Course: Combined Science + Separate Physics | Tier: Foundation + Higher
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Aim
To investigate how the amount of infrared radiation emitted (or absorbed) by a surface depends on the nature of that surface.
Method
- Place a Leslie cube on a heatproof mat. Its four faces are different: e.g. matt black, shiny black, matt white and shiny silver.
- Fill the cube with very hot water and put the lid on.
- Point an infrared detector at one face from a fixed distance (e.g. 10 cm) and record the reading.
- Quickly repeat for each face, keeping the distance the same.
- Compare the readings to rank the surfaces as emitters.
| Independent variable | Type of surface |
| Dependent variable | Amount of infrared radiation emitted (detector reading) |
| Control variables | Distance from detector to face; temperature of water; size of each face |
Questions
Q1 (F) Name the independent and dependent variables.
Show answer
Independent: the type of surface. Dependent: the amount of infrared radiation emitted (the detector reading).
Q2 (F) Give two control variables.
Show answer
Any two of: distance between the detector and the face; temperature of the water; size of each face.
Q3 (F) Which surface emits the most infrared, and which emits the least?
Show answer
Most: matt black. Least: shiny silver (light, shiny surfaces are the poorest emitters).
Q4 (F) State one hazard and how to reduce the risk.
Show answer
Very hot water and a hot cube can scald or burn. Fill the cube carefully on a heatproof mat, and don’t touch or move it until it has cooled.
Q5 (F/H) Why should the readings for all four faces be taken quickly?
Show answer
The water is cooling all the time. Taking the readings quickly keeps the temperature of the water (a control variable) about the same for each face.
Q6 (F/H) Why use an infrared detector rather than feeling the heat with your hand?
Show answer
The detector gives numerical readings that can be compared fairly. Using your hand is subjective, can’t measure small differences, and could burn you.
Q7 (F/H) How are good emitters of infrared related to good absorbers?
Show answer
Good emitters are also good absorbers. Matt black surfaces emit and absorb the most infrared; shiny silver surfaces emit and absorb the least (they reflect it instead).
Q8 (H) Use the results to explain why teapots are often shiny.
Show answer
Shiny surfaces are poor emitters of infrared, so less energy is transferred from the tea to the surroundings by radiation. The tea stays hot for longer.
