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thermodynamics
== Fundamental Thermodynamics ==
Thermodynamics is a basic science that deals with energy. Its most notable application being the generation of power through heat engines. However, the only bit of thermodynamics we need to understand pressure is the '''Ideal Gas Law'''.
=== States and thermodynamic properties ===
Air can be liquified, but for all aviation purposes air is a '''gas'''. We shall not define a gas but the reader should have a good understanding of the concept through life experience.
Any substance of engineering interest can have '''states'''. For example, water can be ice, liquid water, or water vapour. If you are asked the question "What is the state of water at room temperature?" you may intuitively answer "liquid water". However, believe it or not, by reaching this conclusion you are making the assumption that the pressure that this water is at is at everyday atmospheric level. If the same water is put into vacuum while maintained at the same temperature, it will boil.
Therefore, to fully define the state that a substance is at, two thermodynamic quantities must be specified. The most common thermodynamic quantities can be:
*Temperature \(T\)
*Pressure \(p\)
*Density \( \rho \)
and a bunch of others. At least two must be known to fully define the state of a substance.
=== Ideal gas ===
Air can be modelled as an ideal gas. An '''ideal gas''' is a gas for which the following relationship holds:
\[ p = \rho R T \]
where:
*\(p\) is the pressure
*\(\rho\) is the density
*\(R\) is a constant that is a property of the gas itself, and
*\(T\) is the temperature.
Therefore, if we want to find the density of some air, both the pressure and the temperature needs to be specified (it is possible to substitute one or both with something else, but we shall not investigate this complexity). For example, atmospheric air and the air in a tyre are at the same temperature, but since the air in the tyre has a higher pressure (if you remember pumping it in), it has a higher density than atmospheric air.
== Pressure ==
