Pressure is an effect which occurs when a force is applied on a surface. Pressure is the amount of force acting on a unit area. The symbol of pressure is P.[1][2]
Formula
| Conjugate variables of thermodynamics | |
|---|---|
| Pressure | Volume |
| (Stress) | (Strain) |
| Temperature | Entropy |
| Chem. potential | Particle no. |
Mathematically:
where:
- P is the pressure,
- F is the normal force,
- A is the area.
Pressure is a scalar quantity. It relates the vector surface element (a vector normal to the surface) with the normal force acting on it. The pressure is the scalar proportionality constant that relates the two normal vectors:
The minus sign comes from the fact that the force is considered towards the surface element, while the normal vector points outwards.
It is incorrect (although rather usual) to say "the pressure is directed in such or such direction". The pressure, as a scalar, has no direction. It is the force given by the previous relation the quantity that has a direction. If we change the orientation of the surface element the direction of the normal force changes accordingly, but the pressure remains the same.
Pressure is transmitted to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point. It is a fundamental parameter in thermodynamics and it is conjugate to volume.
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