Contains the full lesson along with a supporting toolkit, including teachers’ notes.
Radiation and Radioactivity
You’ve probably heard that heat travels in three different ways: by conduction, convection and radiation. Transfer of heat in solids is by conduction; for example, if you put a metal spoon into a cup of boiling water heat travels from the hot part to the colder part. Heat transfer in fluids (liquids and gases) is mainly by convection — the heated part generally becomes less dense and so convection currents are set up. In this case heat is transferred by bulk movement of the fluid. Heat transfer by radiation involves the emission of electromagnetic waves by a hot source; such waves do not require a medium and so can travel through empty space. The light and heat we get from the sun is of this kind.
Ionising and non-ionising radiation
The electromagnetic radiation generally associated with heat transfer is known as infrare radiation. Some other kinds of electromagnetic radiation can break the bonds that hold molecules together or knock electrons off them forming ions; this is known as ionising radiation. The minimum energy required to break the bonds in molecules or to produce ions is of the order of 5 x 10−19 J. The energy of electromagnetic waves is a function of their wavelength; the shorter the wavelength the greater the energy.
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