What is Pressure? | Type of Pressure

What is Pressure ?

We know that whenever liquid ( such as water, oil etc.) is contained in a vessel, it exerts force ( pressure )  at all points on the sides and bottom of the container.
This force per unit is called as pressure. If “F” is the force acting on area “a” then intensity of pressure is
P =  F/a ( N/mm2)
The direction of this pressure is always at right angles to the surface, with which the fluid at rest is in contact.

Type of pressure 

1) Atmospheric pressure 

A bank of air surrounding the earth surface is called as atmosphere. The height of atmosphere above the earth surface varies from 100 to 1000 km.
At the earth surface, the pressure due to the weight of air above the earth surface is called as atmospheric pressure.
It is measured by mercury barometer.

2) Gauge pressure 

If the pressure is measured above or below the atmosphere pressure it is called as gauge pressure.
If the pressure is measured above the atmosphere pressure, it is called as positive gauge pressure.
If the pressure is measured below the atmosphere pressure it is called as negative gauge pressure or vacuum pressure.

3) Absolute pressure 

If the pressure is measured above the absolute zero (or complete vacuum) then it is called as Absolute pressure.
Absolute pressure = Atmosphere pressure + or – Gauge pressure

Absolute Vaccum 

It is the emptiness or space which contains no form of matter. Instead of complete or absolute vacuum is not used generally. So partial vacuum is used. The relationship between absolute pressure gauge pressure and atmospheric pressure is –