Download Heating and cooling

Heating and cooling
You probably know that when you use a simple pump to pump up a bicycle tyre the pump
gets hot. This is because when a gas gets compressed its temperature rises; the reverse of
this also happens when a gas is expanded, its temperature falls. The gas molecules gain
energy from, or lose energy to, the moving walls of their container.
These effects can be noticed in the refrigerator, in weather and in the cloud chamber used in
nuclear physics.
Refrigeration
In a refrigerator, heat is removed from
the surroundings at one point, the
freezing compartment, and given out to
the surroundings at another, the heat
exchanger. A volatile liquid called freon
evaporates through a small hole (A) into
the coils (B) because of the low pressure
in B. As it does so it cools down taking
latent heat from its surroundings and so
the freezing compartment gets colder.
The pump then forces this cold vapour
into the heat exchanger coils where it is
compressed and liquifies giving out
latent heat to the surroundings. The
whole cycle is then repeated. The heat
exchanger coils get quite hot and for this
reason ‘fridges and freezers should not
be placed with their heat exchangers
close to a wall otherwise they may
overheat.
B
A
freezing compartment
heat given
out
heat
exchanger
cold air
electric
pump
Notice that the freezing compartment is at the top of the fridge, the cold air produced there
will sink to the bottom keeping the whole fridge cold.
Freon is a name for a collection of different gases that are suitable for refrigerators. These
gases used contain harmful CFCs but these have now been banned and replaced with a
safer alternative.
How much water vapour can the air hold?
A given volume of air at a certain temperature can only hold a certain amount of water
vapour. In Figure 2(a) there is an unlimited volume of air above the open beaker and so
water is lost by evaporation. In (b) there is a limited volume of air above the water in the
enclosed space and so after a while the water vapour creates a pressure that prevents any
further net evaporation.
saturated vapour
unsaturated vapour
The water vapour at A is called an
UNSATURATED VAPOUR and that at B a
SATURATED VAPOUR. The hotter the air
the more water vapour it can hold. The
pressure of the saturated vapour will be
greater at higher temperatures.
B
A
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The amount of water vapour in the air is called the HUMIDITY. If the humidity is high there is
a lot of water vapour in the air and so further evaporation is difficult. This is why it is difficult
to lose water vapour by sweating when the humidity is high. If a saturated vapour is cooled
then some of the liquid will condense - a good example of this is in the way clouds form.
As warm, moist air rises it cools, is unable
to hold so much water and so the water
condenses as millions of minute droplets clouds. As more condenses the cloud
cannot support itself and so the water falls
to the ground as rain.
cools
warm, moist
air
rain
The bobbing bird
This lovely toy also works because of evaporation - this
time the evaporation of ether. Ether evaporates from the
body of the bird pushing liquid ether into the head making
it top heavy and so the bird tilts forwards. The head is kept
cool by water and so the liquid does not evaporate but
runs back into the body and so the whole process starts all
over again. Using meths in the glass will make the bird
bob up and down quicker because the meths evaporates
more rapidly.
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