Heat transfer by radiation
Heat can be transferred by infrared radiation.
Unlike conduction and The transfer of heat energy through a moving liquid or gas. - which need the vibration or movement of A general term for a small piece of matter. For example, protons, neutrons, electrons, atoms, ions or molecules. - Electromagnetic radiation emitted from a hot object. is a type of electromagnetic radiation.
When infrared radiation is absorbed by an object it is heated and its temperature rises.
Because no particles are involved, Energy carried by particles from a radioactive substance, or spreading out from a source. can work through the A volume that contains no matter. of space. This is why we can still feel the heat of the Sun even though it is 150 million km away from the Earth.
Heat radiation investigation
The transfer of infrared radiation from a hot object to cooler surroundings can be investigated using a piece of apparatus called Leslie’s cube.
This is a metal cube with four side prepared in different ways: black, white, shiny, or dull.
It can be filled with hot water or heated on an electrical hot plate so that all four sides are at the same temperature.
Method
- Measure the temperature a fixed distance from each side of a Leslie's cube using four identical thermometers.
- Heat the Leslie’s cube by filling it with boiling water.
- Continue to measure and record the temperatures every 30 seconds for five minutes, then plot a graph of temperature on the y-axis, against time on the x-axis, for each side.
- Compare the four graphs obtained.
Control Variables
Distance of each thermometer from the sides of the cube, the type of thermometer used and the time taken for each reading.
Results
The temperature of the thermometer opposite the dull, black side reaches the highest temperature in the same time interval.
From this we can say that dark matt surfaces are better at radiating heat energy than light shiny surfaces.
Key points
- Dark matt surfaces are better at absorbing heat energy than light shiny surfaces.
- Dark matt surfaces are better at radiating heat energy than light shiny surfaces.
| Surface | Absorption | Emission |
| Dull, matt or rough | Good absorber of heat radiation | Good emitter of heat radiation |
| Shiny | Poor absorber of heat radiation | Poor emitter of heat radiation |
| Surface | Dull, matt or rough |
|---|---|
| Absorption | Good absorber of heat radiation |
| Emission | Good emitter of heat radiation |
| Surface | Shiny |
|---|---|
| Absorption | Poor absorber of heat radiation |
| Emission | Poor emitter of heat radiation |
Radiators are often painted with gloss paint, but they would be better at radiating heat if they were painted with matt black paint instead.
They are painted white to make them look nicer.
However, despite their name, radiators actually transfer most of their heat to a room by convection, not radiation.
Normally radiant heaters must be at least red hot before they are effective.