Pressure
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5. Pressure
PRESSURE, FORCE & AREA
Pressure is the concentration of a force:
o If a force is spread over a large area it will result in a small pressure.
o If it is spread over a small area it will result in a large pressure.
When you push a drawing pin, it goes into the surface (rather than your finger) because the force on
the surface is more concentrated
• Pressure is related to force and area by the equation:
• You can rearrange this equation with the help of the formula triangle:
Use the formula triangle to help you rearrange the equation
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The units of pressure depend on the units of area:
o If the area is measured in cm2 (and the force in N), then the pressure will be in N/cm2.
o If the area is measured in m2 (and the force in N), then the pressure will be in N/m2.
Pressure can also be measured in pascals, Pa.
o 1 Pa is the same as 1 N/m2
•
•
• Applications of High Pressure
• The examples of application of high pressure are:
•
• Applications Of Low Pressure
• The examples of application of low pressure are
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Question
Example 1:
A force F is acting on a surface of area 20cm², produces a pressure 2500Pa on the surface. Find the
magnitude of the force.
Example 2:
A block of wood 3 m long, 5 m wide and 1 m thick is placed on a table. If the density of the wood is 900
kgm-3, find
a. the lowest pressure
b. the highest pressure
on the table due to the block.
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Pressure in Liquids
• When an object is immersed in a liquid, the liquid will exert a pressure, squeezing the object.
• This pressure is exerted evenly across the whole surface of the liquid, and in all directions.
Pressure, at a point in a liquid, acts equally in all directions
• The pressure in the liquid depends upon a couple of factors:
o The depth of the liquid
o The density of the liquid
• The bigger either of these factors, the greater the pressure.
Pressure in Liquids
• The size of this pressure depends upon the density (ρ) of the liquid, the depth (h) of the object
and the gravitational field strength (g):
• In the above equation:
o Pressure is in pascals (Pa) – where 1 Pa if the same as 1 N/m2.
o Depth is in metres (m)
o Density is in kg/m3.
• The gravitational field strength on Earth is approximately 10 N/kg.
(You will always be given this figure).
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BAROMETERS & MANOMETERS
• Barometers and Manometers are devices that measure pressure:
o A barometer is a device that is used to measure air pressure.
o A manometer is used to measure other pressures.
• A simple barometer consists of a column of mercury in an inverted tube, sat in a tray of mercury
exposed to the atmosphere.
A simple mercury barometer, used to measure atmospheric pressure
• The weight of the mercury in the tube is balanced by atmospheric
pressure pushing down on the mercury in the tray.
o If atmospheric pressure increases, a greater length of
mercury can be supported in the tube.
o If atmospheric pressure decreases then less mercury will be
supported in the tube.
• A simple manometer consists of a U-tube containing mercury.
A simple manometer, consisting of mercury in a U-tube
• One end of the U-tube is open to the atmosphere.
• The other end is connected to the pressure that is to be
measured.
• As the measured pressure increases, the mercury is pushed
around the U-tube:
The greater the pressure, the further it is pushed.
Question
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Example 1:
The diagram shows 2 fishes in water. The density of the water is 1025
kg/m³. The surface area of fish A is 300 cm² and the surface area of fish B
is 2000cm². Find
a. the pressure exerted by the water on fish A.
b. the pressure exerted by the water on fish B.
c. the force exerted by the water on fish A.
d. the force exerted by the water on fish B.
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