Science

A hair-dryer makes a further step in convective heat transfer by using a fan to blow cold air past the hot wire and produce a blast of hot air. This process is called forced convection as distinct from the natural convection described above.

In our experiments we examined the various aspects of convection discussed above.

If we place a block of wood in a bowl of water, it floats (fig 1) because a volume of wood weighs less than the same volume of water. (We say the density of wood is less than the density of water.)

A volume of a hot substance usually weighs less than the same volume of the substance when it is cold. (The density of the hot water is less than the density of the cold water.) If, for example, some cold water is poured carefully into a beaker of hot water, it sinks to the bottom; the heavy cold water pushes the lighter hot water out of its way as it sinks. If we heat a pan of water at the bottom, as on a cooker, the water on the bottom becomes hotter and rises to the top, because its density becomes less than the cold water above it.

The same thing happens with air. When air is heated, its density becomes less and it rises up. We have seen heat moving by conduction through a length of copper and by radiation from an electric fire. Hot air rising is another way for heat to be moved from one place to another. This is called (natural) convection. A convector heater heats a length of wire by passing electricity through it. Air is allowed to come into the bottom of the heater and pass the heated wire where the air becomes hotter. Its density then becomes less and it rises up into the room which it heats up. This can be demonstrated by holding a piece of tissue or some hairs above a convector heater and they will be seen to be blown about by the hot air (shown in figures 2 and 3). Radiators in our central heating systems might better be called convectors because most of their heat reaches the room by convection.

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