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# UNIT 7: PRESSURE

• Pressure: It is the force per unit area.

P = F / A

• Unit: Pa

• Examples

• Swords exert high pressure because they concentrate the force on a small area.

• Skis exert low pressure on the snow because the weight is spread over a large area.

• Pressure of liquids

• Pressure of a liquid is due to weight of the liquid.

P = h ρ g

When something is immersed in a liquid, the liquid pushes it up. This upthrust is because the pressure near the top of the object is less and the pressure near the bottom of the object is more.

• Hydraulic systems: In such systems, pressure is transmitted between pistons using liquids because liquids are incompressible.

Py = Px
Fy / Ay = Fx / Ax
Fy = Fx ( Ay / Ax )

This means that force Fx will result in a bigger force Fy if Ay > Ax.

• Pressure of gases

• Pressure of gases is due to molecular collisions.

• Pressure of a gas can be changed by

• changing temperature

• changing mass

• changing volume

• Atmospheric pressure

• At sea level it is:

1 atmosphere = 1.0 x 105 Pa = 76cm of Hg = 760mm of Hg

• On a mountain top it is less.

• In a deep mine it is more.

• Simple Mercury Barometer: A simple mercury (Hg) barometer can be used to measure atmospheric pressure.

• Working: Pressure of atmosphere is equal to the pressure of (additional height of) liquid.

• Manometer: A manometer can be connected to a gas container to measure the pressure of the gas inside the container.

• Working:

• PG = PL + P0

OR

• PG + PL = P0

• Boyle’s Law: P1 V1 = P2 V2

• This formula assumes that temperature or mass does not change.

• Total pressure beneath the surface of a liquid is the sum of atmospheric pressure and the liquid’s pressure.

• When a molecule hits a surface it exerts a force.

• When many molecules hit a wall at the same time, their collective force per unit area is pressure ( P = F / A ).

• Pressure depends on frequency of collisions.

• Frequency of collisions is affected by change in mass, volume or temperature:

• When mass increases, the average separation between molecules decreases. So more frequent collisions.

• When volume decreases, the average separation between molecules decreases. So more frequent collisions.

• When temperature increases, the speed of molecules increases. So more frequent and more forceful collisions.