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The Pressure Law
Using the apparatus shown in the diagram we can
investigate what happens to the pressure of a gas when
it is heated, the volume of the gas being kept constant.
Bourdon gauge
From what you know about the structure of a gas you
should expect that the molecules would move faster
and faster as the gas is heated. Heat energy is being
converted into kinetic energy of the molecules.
Air
This means that they will collide with the walls of the
container more violently and therefore the pressure will
rise.
If you plot a graph of pressure against temperature then
you should get a result similar to the one shown in
graph 1. It shows that the pressure increases steadily
with increasing temperature.
water
heat
If we draw the line back to where it cuts the temperature
axis (graph 2) we reach a point where the pressure if
the gas is zero – in other words the molecules have Pressure
stopped moving. They have no velocity and so no
kinetic energy. This is the lowest temperature that it is
possible to reach and is called ABSOLUTE ZERO.
In fact the best definition of absolute zero is to say
that it is the temperature where the gas molecules
have their minimum energy. It is not quite zero energy
but very close to it.
Temperature (oC)
Pressure
On the Celsius scale absolute zero is
therefore -273 oC.
It is useful to use a scale of temperature
that starts at this point and we call this
scale the Absolute or Kelvin scale of
temperature.
On this scale 0 K = -273oC and
0 oC = 273 K.
Temperature (K)
Absolute zero
It follows that 100 oC = 373 K.
The pressure law can be written as:
Pressure/Temperature (K) = constant
or
Pressure 1/Temperature 1 = Pressure 2/Temperature 2
P1/T1 = P2/T2
for a given mass of gas
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