Pressure

Pressure
The force acting normally on per unit area of the surface is called pressure. It is denoted by 'P'. Mathematically it can be written as
          Pressure= Force/Area
                       P = F/A
Its SI unit is N/m*2 or pascal(Pa).
From the above relation, pressure is directly proportional to the applied force and inversely proportional to the surface area.
If one newton force is applied to the surface area of one metre square then the pressure exerted on that surface is known as one Pascal pressure.
Atmospheric presssure
The mixture of different gases which is present in our surrounding is called atmosphere. The pressure exerted by the mixture of different gases on the earth's surface is called atmospheric pressure. The normal atmospheric pressur at sea level is 760 mm of Hg.
Application of atmospheric pressure
. It is used to fill medicine in a syringe.
. It is used to fill ink in a pen.
. To use a vacuum cleaner.
. To make a water pumping machine.
. To run a air filling machines.
Liquid pressure
The pressure exerted by liquid on the walls  and bottom of the vessel in which it is kept is called liquid pressure.
Properties of liquid
. Liquid pressure increases with its depth.
. Liquid finds its own level.
. Liquid transmits pressure equally in all the directions.
. The liquid pressure at any point depends on its density.

Let us consider a liquid of density 'd' is kept in a vessel having the base area 'A' up to the depth 'h' from the surface of the liquid .
We know that,
  Pressure(P) = Force F/ Area A
                        = mass(m) x g / A
                        = Density(d) x volume(v) x g/A
                        = d x A x h x g/ A
                     P = dgh
The above relation shows that liquid pressure depends on density of the liquid, acceleration due to gravity and depth of the liquid from its free surface.
Density
The amount of mass per unit volume is called density. It is denoted by (d). Its SI unit is kg/ m*3. ie.
              Density(d) = mass(m)/ volume(v)
The density of pure water is 1000 kg/m*3.
Relative density
Relative density of any substance is defined as the ratio of the density of a substance to the density of water at 4 degree celcius . If the relative density of gold is 19.2 then it means that the density of gold is 19.2 timesthe density of water at 4 degree celcius.

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