Pressure is defined as the perpendicular force per unit area of distribution and is measured in N/m2. The SI unit of pressure is pascal (1 Pa = 1 N/m2). As pressure acts normal to the surface over a certain area, it is a type of stress. Pressure can also be interpreted as potential energy per unit volume in some cases. The following is the mathematical expression for pressure.
Where is the pressure, is the force acting on the surface, and is the area of the surface the force is acting upon.
Apart from solids, fluid can exert pressure too. For example, all the air molecules in our atmosphere exert pressure on the surface of the earth at about 1.01325 × 105 Pa. This is known as the standard atmospheric pressure of the earth. In the same way, if you go swimming deep inside a swimming pool, the water body above you will exert pressure on your body from above in addition to the atmospheric pressure. Adding the two pressures will give you the absolute pressure on your body. If you subtract the atmospheric pressure from your absolute pressure, you get the gauge pressure.
Frequently Asked Questions
How is pressure different from stress?
Pressure is a scalar quantity, which is simply a normal force acting per unit area on a surface, whereas stress is a tensor quantity that acts in all three directions. Scalar and tensor could be better understood with a simple example. Let's take a simple and pretty little cube. If a force acts normal to all the six faces, then there would be a definite pressure acting on each of the faces, distributed evenly (isotropic). But in the case of stress, the stress could act on all directions. There is the shear stress acting parallel to the faces, and there is the normal stress acting perpendicular to the faces. Therefore, pressure is a type of stress where the direction of it is definite at all points. Technically, pressure is a tensor of order 0.
Pressure is directly proportional to temperature of a system. This has been explained in the past with several gas laws like Boyle's law, Charle's law, Gay-Lussac's law, which related pressure, volume, and temperature. Later, these laws were combined into a single law and with addition of Avogadro's law, the ideal gas law was formulated.