Pressure is the effect of force which depends on the surface area on which the force is applied. Larger area apply low pressure and smaller are apply high pressure. (if force remain constant)

Force acting per unit area is called pressure

Pressure is the force exerted on an object. The force applied is perpendicular to the surface of objects per unit area.Units of pressure is newtons per square meter (N/m2). SI Unit of pressure is  pascals “

Pressure = F/A = Force/Area

To create big pressure, you need

  • large force
  • small area

If both, the pressure will be maximum.

Example: If you hold the flat part of the knife against the fruit and apply force, it won’t cut the fruit. The force will spread out of a large area and pressure will be reduced. Now,If you hold the sharp part of the knife against the fruit and apply force, it will cut the fruit. The force is limited for a small area and pressure will be increased.

Pressure of  Liquid

All matter is made up of tiny particles called atoms. The earth gravitational pull acts on all objects and cause them to have weight. Liquid particles caused pressure (Due to their weight) on the container and on any object in the liquid.

The pressure exerted by the fluid depends on

  • The depth of the fluid
  • The density of the fluid
  • The acceleration of gravity (Gravitational field strength).

Pressure in liquid = depth * density * gravitational field strength

P = hpg

Imagine being submerged to some depth in water. You will feel some pressure on your eardrums. The water above you will push you downward because of the force of gravity. If you go deeper, there will be more water above you, so the weight and pressure from the water would increase too.

Pressure of gas

A gas consists of molecules and every molecule has some kinetic energy.Gas particles caused pressure on the container and on any object in the gas.These molecules when colliding with the walls of a container, apply pressure on it.

The pressure exerted on the container depends

  • The amount of gas in the chamber
  • The temperature of the gas
  • The volume of the chamber.

Boyle’s Law

For a fixed mass of gas, pressure is inversely proportional to the volume.(at constant temperature)

We can write Boyle’s law as “P1 * V1 = P2 * V2

Atmospheric pressure

Atmosphere is just like an ocean of gases which surrounds the air. The atmosphere is the result of gravity by earth.It extends up to 1000 Km above the surface of the earth.

The atmospheric pressure is the weight exerted by the overhead atmosphere on a unit area of surface.

This pressure is (almost ) equal to 1 Kg mass on an area of 1 cm2. A mercury barometer is used to measure the atmospheric pressure.

Mercury Barometer

An instrument that measures air pressure is called a barometer. It contains mercury.

It consist of a tube (calibrated in mm) and a dish filled with mercury. The open end (vacuum filled) of a glass tube is placed in dish of mercury.  Atmospheric pressure forces the mercury to rise up the tube. The column of mercury will rise to a height of 760 millimeters.

Standard atmospheric pressure or 1 atm

The atmospheric pressure at the sea level is equal to 760 mm of height of the column of mercury called 1 atm

The standard atmospheric pressure (1 atm) is a unit of pressure equal to 101325 Pa. Equivalent to 760 mm of mercury .


A manometer is a scientific instrument used to measure gas pressures.

It measures the gas pressure relative to atmospheric pressure (pressure difference). Its basic structure consist of calibrated U shaped tube filled with mercury , oil or water. A column of mercury is open at one end to the atmosphere and exposed to the pressure to be measured at the other end.The height difference due to gas pressure is equal to atmospheric pressure + mercury pressure.

It is commonly used in industries to measure the pressure in gas pipeline.

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