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| GIF image courtesy NASA's Glenn Research Center, Public domain, via Wikimedia Commons |
A
nice little animation from Wikipedia showing the relationship between
pressure and volume of a gas. If you studied science at school, you may
remember this relationship is described by Boyle's Law which states that
at constant temperature, the absolute pressure of a gas is inversely
proportional to the volume. So if you squash a gas into half the
original volume, the pressure doubles. If the volume is made ten times
smaller, the pressure becomes ten times higher and so on. The law was
formulated by Robert Boyle, a 17th century scientist from Lismore in
Waterford. If P is the pressure and V is the volume of a gas, then:
P ∝ 1/V
Absolute and gauge pressure
Absolute
pressure is pressure above vacuum and gauge pressure is pressure above
atmospheric pressure. So for instance if I have a container and I put a
lid on it and the container is fitted with a pressure gauge, the gauge
will read zero because it only reads pressure above atmospheric. (A tyre
pressure gauge does this and if a tyre is completely flat, the gauge
will read zero) However the air inside the container and all the air
around us is at at an absolute pressure greater than zero due to the
weight of the atmosphere. That pressure is approximately 15 pounds per
square inch (psi) absolute or in metric 101,325 pascals (Pa).
Atmospheric pressure is also indicated as 1 bar or 1 atm. See this link:
Constant temperature in the law
The
law states that the inverse pressure/gas relationship is at constant
temperature. This means that it's only true if temperature is kept
constant. In reality, if a gas is compressed, it actually gets hot
because of gas molecules becoming closer together (just like the way a
bicycle tyre gets hot when you pump it up).
GIF image courtesy NASA's Glenn Research Center, Public domain, via Wikimedia Commons