Energy Transfers » Practical: Conduction, Convection and Radiation
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 4.9
- How to compare metals as conductors using rods, wax and drawing pins
- How a coloured crystal in a beaker of water shows a convection current
- How black and silver cans of hot water compare surfaces as emitters of radiation
- The variables, results, errors and safety points examiners ask about
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Unlock This CourseThree experiments, one practical
This practical has three parts, one for each way that thermal energy is transferred. You do not need a school laboratory to answer questions on it. You need to describe each method, say what is seen or measured, and explain how to make it a fair test.
Key terms:
- Potassium manganate(VII): a dark purple crystal that dissolves in water and colours it purple, so you can see the water move.
- Smoke box: a closed box with a glass front and two chimneys in its lid, used to show convection in air.
- Leslie cube: a hollow metal cube whose four side faces have different surfaces, used to compare emitters.
- Infrared detector: an instrument that gives a reading for the amount of infrared falling on it.
Conduction: metal rods and wax
Remember, conduction passes energy along a solid from the hot end to the cold end.
Apparatus: four rods of different metals (copper, aluminium, brass and iron), all the same length and thickness. They rest on a tripod with one end of each rod meeting at the same point, like the spokes of a wheel.
- Stick a drawing pin to the outer end of each rod, using the same sized blob of wax each time.
- Arrange the rods on the tripod with their inner ends close together.
- Heat the inner ends with a Bunsen burner flame so that all four are heated equally. Start a stopwatch.
- Energy is conducted along each rod. When the far end is hot enough, the wax melts and the pin drops off.
- Record the time at which each pin falls, or just the order in which they fall.
| Metal | Time for the pin to fall (s) | Order |
|---|---|---|
| Copper | 40 | 1st |
| Aluminium | 65 | 2nd |
| Brass | 130 | 3rd |
| Iron | 210 | 4th |
In these typical results the pin on the copper rod falls first, so copper is the best conductor of the four. Iron is the poorest. A simpler version uses wax alone and watches the order in which it melts.
Fair test and errors
Independent variable: the metal. Dependent variable: the time for the pin to fall. Control variables: the length and thickness of the rods, the size of the wax blob, the distance of the wax from the heated end, and equal heating of each rod.
Errors: different sized blobs of wax melt at different times. A flame that is off-centre heats one rod more than the others. Repeat the experiment and compare the order.
Convection: a coloured crystal in water
Remember, in convection the heated part of a fluid rises and cooler fluid sinks to take its place. Water is clear, so a purple crystal is used to make the moving water visible.
- Fill a large beaker with cold water and stand it on a tripod and gauze. Leave it until the water is still.
- Hold a glass tube upright in the water with its lower end on the bottom, near one side of the beaker.
- Using tweezers, drop one crystal of potassium manganate(VII) down the tube. Put a finger over the top of the tube and lift it out gently. The crystal is left at the bottom without colouring the rest of the water.
- Heat the beaker gently with a small flame directly under the crystal.
- Watch the purple colour and draw the path it takes.
Results: a purple stream rises straight up from the crystal above the flame. At the surface it spreads across the top of the water. It then sinks down the far, cooler side and moves back along the bottom towards the flame. The colour traces out a complete loop: a convection current.
Keep the same: one small crystal, cold water that is still at the start, and a small flame under the crystal only.
Errors: nothing is measured here. It goes wrong if the crystal is dropped straight in (it leaves a purple trail all the way down), if the water is still swirling, or if the flame is too strong, which mixes the water so fast that the whole beaker turns purple and the loop cannot be seen.
Radiation: black and silver cans
Remember, a dull black surface is the best emitter of radiation and a shiny silver surface is the worst.
- Take two metal cans of the same size and shape. One has a dull black outside and the other a shiny silver outside. Stand both on a heatproof mat.
- Pour the same volume of hot water from a kettle into each can.
- Put a lid on each can, with a thermometer through a hole in the lid.
- When both thermometers show the same temperature, start a stopwatch.
