Pressure in Liquids:

Consider a surface of area A in a liquid at a depth of h. The length of the cylinder of liquid over this surface will be h.

The force acting on this surface will be the weight w of the liquid above this surface. If r is the density of the liquid and m is the mass of liquid above the surface, then

Mass of the liquid cylinder = m = $volume\times density$
= $(A\times h) \times \rho$ 

Force acting on Area F = w = mg
= $A h \rho g$
As, Pressure P = $\frac{F}{A}$     
= $\frac{(A h \rho g)}{A}$
Liquid pressure at depth h = P = $\rho g h$    ………(i)

Equation i) gives the pressure at a depth h in a liquid of density $\rho$. It shows that the pressure in a liquid increase with depth. $(P \alpha h)$

DO YOU KNOW?

The piston of the syringe is pulled out. This lowers the pressure in the cylinder. The liquid from the bottle enters the piston through the needle.