10.10 - Surface & Rate of Cooling
The experiment
Aim
To find out how the nature of a surface affects the amount of thermal energy it radiates.
Background
- Every object gives out radiation and takes it in from its surroundings, all the time.
- An object that radiates more power than it absorbs cools down.
- Dark, dull surfaces are better emitters of radiation than pale, shiny ones.
- You can't measure the energy radiated directly, but you can measure the water's temperature as it falls.
- So in this practical you record how fast hot water cools in containers with different surfaces. The faster the temperature falls, the better the surface is at radiating.
Variables
- Independent variable - The surface covering the container (its colour, and whether it's shiny or dull).
- Dependent variable - The temperature of the water, every two minutes.
- Control variables - The volume and starting temperature of the water, the size and shape of the containers, and a lid on each one.
Method
- Set up - Cover four identical beakers in matt black paper, white paper, and shiny aluminium foil, and leave one plain.
- Add the hot water - Measure the same volume into each beaker, one straight after another, and cover each with a lid, with a thermometer through the lid and its bulb under the water.
- Take readings - Record all four starting temperatures and start the stopclock, then record them every two minutes for ten minutes, in the same order each time.
- Plot cooling curves - Temperature against time, with all four surfaces on the same axes.
Safety
- Hot water scalds - Pour it with the beakers standing on the bench, and use water that has stood for a minute or two after boiling rather than straight off the boil.
Worked example - Cooling curves for four surfaces
A student covered four identical beakers with different surfaces. Each time, they poured in 100 cm3 of water at 80.0 °C and recorded its temperature every two minutes.
| Time (min) | 0 | 2 | 4 | 6 | 8 | 10 |
|---|---|---|---|---|---|---|
| Matt black (°C) | 80.0 | 71.0 | 64.0 | 58.5 | 54.0 | 50.5 |
| White (°C) | 80.0 | 73.0 | 67.5 | 63.0 | 59.0 | 56.0 |
| Plain glass (°C) | 80.0 | 74.5 | 70.0 | 66.0 | 62.5 | 59.5 |
| Shiny foil (°C) | 80.0 | 76.5 | 73.5 | 71.0 | 69.0 | 67.0 |
Rank the surfaces, describe the curves, explain the results in terms of radiation, and say what the control variables protect against.
Step 1 - Find each temperature drop
Subtract the temperature after 10 minutes from the 80.0 °C start.
- With matt black, the drop is 80.0 − 50.5 = 29.5 °C.
- With white, the drop is 80.0 − 56.0 = 24.0 °C.
- With plain glass, the drop is 80.0 − 59.5 = 20.5 °C.
- With shiny foil, the drop is 80.0 − 67.0 = 13.0 °C.
Step 2 - Rank the surfaces
The bigger the drop, the more energy the water radiated away. So matt black is the best emitter, followed by white and then plain glass, and shiny foil is the worst.
Step 3 - Describe the shape of the curves
All four curves start together at 80.0 °C and spread apart. Every curve is steepest at the start and flattens over time. The water cools fastest when it's much hotter than the room, and more slowly as it gets closer to room temperature.
Step 4 - Explain the results in terms of radiation
- All four beakers emit and absorb radiation at the same time, but they're much hotter than the room, so they radiate far more power than they absorb.
- Matt black emits radiation fastest, so it cools fastest. Shiny foil emits it slowest, so it cools slowest.
- Left long enough, all four reach room temperature. Then each emits radiation at the same rate as it absorbs it.
Step 5 - Say what the control variables protect against
- The same volume of water means each beaker starts with the same thermal energy store. A larger volume cools more slowly whatever the surface.
- Beakers of the same size and shape have the same surface area to radiate from.
- The same starting temperature means every beaker starts with the same temperature difference from the room.
Common exam questions
Question 1
Which surface radiates thermal energy best, and how do you tell from the graph?
Matt black. Its cooling curve falls most steeply and finishes lowest, so it lost the most energy in the same time.
Question 2
Why is a lid put on each beaker?
So energy can't escape by evaporation and by warm air rising out of the top. Then the surface is the only thing being compared.
Question 3
Why is the hot water poured into all four beakers one straight after another?
So none has been cooling longer than the others before the timing starts, and all four begin at the same temperature.
Question 4
A student leaves one of the four beakers plain, with no covering. Is that a fault in the experiment?
No. A plain beaker is itself a surface, and it gives a middle result to compare the others against. Leaving the lid off it would be a fault, since that changes something other than the surface.
Question 5
How would you change this experiment to investigate absorption rather than emission?
Fill the four covered beakers with cold water, stand them the same distance from a radiant heater, and record how fast each one warms up. The surface that emits best also absorbs best, so the matt black one should warm fastest.