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Pressure -

posted May 14, 2015, 7:32 PM by Upali Salpadoru   [ updated Oct 30, 2016, 10:04 PM ]

Atmospheric. Pressure

Fig.1. Earth is immersed in air.

It is a wellknown fact that the Earth is immersed inside a bigger globe of air called the ‘Atmosphere’. 

This does not have a definite height as it gradually becomes thinner as we go up. Just like any other substance, this mass of air is also attracted toward the earth. So this causes a pressure on every one and everything. It is this pressure that sends air into our lungs.

There are so many simple experiments to demonstrate the existence of atmospheric pressure.

Demonstration 1

 Dip a straw into a liquid as shown and draw the air out. The liquid will come to your mouth. Canvas.jpg


 Fig.2 Why does the milk come up?

Explanation 1.

Nelly:- Could it be that when the air is taken out a liquid  tries to fill it up.


Explanation 2.

Ali: - As the air is taken into the mouth, inside the straw air pressure decreases. But the outside air pressure remains the same . So this pressure pushes the liquid into the mouth.

Who is right?

An experiment to find out.

Take a glass tube with the two ends open. Press one end on to your hand and fill about half of it with water. Then suck the air out and create a low pressure area. Does the water come up.

Demonstration 2.

PRECAUTION Keep a basin to collect any water that may spill

Take a plastic drinking cup and make a hole at the bottom.

Closing the hole with your finger, fill it with water, up to the brim. Then take a piece of paper from a picture card and place it on the top. Press it slightly making sure that no air gets in.

Now invert it as shown here, while keeping the finger tight.

You will be surprised to see that the water does not come down. Then release the finger and watch what happens.

Fig.3. Air supports water.

Demonstration 3

Take an empty metal oil can. Add some boiling water to it. Wait for a few seconds for the steam to repel the air out. Then close the lid tightly. And sprinkle some cold water.

What happens?  The tin gets crushed as the steam condenses back to water.

What is the Atmospheric pressure?

Nelly and Ali live in an up stair house. Just below the window of Ali’s room there is a pond. They dipped a transparent plastic tube into the pond and filled it completely with water. They closed one end of it until it was not leaking any water. Then Ali tied a string to it and let Nelly pull it up through the window. This is shown in the picture. Then they discovered that the water level came down a bit.

There is a limit to which water can ascend. When the water pressure become equal to the atmospheric pressure the height of water column is equal to the atmospheric pressure. This would be about 10 m. at sea level.

As you go up to hills will this increase or decrease?


These are the instruments that can measure atmospheric pressure. There are two main types. Liquid barometers and Aneroid barometers.

Fig. 4 . Measuring atmospheric pressure.

Some Alternative Units of Pressure

  • 1 bar - 100,000 Pa
  • 1 atmosphere - 101,325 Pa
  • 1 mm Hg - 133 Pa

Fig. 5.  Principle of Mercury barometer


Model Mercury barometer shown here is one of the simplest and if made properly it is very accrate and long lasting. The tube shown here has to be very long at least 80cm. Professional type barometers use a capillary tube fixed to a wooden stand. There is a device to keep the mercury level in reservoir constant. In order to read slight changes in level there is a vernier scale attached.


The normal pressure at sea level will be 76 cm Hg.  (Hg is Mercury)

The mercury in the cup is exposed. So it is not easy to make this mobile.

Fig.6 Aneroid barometer

Aneroid Barometers do not use a liquid.

This is a very convenient instrument that can be used in ships and some other vehicles.

Fig.7. This is how it works.

 It has a small metal capsule from which air has been removed. The diapragm on top is very thin and flexible. As the air pressure varies it moves up or down. This is magnified bythe  lever and the chain attached to a wheel.

Expressing the pressure in Pascals

One pascal is a pressure caused by a force of 1 newton on 

How the Pressure affects the Weather.

Pressure change.

Weather to be expected.


Storms, rain and windy


Good, dry, and cold.

Slow, regular falls


Small rapid decrease

Windy and some showers.

A quick drop.

May be a storm in a few hours.

A rapid increase and continuous decrease.

Long period of poor weather.


Pressure - Hydraulics

 An introduction from Wikipedia.

" In ancient Sri Lanka, hydraulics were widely used in the ancient kingdoms of Anuradhapura and Polonnaruwa.[3] The discovery of the principle of the valve tower, or valve pit, (Bisokotuwa in Sinhalese) for regulating the escape of water is credited to ingenuity more than 2,000 years ago.[4] By the first century AD, several large-scale irrigation works had been completed.[5] Macro- and micro-hydraulics to provide for domestic horticultural and agricultural needs, surface drainage and erosion control, ornamental and recreational water courses and retaining structures and also cooling systems were in place in Sigiriya, -"

Hydraulics is the study of the flow and pressure in liquids and gases. A gas has been collected by the displacement of water. According to the diagram, what is the pressure of the gas?

Gas p.jpg

Fig.1. Finding the pressure of a gas supply.

The conditions were as follows:-

·         Atmospheric pressure    .    .     = 10.26 m

·         The difference in water levels = 0.20 m.

From the diagram it should be clear to you that the gas pressure is equal to water in the right arm plus the atmospheric pressure.

Gas Pressure      = Atmospheric pressure + water pressure.


This can be converted to the standard units as follows:

The pressure of a liquid = height x density x gravity.

                                             = 10.06 x 1000 x 10 newton per square meter   or Pascal.

      Written as    ..      .            = 100 600. Pa.


These are very simple devices consisting of ‘U’ tubes, ‘J’ tubes or simply plastic tubing used to measure or compare the pressure of liquids or gases.

It is basic fact here to note that in any liquid , the pressure at the same level is the same.

Fig. 2  Manometer to find the pressure of  gas.

Readings available:  Atmospheric pressure (A) = 0.76 m.

Height           .         .             .              .         .        (h)=  0.25 m

According to this P1 = P2.

P2  .     .      .       .      .  = A + h

                                       = 0.76 + 0.25 mHg. = 1.01 mHg.

“A common operating pressure for natural gas appliances is around 7 inches of water column (WC) or re-stating this in equivalent measure, that's 14.9 millibars or 1743 Pascals or Pa, or about 0.25 psi (pounds per square inch) or about 4 ounces of pressure per square inch”.

Converting to Pascals.  Pressure = hdg.

                                                     = 1.01 x 13593 x10  = 137289 Pa.

                               The difference  =  35964 Pa

Manometers may be used to compare the densities of liquids.

A basic fact that should be remembered here is that in a liquid the pressure at the same level is always the same. This applies only for the same liquid.

Let us find the density of oil assuming that the density of water as 1000kg.m3.



                  Height of oil ho   = 0.5 m.

                  Height of water = 0.35 m.   

The pressure at P1 = the pressure at  P2.

ho x density of oil x10 = hw x 1000 x 10  ( cancel 10)

Density of oil  = (0.35 x 1000 x )  / 0.5

                              =  700 kg.m3.

Fig. 3.  An oil column being balanced by water.

Hydraulic Lifting

Gas p.jpg

Hydraulics is often employed to lift very heavy loads. This diagram explains how a man can lift a car single handed.

The base of the two pistons are at the same level. As the liquid is the same the pressure at the same level is the same.

Using the given Force and area we can determine the pressure by the small poston.

P small piston    = 750/ 0.001

P large piston   =  / 0.02

As the liquid pressures have to be the same at the same level.  / 0.02 = 750/ 0.001

Force of car =  (750/ 0.001) x 0.02 =15000 N


 Fig. 4 Find the weight of the car.