Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. This definition can be stated for the generic point of any stressed surface (not necessarily flat but without angular points):
- considering a surface element small enough to be considered flat (as it can be confused with the corresponding element of the tangent plane locally to the surface);
- considering the ratio between the stress component according to the normal to the element and the area of the latter and defining then the pressure in the given point as the limit of this ratio when the area of the element.
When describing a container of gas, the term pressure (or absolute pressure) refers to the average force per unit area that the gas exerts on the surface of the container.
Absolute pressure is zero-referenced against a perfect vacuum, using an absolute scale, so it is equal to gauge pressure plus atmospheric pressure.
- Sound pressure