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Pressure
16th December 2015
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Recap: What do these words mean?
• Force:
– a push or a pull
• Surface Area:
– a 2-D space over a surface
• Pressure:
– a force acting over a given surface area
– P = F/A
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Units for Pressure, Force and Area
• Force:
– measured in Newtons (N)
• Area: – measured in square metres (m2)
•Pressure: – measured in Newtons per square metre (N/m2)
– this is called a Pascal (Pa)
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What is Pressure?
• Pressure = Force ÷ Area
• Recall that force is weight of an objectacting over a surface area and this causes
pressure. Hence;
Pressure = Weight ÷ Area
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Units of Pressure
• Pressure is measured in Pascals (Pa).
• We sometimes see pressure beingmeasured in N/m2 or N/cm2
•1 Pa is the same as 1N/m
2
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What is Pressure?
• Any object that possesses weight, whether atrest or not, exerts a pressure upon the surfacewith which it is in contact.
• The magnitude of the pressure exerted by anobject on a given surface is equal to its weight
acting in the direction perpendicular to thatsurface, divided by the total surface area of contact between the object and the surface.
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Force of object acts perpendicular to
the surface area of contact
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Sensible shoes?• The force (weight) of our body
pushing down will exert a pressure.
• The surface area of our shoes incontact with the ground will
affect the pressure.
• These stiletto shoes willconcentrate the force over a
small area.
• These shoes will give a larger pressure and would not besuitable for walking on soft
ground like sand.
Surface area
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More sensible shoes?• These boots have a large surface area.
• The weight of a person is spread out
over the larger surface area.
• The pressure exerted by the person is
smaller than if they were wearing
stiletto shoes.
• A small pressure means you will less
likely sink into soft ground.
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Examples of where a large area is
needed to reduce pressure.
Caterpillar tracks on diggers and on tanks
Tyres on a tractor
Skis and snowboards
snowshoes
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Examples of where a small area is
needed to increase the pressure
• Nails
• Needles
• Kitchen knives
• Scissors
• Drawing pins
• Spade
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How to Measure Pressure
• Manometer
•Elastic Transducers
• Bourdon Gauge
• Electrical Pressure Transducers
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Manometer
With both legs of a U-tube
manometer open to the
atmosphere or subjected to the
same pressure, the liquid
maintains the same level in
each leg, establishing a zero
reference.
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Manometer
•With a greater pressureapplied to the left side of a U-tube manometer, the liquidlowers in the left leg andrises in the right leg.
• The liquid moves until theweight of the liquid, asindicated by h, exactly
balances the pressure appliedat P2.
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Measuring Pressure – Bourdon Gauge
•
A Bourdon gauge uses a coiled tube, which, as it expands due to pressure increase causes a rotation of an arm connected to the tube.
• In 1849 the Bourdon tube pressure gauge was patented
in France by Eugene Bourdon.
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What is Atmospheric Pressure?
• Atmospheric pressure is the pressure exerted by the weight of air in the atmosphere of Earth.
• Gravity pulls the air molecules toward theearth, giving them weight.
• The weight of the air molecules all around usover a given surface area is called the air pressure or atmospheric pressure.
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The air is made up of molecules.
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Illustration of Atmospheric Pressure
Surface area thatweight of air is
acting on
Force = Weight of Air
= Mass x Gravity
Atmospheric Pressure is:
Weight of Air / Surface Area
Gravity
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Atmospheric Pressure
Gas pressuredepends on both
density and
temperature.
Adding airmolecules
increases the
pressure in a
balloon.
Heating the airalso increases
the pressure.
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Air pressure isequal in all
directions.Pressure = force per unit area
Because air is a fluid, force applied in one direction is distributed
equally in all directions.
Thus the downward pull of gravity on air molecules produces air
pressure in all directions.
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Torricelli’s barometer
used a glass columnsuspended in a bowl of
mercury. The pressure
of the air molecules
pushed the mercury upinto the glass tube.
The weight of the mercury in
the tube was equal to the
weight of the air pressing
down on the mercury in the
dish.
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As atmosphericpressure
increases…
The mercury in
the tube rises.
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a) When a straw is put into aglass of orange soda, the
pressure inside and outsidethe straw is the same, sothe liquid levels inside andoutside the straw are the
same.
b) When a person sucks onthe straw, the pressure
inside the straw is lowered.The greater pressure on thesurface of the liquidoutside the straw pushesthe liquid up the straw.
Drinking Straws
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Suction Cups
The cardboard does not fall
and the water remains in the
glass even though it’s not
supported by anything.
The force caused by the
atmospheric pressure acts onthe surface of the cardboard is
greater than the weight of the
water in the glass.
Li id P
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Liquid Pressure• Liquids are made up of particles
• The particles can move in all directions
• The pressure exerted by a liquid acts in all directionstoo
• Liquids with a high density will exert a higher pressure
• As you go deeper, the pressure increases
• This is due to the weight of all the water above pushingdown
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Water Pressure and Body Pressure
• Water pressure
acts in all
directions on this
fish
• The fish’s body
pushes back with
an equal and
opposite pressure
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Submarine and Pressure
Air is vented out by opening the
top of the plastic tubing, to allow
water to “flood” the submarine.
Air is blown into the plastic
tubing, to push water out of the
submarine, so it can float
upwards.
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How Submarine WorksValves are used to
control flow of water
into or out of the
exterior of the
submarine.
By allowing theballast tank to fill up
with water, the lower
valves will be open, as
more water fills up,the submarine will
start to submerge.
The upper valves are
also open to vent air
out.
Lower valves
Upper
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How Submarine Works
Valves are used tocontrol flow of air
into or out of the
exterior of the
submarine.
To surface the
submarine, the upper
valves must open to
allow air into the
ballast tank to push
the water down pass
the lower valves.
Upper
P ti Q ti
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Practice Questions
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