Force experienced per unit area is known as pressure.

Pressure = Force / Area

Units of pressure is Newton / metre² or N / m²

Example – Liquids and gases exert pressure on the walls of the container.

Absolute pressure (p_a): The pressure measured above absolute zero of pressure is called absolute zero pressure. A absolute zero pressure occurs only in complete volume.

Absolute pressure = Gauge pressure + Atmospheric pressure

p_{a =} p_{g +} p_atm

Gauge pressure (p_g): The instrument used to measure the pressure generally measure the difference between the system pressure and atmospheric pressure. This is known as gauge pressure.

Vaccum pressure: It is difference between the atmospheric pressure and absolute pressure.

Vaccum pressure = Atmospheric pressure – absolute pressure

1 atmospheric pressure = 1.01325 bar

The pressure exerted by the surrounding air is known as atmospheric pressure.