Pressure

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It is defined as force per unit area

It is measured in SI units of Pascal (Pa)

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Barometer

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 To measure atmospheric pressure an instrument called a Barometer is used.

Standard atmospheric pressure is 100 000 Pascal and this will cause the column of Mercury (Hg) to rise up the evacuated tube by 760 mm.

In other words: 100 kPa = 760 mmHg = 1 atmosphere

How pressure of a gas results from the kinetic energy of particles

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• Gas particles move about quickly and randomly
• They collide elastically with the walls of the container and change their momentum each time they collide.
• This change of momentum per unit time imparts a force
• Many particles exert a total force on each unit area of the container walls
• In this way pressure is exerted by the sum of the many particle collisions

Pressure in a liquid

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The pressure in a liquid depends upon:

• the density of the liquid
• the acceleration due to gravity
• the height of water above the level being considered

These three factors go together to produce the equation:

p = ρgh               p=pressure

                          ρ=density

                          g=acceleration due to gravity

                          h=height of water above

 Manometer

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An instrument used to measure the difference in pressure between two fluids

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How can pressure be used to transmit and magnify force?

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• Since the liquid is in-compressible (no volume change) the pressure exerted by the first piston is transmitted directly to the second piston
• In other words pushing piston 1 down will push piston 2 up
• As this pressure remains constant the ratio of F1/A1 is equal to the ratio of F2/A2
• If A2 = 50xA1 then a force of 1 Newton at F1 will produce a 50 Newton force at F2