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objectives
Thermal Radiation
Thermal Radiation
- On Earth, we are warmed by the Sun. Its energy travels to us in the form of electromagnetic waves. These include invisible infrared waves as well as light, and they can travel through a vacuum (empty space). They heat up things that absorb them, so are often called thermal radiation.
- All objects give out some thermal radiation. The higher their surface temperature and the greater their surface area, the more energy they radiate per second. Thermal radiation is a mixture of different wavelengths, as shown on the right.
- Warm objects radiate infrared. But if they become hotter, they also emit shorter wavelengths which may include light. That is why a radiant heater or grill starts to glow 'red hot' when it heats up.
Emitters and Absorbers
Emitters and Absorbers
- Some surfaces are better at emitting (sending out) thermal radiation than others. For example, a black saucepan cools down more quickly than a similar white one because it emits energy at a faster rate.
- Good emitters of thermal radiation are also good absorbers, as shown in the chart below. White or silvery surfaces are poor absorbers because they reflect most of the thermal radiation away. That is why, in hot, sunny countries, houses are often painted white to keep them cool inside.
- This chart shows how some surfaces compare as emitters, reflectors, and absorbers of thermal radiation.
Comparing emitters The metal cube is filled with coiling water which heats the surfaces to the same temperature.
The thermal radiation detector is placed in turn at the same distance from each surface and the meter readings compared.
Comparing absorbers The metal plates are placed at the same distance from a radiant heater. To find out which surface absorbs thermal radiation most rapidly, the rises in temperature are compared.
The Greenhouse Effect
The Greenhouse Effect
- When the Suns thermal radiation reaches the Earth, the atmosphere acts as a ‘heat trap'. This happens because some gases (notably water vapour, carbon dioxide, and methane) absorb energy strongly at certain wavelengths in the infrared region of the spectrum.
- The heat-trapping action of the atmosphere is called the greenhouse effect.
- Without it, the Earths surface would be around 25 °C cooler than it is.
- The present concern is that extra carbon dioxide from burning fuels may be adding to the effect and causing global warming.
- Greenhouses act as heat traps, which is how the greenhouse effect got its name. However, they work in a different way. Thermal radiation from the Sun passes easily through the glass or plastic. The ground inside warms up and heats the air. But the hot air is trapped. It cannot escape by rising and flowing away.
- The Sun's thermal radiation passes easily into a greenhouse. But unless you leave the door or a roof vent open, the heated air inside cannot escape.
Solar Panels
Solar Panels
- Some houses have a solar panel on the roof like the one on the right.
- It uses the Suns thermal radiation to warm up water for the house.
- The blackened layer absorbs the radiant energy and warms up the water flowing through the pipes.
Vacuum Flasks
Vacuum Flasks
- A vacuum flask can keep drinks hot (or cold) for hours. It has these features for reducing the rate at which thermal energy flows out (or in):
- An insulated stopper to reduce conduction and convection.
- A double-walled container with a gap between the walls. Air has been removed from the gap to reduce conduction and convection.
- Walls with silvery surfaces to reduce thermal radiation.
A video explaining how vacuum flasks work
A video explaining how vacuum flasks work
5.09---Vacuum-flasks.mp4thermal radiation worksheet
thermal radiation worksheet
5.08---Thermal-Radiation.docxPage updated
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