Pressure and temperature are the basis of thermodynamics. Pressure allows us to calculate the forces exerted on the object and thus to account for the work and energy exchanged. While the temperature gives an account of the energy stored.

Over time, several tools have been developed to measure pressure and temperature in a relative way. From the manometer or barometer for pressure to the thermometer for temperature. For the temperature, the absolute zero was fixed as the passage from the solid state to the liquid state of water while for the degree Fahrenheit, its reference was based on the temperature of a horse (100°F), which is very relative.

This allows the temperature to take all its importance and surely the law of perfect gases. This law tells us that at a given temperature, the product of pressure and volume remains constant (PV = f(T)). The problem is that now there is not really an absolute 0 for the degree Celsius in the sense that the melting temperature of ice can change according to the pressure. Today, the reference frames for these temperatures have fortunately changed. And science uses the kelvin for the temperature with a real absolute zero in the sense that it is the lowest temperature that can exist.

Gases can be seen at two different levels:
• From a microscopic point of view, they are particles moving in all directions and separated by empty space
• From a macroscopic point of view, it is only a continuous fluid which exerts a pressure on walls
Statistical physics will study from a microscopic point of view the particles and calculate their forces on the walls to deduce a new parameter which will be studied from a macroscopic point of view which is the pressure.

Tomorrow, we will approach the first principle of thermodynamics, i.e. the conservation of energy.

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