- Record the temperature of the water in each can every 5 minutes for 20 minutes.
| Time (minutes) | 0 | 5 | 10 | 15 | 20 |
|---|---|---|---|---|---|
| Dull black can (°C) | 85 | 75 | 67 | 61 | 56 |
| Shiny silver can (°C) | 85 | 79 | 74 | 70 | 66 |
Worked example
Use the table to compare the two cans.
Step 1: Temperature fall of the black can = 85 − 56 = 29 °C.
Step 2: Temperature fall of the silver can = 85 − 66 = 19 °C.
Step 3: The water in the black can cooled more in the same time. So the dull black surface emitted more radiation each second: it is the better emitter.
Fair test and errors
Independent variable: the surface of the can. Dependent variable: the temperature of the water (or its fall in 20 minutes). Control variables: the size, shape and material of the cans, the volume of water, the starting temperature, the time, and the same place in the room, away from draughts.
Why the lids? Without a lid, a lot of energy leaves from the open water surface. That loss would be the same for both cans and would hide the difference made by the two surfaces.
Errors: read each thermometer at eye level, and at exactly the right time.
Three other versions to know
💨 Smoke box (convection)
A lit candle stands under one chimney. A smouldering taper (a thin waxed wick that gives off smoke once its flame is blown out) is held above the other. The smoke is drawn down that chimney, across the box and up the chimney above the candle. This is a convection current in air: hot air rises above the candle and cooler air sinks to replace it.
📦 Leslie cube (emission)
The cube is filled with hot water, so all four faces are at the same temperature. An infrared detector is held the same distance from each face in turn. The dull black face gives the highest reading and the shiny silver face the lowest.
🔥 Plates near a heater (absorption)
A dull black metal plate and a shiny silver one stand the same distance either side of a heater. A drawing pin is stuck to the back of each with wax. The pin on the black plate falls first, so dull black is the better absorber.
Safety
- Hot apparatus: rods, tripods and beakers stay hot long after heating stops. Leave them to cool before touching them, and work on a heatproof mat.
- Hot water: can scald. Pour it carefully.
- Eye protection: wear it whenever a Bunsen burner is in use or hot water is being heated.
- The crystal: potassium manganate(VII) is harmful and stains skin, so handle it with tweezers.
Common mistakes
- Saying the black can 'loses heat faster' with no reason. Say that the dull black surface is a better emitter of (infrared) radiation.
- Mixing up the two radiation experiments. Cans of hot water and the Leslie cube test emission. Surfaces placed near a heater test absorption.
- Vague control variables. 'Same amount of water' is vague and may not get the mark. Write 'same volume of water'.
Exam-style question
A student investigates thermal energy transfer.
(a) She fixes a drawing pin to one end of each of four metal rods with wax. She heats the other ends equally and times how long each pin takes to fall.
| Metal | Aluminium | Brass | Copper | Iron |
|---|---|---|---|---|
| Time for the pin to fall (s) | 55 | 115 | 35 | 190 |
(i) State which metal is the best conductor. Give a reason for your answer. [2 marks]
(ii) State two variables the student should control. [2 marks]
(b) The student places a crystal of potassium manganate(VII) at the bottom of a beaker of cold water, near one side. She heats the beaker gently below the crystal. Describe the path taken by the coloured water. [3 marks]
(c) A dull black plate and a shiny silver plate are placed the same distance from a heater. A drawing pin is fixed to the back of each plate with wax. Explain why the pin on the dull black plate falls first. [2 marks]
Model answer
(a)(i) Copper (1). Its pin fell in the shortest time, so energy was conducted along the copper rod fastest (1).
(a)(ii) Any two from: length of the rods (1), thickness of the rods (1), size of the wax blob (1), distance of the wax from the heated end (1). Maximum 2 marks.
(b) The coloured water rises above the flame (1). It moves across the top of the water (1). It sinks at the other (cooler) side and returns along the bottom (1).
(c) Dull black is a better absorber of (infrared) radiation than shiny silver (1). So the black plate heats up faster and its wax melts first (1).
Exam tip
Part (b) says describe, so write what you would see, in order: up, across, down, back. In part (c) the plates are taking energy in, so the key word is absorber, not emitter.