Pressure is the amount of perpendicular or normal force per unit area acting on a surface. It is also known as the effect of a force applied to a surface. The symbol of pressure is p.
can be used to cut an object easily because a small force applied on a very small area of the knife edge can produce a large pressure for cutting the object.
2. Spiked Running Shoes
can provide a better grip for athletes running on a track because these shoes have spikes that produce large pressure to pierce the track.
3. Ice-skates
designed to have a small area of contact between the skates and the ice.
while skating, the weight of the skater produces a large pressure on the ice and melts it, thus enabling the skates to slide easily over a thin film of water.
4. Saws, Chisels and Planes
have sharp edges to produce a large pressure for cutting.
nails, pins and needles also have sharp points to produce a large pressure for piercing an object easily.
Applications of Low Pressure
1. Bulldozers
usually moved by a chain with sharp edges in between surfaces of large area to decrease the pressure exerted on the ground.
in this way, the chin can prevent the bulldozers from sinking into soft ground.
2. Heavy lorries and buses
usually fitted with more than four wheels of larger surface area to decrease the pressure exerted on the road.
this can prevent the tyres from bursting and sinking into the soft ground.
3. Tall buildings
usually built on a base of large surface area to decrease the pressure and to prevent the buildings from sinking to the ground.
4. Skis
designed to have a larger surface area to decrease the pressure exerted on the snow.
this enables the skiers to move smoothly without sinking the snow.
Science Corridor
Research on atmospheric pressure enables meteorologists to make reports on weather forecast. Image 1.0 shows a weather map consisting of isobars which are lines representing regions having the same atmospheric pressure. The lines are labelled with numbers that represent the atmospheric pressure in units of millibars. The value of 1000 millibars equals 760 mm Hg or 1 standard atmosphere.
Understanding Pressure in Liquid
1. At the same level or depth, the liquid pressure must be the same, otherwise liquid
will flow to equalize any pressure differences.
2. This confirms the fact that the pressure in a liquid :
depends on vertical depth
is dependent of the shape and cross-sectional area of the containing vessel
h = height of liquid column or depth within a substance
Applications of Pressure In Liquids
1. Public Water Supply System
The reservoir is placed at an elevated location so that the water will have sufficient pressure to flow to consumers located at lower grounds.
A water tank is usually built atop the roof of each house to store water and maintain constant water pressure.
A high-rise building is supplied with water from its own water tank which is situated at the highest level.
2. The Walls of a Dam
A dam is built across a river to collect water for the purpose of water supply, irrigation and generation of hydroelectric power.
The walls of a dam increase in thickness downwards.
This is because a thicker wall is required to withstand greater pressure since water pressure increases with depth.
3. Fire Hose Used by a Fire-fighter
A fire-fighter requires a fire hose for directing a water jet of very high pressure to put out fire in a high-rise building.
To produce a pressurised water jet, the water is accelerated by an electric pump before flowing through the fire-hose.
4. IV Injection under Gravity
IV injection is used for injecting solutions of salt or medicine into the vein of a patient.
IV bottle should be suspended at the optimum height, h to ensure that the fluid (solutions) gauge pressure of the IV bottle is greater than the pressure in the vein.
Science Corridor
On the 12th of August 2000, a Russian nuclear-powered submarine called Kursk was stranded at the bottom of the Barents Sea. At a depth of more than 100 m below the sea, it was no easy task to investigate the actual cause of the tragedy that took 118 lives.
Many theories were put forward by different factions to explain the disaster. Basically, these theories were based on two assumptions. It seemed that the Kursk might have collided with another object of mass equal to, or mare than, 8000 tonnes resulting in damage to its front portion. While experts were divided on the first assumption, another possibility was opened up. The powerful Kursk might have dived too fast until its front portion was crushed and damaged by very hugh water pressure at great depth. However, the mystery of the Kursk's disaster remains unsolved until today.
The Russian Nuclear Powered Submarine, Kursk
Understanding Gas Pressure and Atmospheric Pressure
1. Gas Pressure
according to the Kinetic Theory Of Gases, molecules in a gas are always moving randomly and constantly colliding with the walls of its container.
collisions of gas molecules with the walls of the container occur frequently, resulting in a change of momentum which exerts a force on the wall of the container.
the force per unit area produced by collisions of gas molecules on the walls of the container is the gas pressure.
2. Atmospheric Pressure
it is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth.
it can extend up to 1000 km above Earth's surface and has a total mass about 5 million billion tonnes.
the air molecules near Earth's surface are subjected to compression that produces higher air pressure which supports the atmosphere and prevents it from collapsing.
we do not normally feel the large atmospheric pressure because the blood pressure inside our bodies can balance the external pressure.
Effect of Altitude on the Magnitude of Atmospheric Pressure
1.At sea level, the atmospheric pressure is 760 mm Hg.
2.At high altitudes, the density and temperature of air become lower. Therefore, the frequency of collision of the air molecules decreases, producing a lower atmospheric pressure.
3. Effects on humans :
breathing is difficult.
nose-bleeds may occur due to a slightly higher pressure of the blood in our body than the external pressure.
