diode's switch-on p.d.	Shape of the graph unclear	Take readings at smaller p.d. steps around 0.5–0.8 V
Meter zero error	All readings shifted	Check meters read zero with no current

Risk assessment

Hazard	Risk	Control measure
Lamp or resistor gets hot	Burns	Do not touch components while on; let them cool
Diode passes a large current	Diode overheats	Always include a protective resistor

Common mistakes

Connecting the voltmeter in series.
Calculating resistance from the gradient of a curved graph instead of \(\dfrac{V}{I}\) at a point.
Forgetting the negative readings when the supply is reversed.

Worked analysis

At 6.0 V the lamp current is 0.29 A, so R = \(\dfrac{V}{I}\) = 6.0 ÷ 0.29 ≈ 21 Ω. At 1.0 V and 0.10 A it is 10 Ω, so the resistance has doubled as the filament heated.

Common misconception

The resistance of a filament lamp is not the gradient of its I–V graph. Resistance at any point is V ÷ I at that point.

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