Section 1 of 7The idea being tested
Emission of infrared
- Emission of infrared Every object emits infrared radiation, and the hotter it is the more it emits.
- Surfaces differ in how well they emit: dull black surfaces are the best emitters, while shiny white and silver surfaces are the worst emitters.
Absorption is the mirror image
- Absorption is the mirror image A surface that is a good emitter is also a good absorber of infrared,
- so dull black surfaces absorb best and shiny silver surfaces reflect most of it.
- The same investigation idea can be turned round by placing identical objects at an equal distance from a heater and measuring how fast each one warms up.
Aim
- Aim To find out how the colour and finish of a surface affect the amount of infrared radiation it gives out as it cools.
Section 2 of 7Variables
| Type | Variable | How it is handled |
|---|---|---|
| Independent | Colour and finish of the flask surface | Four flasks painted black, dull grey, white and shiny silver |
| Dependent | Temperature of the water, measured at regular intervals | Thermometer in each flask, read to 1 °C |
| Control | Size, shape and material of the flasks | Use identical flasks that differ only in their outer surface |
| Control | Volume and starting temperature of the water | Measure the same volume of freshly boiled water into each flask |
| Control | Time interval and room conditions | Same stopwatch intervals, all flasks placed together away from draughts |
Section 3 of 7Method
1
Stand the four identical flasks on a heatproof mat, well spaced and in the middle of the bench, and fit a lid with a snug thermometer hole to each.
2
Boil water in a kettle and use a measuring cylinder to pour exactly the same volume into every flask.
3
Place a thermometer in each flask and record the starting temperature, checking that all four are as close as possible to the same value.
4
Start the stopwatch and read each thermometer at eye level every 30 seconds for about 10 minutes.
5
Record every reading in a table with one column for each colour.
6
Repeat the whole investigation at least once and calculate mean temperatures at each time.
Section 4 of 7Processing and interpreting the results
1Plot the data
- Plot the data Draw temperature on the y-axis against time on the x-axis, with a smooth curve of best fit for each flask on the same axes.
2Compare the gradients
- Compare the gradients A steeper curve means the water is losing energy faster.
3Rule out other causes
- Rule out other causes Conduction and convection losses are the same for all flasks because the flasks are identical, so colour does not change them.
4Conclude
- Conclude Any difference in cooling must be due to infrared radiation: the black flask cools fastest and the shiny silver flask cools slowest.
Section 5 of 7Evaluation and safety
Reducing errors
- Reducing errors Make sure every flask starts at the same temperature,
- because water cools quickly at first and a small difference spoils the comparison.
- Keep the thermometer hole small so that thermal energy does not escape through it, and read scales at eye level to avoid parallax error.
- A data logger with digital temperature probes gives more accurate and more frequent readings than reading by eye.
Safety
- Safety Wear safety goggles when pouring boiling water, do not overfill the kettle and keep water away from electrical sockets.
- Keep equipment away from the edge of the bench and work standing up so that you can react quickly to a spill.
- Treat any scald by holding the area under cold running water for at least five minutes.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Infrared radiation | Electromagnetic waves emitted by all objects because of their temperature; the hotter the object the more it emits. |
| Emitter | A surface that gives out radiation; dull black surfaces are the best emitters. |
| Absorber | A surface that takes in radiation; dull black surfaces absorb infrared best. |
| Control variable | A quantity kept the same in every trial so that only the independent variable affects the result. |
| Parallax error | A reading error caused by looking at a scale from an angle instead of straight on. |
| Curve of best fit | A smooth line drawn through plotted points that shows the overall trend. |
Section 7 of 7Exam technique
1
Saying that black flasks 'attract' heat. Black surfaces emit and absorb infrared better; nothing is being attracted.
2
Forgetting to control the starting temperature and volume of water, which makes the comparison unfair.
3
Claiming that radiation is the only way the flasks lose energy. Conduction and convection also occur but are equal for all flasks.
4
Reading the thermometer from above or below instead of at eye level, which introduces parallax error.
5
Describing the shiny silver flask as cooling fastest. Shiny surfaces are poor emitters, so that flask cools most slowly.