Investigating Thermal Radiation
Aims of the Experiment
The aim of the experiment is to investigate how the amount of infrared radiation absorbed or radiated by a surface depends on the nature of that surface
Variables
- Independent variable = Colour
- Dependent variable = Temperature
- Control variables:
- Identical flasks (except for their colour)
- Same amounts of hot water
- Same starting temperature of the water
- Same time interval
Equipment List
| Equipment | Purpose |
| Kettle | To boil water |
| 4 thermometers | Measure the temperature of water |
| Flasks painted different colours (black, dull grey, white, silver) | Investigate heat loss in flasks of different colours |
| Heatproof mat | Protect surface and prevent heat loss from the bottom of the flasks |
| Stopwatch | Record time taken for water to cool |
- Resolution of measuring equipment:
- Thermometer = 1°C
- Stopwatch = 0.01 s
Method
Four Flasks with Thermometers
- Set up the four identical flasks painted black, grey, white and silver
- Fill the flasks with hot water, ensuring the measurements start from the same initial temperature
- Note the starting temperature, then measure the temperatures at regular intervals e.g. every 30 seconds for 10 minutes
Analysis of Results
- All warm objects emit thermal radiation in the form of infrared waves
- The intensity (and wavelength) of the emitted radiation depends on:
- The temperature of the body (hotter objects emit more thermal radiation)
- The surface area of the body (a larger surface area allows more radiation to be emitted)
- The colour of the surface
- Most of the heat lost from the beakers will be due to conduction and convection
- This will be the same for each beaker, as colour does not affect heat loss in this way
- Any difference in heat loss between the beakers must, therefore, be due to infrared (thermal) radiation
- To compare the rate of heat loss of each flask, plot a graph of temperature on the y-axis against time on the x-axis and draw curves of best fit
- The expected results are shown on the graph below:
Expected Results and Conclusions
- An example table of results might look like this:
Example Results Table
Evaluating the Experiment
Systematic Errors:
- Make sure the starting temperature of the water is the same for each material since this will cool very quickly
- It is best to do this experiment in pairs to coordinate starting the stopwatch and immersing the thermometer
- Use a data logger connected to a digital thermometer to get more accurate readings
Random Errors:
- Make sure the hole for the thermometer isn’t too big, otherwise the heat will escape through the hole
- Take repeated readings for each coloured flask
- Read the values on the thermometer at eye level, to avoid parallax error
Safety Considerations
- Keep water away from all electrical equipment
- Make sure not to touch the hot water directly
- Run any burns immediately under cold running water for at least 5 minutes
- Do not overfill the kettle
- Make sure all the equipment is in the middle of the desk, and not at the end to avoid knocking over the beakers
- Carry out the experiment only whilst standing, in order to react quickly to any spills
