IGCSE CP8 – Thermal Energy Transfer | Edexcel International GCSE Physics

Theory — Thermal Energy Transfer

Thermal energy moves from hotter regions to cooler ones by three mechanisms: conduction, convection, and radiation.

Conduction

Conduction is thermal energy transfer through a solid without bulk movement of the material. In metals, free electrons carry energy rapidly from hot to cool ends — metals are good conductors. The rate depends on the material's thermal conductivity k.

Copper: k ≈ 400 W/m·K — best conductor
Aluminium: k ≈ 205 W/m·K
Brass: k ≈ 109 W/m·K
Iron: k ≈ 80 W/m·K — slowest of the four

Convection

Convection occurs in fluids (liquids and gases). Heated fluid expands, becomes less dense, and rises. Cooler, denser fluid sinks to replace it — forming a convection current. Convection cannot occur in solids because particles cannot move freely.

Radiation

All objects emit and absorb infrared radiation — electromagnetic waves that need no medium. Rate of emission depends on the surface:

Matt black — best emitter and best absorber
Shiny silver — worst emitter and worst absorber (best reflector)
Higher temperature → greater rate of emission

Procedure

Equipment

4 metal strips (copper, aluminium, iron, brass) of equal dimensions · Insulated central disc · Ball bearings · Candle wax · Bunsen burner · Stopwatch · Beaker · Potassium permanganate · Leslie cube · IR detector

1

Conduction: set up the apparatus

Attach the four metal strips to the insulated disc. Fix a ball bearing to each strip end using equal amounts of wax, at the same distance from the centre. Invert so ball bearings hang below.

💡 Equal wax amounts and equal distances from centre = fair test.

2

Heat centre and record drop times

Apply the Bunsen flame to the centre. Start stopwatch. Record the time for each ball bearing to fall. Shorter time = faster conduction = higher k. Repeat and average.

3

Convection: observe fluid flow

Place a crystal of potassium permanganate at the bottom of a beaker of cold water. Heat gently below one side. Observe the purple colour tracing the circular convection current.

4

Radiation: Leslie cube

Fill the Leslie cube with boiling water. Hold an IR detector at equal distances from each face. Compare readings — matt black face emits most, shiny silver emits least.

🔥 Select an experiment below. In Conduction: press Start Heating to begin — watch ball bearings drop as heat travels along each bar.

Experiment

Timings

Elapsed0.0 s

Copper—

Aluminium—

Brass—

Iron—

Questions

Question 1

In the conduction experiment, the copper ball bearing drops after 14 s and the iron one after 52 s. Explain why copper conducts faster than iron, and state one variable that must be controlled to make this a fair test.

Copper has a much higher thermal conductivity (≈400 W/m·K vs ≈80 W/m·K for iron). In metals, free electrons carry energy from hot to cold — copper has more freely-moving electrons, so energy transfers faster. Iron has fewer free electrons and they move less freely, so conduction is slower. Fair test variables: the length of each metal strip from centre to ball bearing; the cross-sectional area of each strip; the amount of wax used; or the distance of the heat source from the centre must all be kept equal.

Question 2

Explain why convection can occur in water but not in a solid metal block, even if both are heated from below.

Convection requires the bulk movement of particles from one region to another. In water (a liquid), particles can move freely past each other. When heated, water at the bottom expands, becomes less dense and rises; cooler denser water sinks to replace it, setting up a convection current. In a solid metal block, particles are held in fixed positions by strong bonds — they can only vibrate about their fixed positions and cannot flow. So bulk movement is impossible, and convection cannot occur; energy in the solid is transferred only by conduction.

Question 3

A Leslie cube has four faces at the same temperature. Predict the order of IR detector readings (highest to lowest) and explain why the surfaces differ.

Order: matt black > dull grey > white > shiny silver. Matt black surfaces emit infrared most effectively — they are the best emitters because they absorb all wavelengths efficiently and emit equally well across all wavelengths. Shiny silver surfaces reflect most infrared rather than emitting it — they are poor emitters. At the microscopic level, rough dark surfaces interact with radiation more strongly than smooth shiny ones. Since all faces are at the same temperature, only surface properties differ. The readings confirm that surface type, not temperature, determines emission rate in this comparison.