Pressure in Liquid and Gas

In Physics Pressure can be defined as the force acting perpendicularly on a surface per unit area.

The unit of  Pressure is the Pascal or Newton Per Square Metre.

Mathematically,

                            Example

A force of 80N acts on an area of . What is the pressure exacted on the surface ?

Pressure In A Liquid

Consider the height h1 and h2 of a column of liquid in a rectangular tank as shown in figure 1 above.

The volume of liquid resting on area A at height h1 = Ah1

The volume of liquid resting on area A at the base where height is h2 = Ah2

Density ρ  x volume

But

Weight W = Mass m x acceleration due to gravity g

W=mg

And

Where  = acceleration due to gravity

CHARACTERISTIC OF PRESSURE IN A LIQUID

1. The pressure in a liquid is directly proportional to the height or depth of the liquid.

2. The pressure in a liquid is directly proportional to the density of the liquid. The pressure in different liquids at the same depth varies directly with density ρ.

3.  The pressure of a liquid with same density and depth on Earth  is higher than that of the moon. This is because of lower acceleration due to gravity   on the moon.

4. The pressure at any point in the liquid acts equally in all directions.

5. The pressure at all points at the same level within a liquid is the same.This is illustrated in fig2 above. The pressure at the bottom of the containers are the same. This is because pressure is not affected by the breadth of the joined container but is affected  by the depth of the liquid. The pressure at A is the same at all container, The pressure At B is the same at all container.

The pressure at C is the same at all container. The pressure at C is higher than pressure at A and B

Example

A  Tank of height 30m is half full of water . What is the Pressure at the bottom of the Tank ?

  and density of water

Solution