Required Practical 10: Investigating Infrared 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
- Resolution of measuring equipment:
- Thermometer = 1°C
- Stopwatch = 0.01 s
Method
- 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
- An example table of results might look like this:
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:
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
- 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
When teaching this investigation to my GCSE students, I always remind them that the key is the control variables. In physics, we tend to have handy lists of 'factors affecting...'. These are the factors that should be controlled within the investigation. If we fail to control these variables, then we have no way of knowing if the effect we measure is due to those variables or the independent variable (the variable we change on purpose). By controlling these external factors we ensure that our data is valid.
If surface area affects the rate of energy transfer, then we need to make sure the surface area of each container is the same. If hotter substances emit more thermal radiation than cooler ones, then we need to make sure the initial temperature of each flask is the same. The only thing we want to be different is the independent variable, which in this case is the colour of the surfaces of the flasks.
