9.10 - Methods of Heat Transfer
The experiment
Aim
To observe energy being transferred by conduction, by convection and by radiation, and to compare how well different materials and different surfaces do it.
Background
- Energy moves from hotter places to cooler ones by conduction, convection, and radiation.
- In conduction, particles at the hot end vibrate faster and pass energy to their neighbours. In a metal, free electrons also carry energy, which is why metals conduct far better than anything else.
- In convection, a liquid or gas itself moves and carries the energy with it, in a loop called a convection current.
- Radiation travels as infrared waves and needs no material. How much a surface absorbs depends on whether it's dark or pale, and dull or shiny.
- You can't see energy moving, but you can see what it does.
- So in this practical you time how long Vaseline takes to melt on rods of different metals, follow a purple colour round heated water, and compare how much two thermometer bulbs warm up in front of a lamp.
Variables
- Conduction - The independent variable is the metal, and the dependent variable is the time for the pin to fall off. The control variables are the size of the rods and of the Vaseline blobs, how far each rod sits in the flame, and heating all four together.
- Convection - This part is an observation, so nothing is changed and nothing is measured.
- Radiation - The independent variable is the surface on the bulb, and the dependent variable is the temperature rise in ten minutes. The control variables are the distance from the lamp, the time, and the starting temperature.
Method - Conduction
- Choose the rods - Take four rods of the same length and thickness, made of copper, aluminium, brass, and iron.
- Attach the pins - Stick a drawing pin to the far end of each rod with a blob of Vaseline, the same size each time.
- Heat the rods - Put one end of each rod the same distance into the same heat source, heat them together, and start the stopwatch.
- Time each pin - Record the time at which each pin drops off.
Method - Convection
- Let the water settle - Fill a beaker with cold water and leave it until it's still.
- Place the crystal - Stand a glass tube in the beaker, drop a crystal of potassium manganate(VII) down it with tweezers, and lift the tube out.
- Heat gently - Heat the beaker with a small blue flame directly under the crystal.
- Watch the colour - Record the path the purple colour takes.
Method - Radiation
- Cover the bulbs - Tape matt black paper over the bulb of one thermometer, and shiny aluminium foil over the bulb of another.
- Face them to the lamp - Stand both thermometers 10 cm from a filament lamp, facing it squarely.
- Take readings - Record both starting temperatures, switch the lamp on, and record both temperatures again after ten minutes.
Safety
- Potassium manganate(VII) - It's harmful and oxidising, so handle the crystals with tweezers and wash your hands afterwards.
- Hot apparatus - The tripod, gauze, and rods stay hot long after the flame is out, so let them cool before touching them.
- Aluminium rod - It melts at a much lower temperature than the other metals, so don't overheat it.
- Eye protection - Wear it throughout.
Worked example - Interpreting the three methods
A student ran all three parts. In the conduction part, the drawing pins fell off at these times.
| Metal | Copper | Aluminium | Brass | Iron |
|---|---|---|---|---|
| Time for the pin to fall (s) | 45 | 60 | 95 | 150 |
In the radiation part, the two thermometers stood 10 cm from a filament lamp.
| Surface on the bulb | Matt black paper | Shiny foil |
|---|---|---|
| Starting temperature (°C) | 20.0 | 20.0 |
| Temperature after 10 min (°C) | 32.0 | 24.5 |
In the convection part, the purple colour rose from the crystal, spread across the surface, sank down the sides of the beaker, and returned along the bottom.
Rank the metals, compare the two surfaces, explain the convection current, and suggest why results may vary between groups.
Step 1 - Rank the metals as conductors
The sooner the pin falls, the sooner energy reached the far end. So from best to worst conductor, the order is copper (45 s), aluminium (60 s), brass (95 s), and iron (150 s). Copper is the best conductor of the four.
Step 2 - Compare the two surfaces as absorbers
Work out each temperature rise.
- With matt black paper, the rise is 32.0 − 20.0 = 12.0 °C.
- With shiny foil, the rise is 24.5 − 20.0 = 4.5 °C.
The matt black bulb warmed more. A dark, matt surface is a good absorber of infrared radiation, while a pale, shiny surface reflects most of it away.
Step 3 - Explain the convection current
- Water heated at the bottom expands, becomes less dense than the water around it, and rises.
- Cooler, denser water sinks down the sides to take its place, and is heated in turn.
- The water itself moves, carrying energy round the beaker in a convection current.
Step 4 - Suggest why results may vary between groups
- A slightly thinner rod heats through faster.
- A smaller blob of Vaseline melts sooner.
- A rod placed further into the flame receives more energy.
Common exam questions
Question 1
Why are all four rods heated at the same time from the same source?
So each rod gets energy at the same rate from the same moment. Heating them one after another would let the flame, the room, or the rods' starting temperature change between runs.
Question 2
Why is the potassium manganate(VII) crystal dropped down a glass tube rather than straight into the beaker?
So it lands on the bottom in one place without colouring the water on the way down. A beaker of purple water would show no current, because there would be nothing to follow.
Question 3
Why doesn't convection happen in a solid?
Convection needs the material itself to move, and the particles in a solid are held in fixed positions. In a solid, energy is passed on by conduction instead.
Question 4
Why must both thermometers be the same distance from the lamp?
Radiation spreads out as it travels, so a thermometer further away receives less of it. One bulb would then warm more for a reason that had nothing to do with its surface.
Question 5
The matt black surface is the better absorber of radiation. What else is it good at?
Emitting radiation. A good absorber is an equally good emitter, so a matt black surface also radiates energy away faster than a shiny one.