4. Solutions :
cabins of modern aircrafts are pressurised by increasing the air pressure to safeguard the crew and the passengers.
it is used for measuring differences in gas or liquid pressure.
consists of a U-tube containing a liquid such as mercury and water.
for example, sphygmomanometer is a mercury manometer designed for doctors to measure the blood pressure of patients.
the pressure of the gas , p, can be calculated by using the expression :
P = Pa + hpg
Pa = atmospheric pressure
h = height difference of liquid column
p = density of liquid in the manometer
g = Earth's gravitational field strength
2. Bourdon Gauge
it is used for measuring very high pressure such as the pressure of steam in a boiler or the pressure of compressed gases.
consists of a flattened and hollow metal tube shaped like a hook.
when a compressed gas enters the curved tube, the tube tends to straighten out thus causing the other closed end to move outwards.
this movement is magnified by the lever arrangement which turns the cog-wheel that moves the pointer over a scale to indicate the pressure reading.
Instruments for Measuring Atmospheric Pressure
1. Simple Mercury Barometer
made from a clean, dry and thick-walled glass tube about 100 cm long which is completely filled with mercury.
the mercury column in the tube drops until its height is above the mercury level in the dish which is 760 mm Hg (standard atmospheric pressure or 1 atm)
the space between the mercury column in the tube is a vacuum known as Torricellian vacuum.
the height of the mercury column in the tube is unaffected by :
a) using glass tubes of different diameter.
b) tilting the tube at different angles
2. Fortin Barometer
its tube is enclosed in a metal case with glass windows at the upper part of the tube for viewing the mercury column.
the height of the mercury column is measured from the tip of the ivory pointer to the mercury level in the tube.
an accurate measurement of the height of the mercury column can be obtained by adjusting a vernier scale over the main scale.
3. Aneroid Barometer
it contains no liquid and can be carried about conveniently
consisits of a corrugated steel box which is sealed after some of the air from inside is pumped out.
changes in the air pressure make the box contract or expand.
the movements of the box are magnified by a system of levers which causes a pointer to move over a a calibrated scale.
example :
a ) weather forecasting - low pressure indicates an approaching storm
- high pressure is associated with fine weather.
b) altimeters - located in aircrafts for measuring height
Magnitude of Atmospheric Pressure
pressure at the base of a mercury column
1 atm = 101.325 kPa
water can also be used in a barometer.
however, a much longer glass has to be used since water has a much lower density than mercury.
h = 10.3 m
meteorologists often express atmospheric pressure in another unit called bar(b)
1 mb = 100 Pa
therefore :
1 atm = 1.013 b
= 1013 mb
Applications of Atmospheric Pressure
1. Drinking Straw
air pressure inside becomes lower when air is sucked from a drinking straw.
the higher atmospheric pressure acting on the surface of the drink pushes the drink into the drinking straw and enables it to be sucked into the mouth.
2. Syringe
consists of a tight-fitting piston which moves in a cylinder with a nozzle at one end.
commonly used to injecting medicine into the body and for watering plants
the pressure is reduced when the nozzle is dipped into a liquid and the piston is pulled up.
the higher atmospheric pressure acting on the surface of the liquid pushes the liquid into the cylinder.
when the piston is pushed down again, the liquid is ejected.
3. Rubber Sucker
it is a rubber cup that can be made to stick on to a smooth wall such as glass or tiled wall.
made air-tight by wetting its rim before pressing it against the wall to create a partial vacuum by driving the air out.
higher atmospheric pressure acting on the outer surface of the cup presses it in position against the wall.
4. Vacuum Cleaner
it applies atmospheric pressure to remove dust particles.
when it is switched on, a fan sucks out a stream of air to create a partial vacuum ( space X )
higher atmospheric pressure outside pushes the air and dust particles into the space X.
dust particles are trapped in the dust bag while the fat moving air is allowed to flow out from the back of the vacuum cleaner.
5. Siphon
Siphon Principle
the difference in air pressure produces a continuous flow through the rubber tube as long as the point is located lower than the liquid level in the tank.
Pascal's Principle
also known as Pascal's principle of transmission of pressure in fluids.
the principle was established by French mathematician Blaise Pascal.
states that pressure exerted on an enclosed fluid (liquid) is transmitted equally to every part of the fluid (liquid).
This principle is stated mathematically as:
where
= hydrostatic pressure (Pa)
ρ = fluid density (kg m^-3)
g = acceleration due to gravity (m s^-2)
= height of fluid above the point of measurement (m)
Idea of Transmission of Pressure in a Liquid
The effects of Pascal's law, as Pascal discovered in his 1646 barrel experiment
2 properties of liquid made use in hydraulic system :
a) it is incompressible b)if a pressure is applied to an enclosed liquid, then the pressure would be transmitted to all parts of it
when the piston is pushed inwards, water is compressed by the applied force.
the force of compression per unit surface area gives rise to pressure exerted on the water.
since the volume of a liquid is constant, the pressure applied is transmitted equally in all directions throughout the liquid thus proving Pascal's principle.
