Interactive practical

Methods of heat transfer

Time pins dropping from copper, aluminium, brass and steel rods in hot water to compare conduction, then watch a convection current carry dye around a heated beaker.

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GCSE Physics

Investigating methods of heat transfer

WJEC 3430QD — matched to your course.

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Practical focus

Investigate conduction, convection and thermal radiation using controlled observations and measurements.

Practical method

Aim

Investigate conduction in different metals, convection in water and radiation from different surfaces.

Variables

Independent variable: metal type (conduction) or surface finish (radiation)
Dependent variable: time for wax markers to fall, or temperature rise
Control variables: rod length and diameter; distance of markers from the flame; distance from the radiant source; starting temperature.

Apparatus

Rods of copper, aluminium, brass and steel of equal size
Wax and small pins
Bunsen burner and tripod
Beaker of water and a crystal of potassium manganate(VII) or food colouring
Two thermometers with matt black and shiny silver bulbs
Lamp or radiant heater
Stopwatch

Method

Conduction: fix a pin with wax to the end of each rod at the same distance from the centre. Heat the centres of the rods equally and time how long each pin takes to fall.
Convection: drop a small crystal of colouring at the bottom edge of a beaker of water and heat gently below it. Observe the coloured water rise above the flame, move across and sink on the other side.
Radiation: place a matt black and a shiny silver thermometer bulb the same distance from a lamp. Record both temperatures every minute for 5 minutes.
Repeat the conduction and radiation tests and compare results.

Sample results

Conduction results (time for the wax pin to fall, mean of two runs).

Metal	Time / s	Rank (best conductor = 1)
Copper	45	1
Aluminium	70	2
Brass	120	3
Steel	210	4

Analysis

Copper's pin fell first (45 s), so copper is the best thermal conductor of these metals. In the convection test, heated water expands, becomes less dense and rises, while cooler, denser water sinks to replace it. In the radiation test the matt black bulb warmed faster than the shiny silver one: matt black surfaces are better absorbers (and emitters) of infrared.

Errors and improvements

Problem	Effect	Improvement
Rods heated unequally	Unfair comparison	Arrange rods symmetrically around one flame (conduction ring)
Different amounts of wax	Pins fall at different temperatures	Use the same small amount of wax
Thermometers at different distances from the lamp	Unfair radiation comparison	Measure and fix equal distances

Risk assessment

Hazard	Risk	Control measure
Bunsen flame and hot rods	Burns	Use a heat-proof mat; let rods cool before touching
Potassium manganate(VII)	Harmful, stains skin	Use one small crystal with tweezers; wear eye protection

Common mistakes

Saying heat rises (it is the warm fluid that rises).
Describing convection in solids.
Saying shiny surfaces absorb more infrared.

Worked analysis

Copper pin falls after 45 s and steel after 210 s. Copper conducts about 210 ÷ 45 ≈ 4.7 times faster in this set-up, so it would be the better choice for the base of a saucepan.

Common misconception

Heat does not rise. In convection the warmer, less dense fluid rises and cooler, denser fluid sinks.

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