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Pressure: It is the force per unit area.
P = F / A
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Unit: Pa
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Examples
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Swords exert high pressure because they concentrate the force on a small area.
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Skis exert low pressure on the snow because the weight is spread over a large area.
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Pressure of liquids
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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.
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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.
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Pressure of gases
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Pressure of gases is due to molecular collisions.
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Pressure of a gas can be changed by
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changing temperature
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changing mass
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changing volume
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Atmospheric pressure
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At sea level it is:
1 atmosphere = 1.0 x 105 Pa = 76cm of Hg = 760mm of Hg
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On a mountain top it is less.
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In a deep mine it is more.
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Simple Mercury Barometer: A simple mercury (Hg) barometer can be used to measure atmospheric pressure.
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Working: Pressure of atmosphere is equal to the pressure of (additional height of) liquid.
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Manometer: A manometer can be connected to a gas container to measure the pressure of the gas inside the container.
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Working:
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PG = PL + P0
OR
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PG + PL = P0
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Boyle’s Law: P1 V1 = P2 V2
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This formula assumes that temperature or mass does not change.
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Total pressure beneath the surface of a liquid is the sum of atmospheric pressure and the liquid’s pressure.
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When a molecule hits a surface it exerts a force.
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When many molecules hit a wall at the same time, their collective force per unit area is pressure ( P = F / A ).
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Pressure depends on frequency of collisions.
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Frequency of collisions is affected by change in mass, volume or temperature:
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When mass increases, the average separation between molecules decreases. So more frequent collisions.
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When volume decreases, the average separation between molecules decreases. So more frequent collisions.
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When temperature increases, the speed of molecules increases. So more frequent and more forceful collisions.
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