Simple Hydraulic System
consists of 2 cylinders with each cylinder is fitted with a piston and filled with a liquid such as water and oil.
when a small force, F1 is applied to the smaller piston X, a large force, F2 is produced on the larger piston Y.
F2 is produced by pressure exerting on the larger piston Y due to the pressure transmitted from piston X through the liquid.
if there are some air bubbles present in the hydraulic fluid, the hydraulic system will not operate fully because the force applied on the small piston is partly used to compress the air bubbles and not fully used to exert a pressure on the large piston.
Applications of Pascal's Principle
1. Hydraulic Jack
it uses a fluid, which is incompressible, that is forced into a cylinder by a pump plunger.
however, oil is used since it is self lubricating and stable.
the pressure produced by the force applied is transmitted through the oil to the large piston thus the large piston is pulled up by a large force.
to lower the large piston,the release valve has to be opened to allow the oil to flow back into the buffer tank
2. Hydarulic Lift
consists of 2 cylinders of different diameters with each of the cylinders is fitted with a piston and filled with oil.
when a switch is on, compressed air is pumped into the small cylinder to exert a force and a pressure on the oil.
this pressure is transmitted through the oil to the large cylinder thus producing a force strong enough to lift a heavy load such as cars.
3. Hydraulic Brakes
used in vehicles to produce a braking force on all the wheels simultaneously at the instant the brake pedal is applied.
2 types of brakes in a car : a) disc brakes at the front wheels b) drum brakes at the back wheels
when the brake pedal is pressed, pressure is exerted on the brake fluid by the large piston in the master cylinder.
this pressure is transmitted through the brake fluid to all the small cylinders of the wheels.
when the brake pedal is released, the small pistons return to their initial positions.
a string connecting the 2 brake shoes at the back wheels contract and pull them away from the brake drum.
the wheel is then free to rotate again.
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states that an object which is partially or wholly immersed in a fluid is acted upon by an upward buoyant force equal to the weight of the liquid it displaces.
Buoyant Force
buoyant force is the weight of liquid displaced by an object
it is created by the increase of pressure with depth.
it depends on :
a) volume of the object b) density of the liquid
formula :
apparent immersed weight = weight - weight of displaced liquid
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statesthat a floating body displaces its own weight of the liquid in which it floats.
for an object floating in a liquid :
the weight of liquid displaced = the weight of the body
Monday, 30 July 2012
Application of Archimedes Principle
1. Ship
a ship stays afloat by displacing the weight of sea water to its own weight.
however, it may sink too deeply due to the overloading or changes in water density.
as a prevention, a plimsoll line is marked on the hull of all ships to show the safe depth to be navigated
2. Submarine
it has large ballast tanks to control its position and the depths it can submerge below sea level.
when afloat, water is driven out from the ballast tanks by compressed air to reduce its weight and produce a larger buoyant force.
when submerged, the ballast tanks are filled with water again to increase the weight of the submarine.
it can submerge to a depth where the buoyant force is equal to its weight.
3. Hot-air Balloons
it is used in :
a) weather forecasting b) sports c) recreation activities d) advertisements
when the envelope of a balloon is filled with a gas of lower density than air such as hydrogen, helium or hot air, its weight decreases thus experiencing a buoyant force.
if the buoyant force is equal to the total weight of the balloon, it remains stationary or continues to move upwards with constant velocity.
if the buoyant force is greater than the total weight of the balloon, it would be accelerated upwards by a net upwards force.
4. Hydrometer
an instrument for measuring densities of liquids.
widely used in :
a) checking the condition of a car battery
b) testing the content of natural rubber in rubber latex
consists of a wide bulb carrying a narrow glass stem.
the lower bulb is weighted with lead shots to keep it floating upright.
the immersed length of the hydrometer in a liquid decreases as the liquid density increases due to an increasing buoyant force.
Bernoulli's Principle
states that the pressure of a fluid decreases at the region where the speed of fluid flow increases.
Applications of Bernoulli's Principle
1. Aerofoil
a curved wing used to produce a lift.
the pressure difference between the air below and above the aerofoil produces a resultant upwards force known as a lift.
if the airplane is moving with constant speed, then : a) lift = weight b) thrust = drag
2. Carburettor
used to produce a mixture of petrol and air which can be burnt easily in the engine of a car.
air flowing at a higher speed through the narrow section causes the air pressure there to become lower
the atmospheric pressure pushes the petrol from the tank to flow out through the jet as a spray of petrol vapour after mixing with air.
3. Bunsen Burner
when it is connected to the gas supply, gas gusts out through the jet with high speed which creates a low pressure around the jet.
the outside air at atmospheric pressure which is higher is sucked into the low pressure region.
the mixture of air and gas which enters the barrel is then burnt to produce a flame.
4. Insecticide Spray
when the plunger is pushed into the cylinder, air gusts out at high speed through the narrow opening which produces a low pressure region.
the air above the insecticide is at atmospheric pressure which is higher, so it pushes the insecticide to rise through the metal tube.
Video Tutoring on Bernouli's Principle
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= hydrostatic pressure (Pa)
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