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Energy Transfer

Heat - Convection (continued from the previous page).

You may be asked to draw the direction of convection currents
and explain why they move in this way.
The picture below shows a radiator heating a room in a house.
Convection of Heat from a Radiator

The hot radiator transfers heat to the nearby air
when air molecules collide with the radiator surface.
The air is also heated by infra-red radiation.

The hot air near to the radiator expands and increases
in volume. Hot air expands because the particles move
further apart as they get hotter (see the structure of a gas).
This makes the density of the hot air decrease and it starts
to rise upwards. The colder air above it gets pushed along
to the right and then circulates as shown by the arrows.
The arrows show how the convection currents move.

As the hot air moves around the room, it loses its
heat by collision with the walls, ceiling and the objects
in the room. Finally the colder air circulates near to
the radiator where it is heated and the whole process
repeats itself. The efficiency of convection can be
improved by placing shiny metal foil behind the radiator.

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