What is pressure?

State the equation for calculating pressure.

A liquid is held in its container by its weight. This creates pressure in the liquid and on the container.Pressure in liquids has the following properties:• The pressure acts in all directions.

• The pressure increases with depth

• Pressure depends on the density of the liquid

• Pressure does not depend on the shape of the container.

Expt.p.64

Pressure in a liquid is calculated using the equation:

P = ρgh

Where P= pressure in Pascal (Pa)

ρ = density in kg/m3

g = acceleration due to gravity (10m/s2 = 10N/kg)

h = height in meters (m)

Example + ex p.65

The air in Earth’s atmosphere has weight and acts on the Earth itself. This pressure created is called Atmospheric pressure.At sea level, atmospheric pressure is about 1.0x105Pa (= 100kPa).Atmospheric pressure can also be measured in:• Mercury height (---mm = --cm)• Atmospheric pressure (1 atm)• Millibar (1000mb)

Atmospheric pressure is measured using a Barometer.Mercury barometers use liquid mercury. Mercury has a density of 13 590kg/m3.

Qn: calculate the height of the mercury column of atmospheric pressure at sea level.

A manometer measures pressure difference.The height difference shows the extra pressure that the gas supply has in addition to atmospheric pressure.The extra pressure is called excess pressure.To find the actual pressure of the gas supply;

Actual pressure = ρgh + atmospheric pressurep.69 + ex.