Interactive practical

I-V characteristics of a lamp and metal wire

Read signed voltage and current for a wire at constant temperature and a filament lamp, then compare their I–V characteristics.

Open interactive simulation

A-Level Physics

I-V characteristics of a lamp and metal wire

WJEC 1420QS — matched to your course.

These revision notes are free. Premium unlocks the full question and retrieval-card bank for this topic.

Revision hub

At a glance

Start practice

Practical focuses1

Questions available2

Retrieval cards available1

Available question marks4

Practical focus

Measure current-voltage characteristics for a filament lamp and a metal wire while controlling temperature where required.

Practical method

Aim

Investigate the current–p.d. characteristics of a metal wire at constant temperature and a filament lamp.

Variables

Independent variable: p.d. across the component
Dependent variable: current
Control variables: temperature of the wire (water bath); the same components.

Apparatus

Metal wire in a water bath
12 V filament lamp
Variable supply or potential divider
Ammeter and voltmeter
Switch

Method

Connect the component with an ammeter in series and a voltmeter in parallel, supplied through a potential divider so p.d. can start from zero.
Record current for p.d. values in both directions (reverse the connections for negative values).
Keep the wire in a water bath so its temperature is constant.
Repeat for the filament lamp, waiting for the filament to settle at each setting.
Plot I against V for each component and calculate R = \(\dfrac{V}{I}\) at several points.

Sample results

Positive values shown; negative values are symmetrical.

V / V	I wire / A	I lamp / A	R lamp / Ω
1.0	0.10	0.14	7.1
2.0	0.20	0.22	9.1
4.0	0.40	0.32	12.5
8.0	0.80	0.45	17.8
12.0	1.20	0.55	21.8

Analysis

The wire at constant temperature is ohmic: I ∝ V with R = 10 Ω. The lamp's graph curves towards the V axis: its resistance rises from 7 Ω to 22 Ω because the filament heats up and lattice vibrations increase.

Uncertainty

The lamp takes time to reach a steady temperature, so readings taken too soon are unreliable. Meter resolution (±0.01 A) gives about 2% at the lowest currents.

Errors and improvements

Problem	Effect	Improvement
Wire heating	Not ohmic at high current	Keep it in a water bath, use small currents
Lamp readings taken before the filament settles	Inconsistent values	Wait for steady readings
Starting p.d. not zero	Missing low-voltage data	Use a potential divider

Risk assessment

Hazard	Risk	Control measure
Hot lamp	Burns	Do not touch the bulb
Exceeding the lamp rating	Bulb failure	Stay within the rated voltage

Common mistakes

Calculating resistance as the gradient of an I–V curve.
Not reversing the p.d.
Forgetting the lamp's resistance depends on temperature.

Worked analysis

Lamp at 8.0 V and 0.45 A: R = \(\dfrac{V}{I}\) = \(\dfrac{8.0}{0.45}\) = 17.8 Ω. At 1.0 V and 0.14 A, R = 7.1 Ω: resistance more than doubles as the filament heats.

Common misconception

Resistance is \(\dfrac{V}{I}\) at a point, not the gradient of the I–V graph.

Assessment coverage

Practice this practical

2 questions available, worth 4 marks in total.

Practical 2

Foundation 1Standard 1

Practise this practical