Radiation is the transfer of heat energy as electromagnetic (EM) waves, without the need for a medium.
STS
The method of convection in fluids explains why heating elements are usually located at the base of containers. This ensures that the rising hot water current moves up through the fluid and is replaced by a cold layer that will be heated in turn.
To demonstrate radiation
1. Set up two containers of hot water with a thermometer or temperature probe in each. The outside of one can is coated black and the other is shiny (Fig. 11.11).
2. Record the temperature readings in each can as time passes.
3. The black can will radiate heat faster and therefore its temperature drops quicker. The shiny can should reflect heat better and therefore keep its energy longer. This means it should stay hotter than the black can.
NOTE
Infrared radiation transfers heat from all objects. The higher the temperature, the higher the frequency of EM radiation.
Heat transfer in action
Having discussed the effects of heat energy and the methods of transferring heat, we will now define the amount of heat that transfers from our main source: the Sun.
Solar constant The solar constant is on average 1.37 kW m–2 but it varies with time. While 1 m2 of perpendicular atmospheric surface can be used for calculations, the solar constant refers to just above the atmosphere, in space (Fig. 11.12).
1.5 ×10 m11
The amount of solar energy striking 1 m2 of the Earth’s atmosphere every second is known as the solar constant, or solar irradiance.
