Atmospheric pressure

Atmospheric Pressure

The thick layer of air covering the entire earth's surfaces is called atmosphere. It is  extended up to a high altitude of about 10,000 km from the earth surface in different layers. Such huge mass of air around the earth is pulled by the earth's gravity. The mass of the atmosphere has certain weights. This weight applies pressure called atmospheric pressure. Thus, the force exerted by the weight of the atmosphere per unit area  on the earth's surface is known as atmospheric pressure.

Atmosphere exerts pressure on liquid and solid surfaces of the earth, but we do not feel this pressure. This is because the internal pressure in our body counter balances the external pressure of the atmosphere. The following activities can show us the effect of atmosphere pressure on the surface of the objects on earth.

Write down the Importance of atmospheric pressure

Atmospheric pressure is  very important to us. We make and use various equipment based on atmospheric pressure. Some equipment and their importance are given below: 

1. Atmospheric pressure is used to fill ink in a fountain pen. 
2. Atmospheric pressure is used to fill medicine in a syringe. 
3. Atmospheric pressure is used to fill air in a bicycle tube or tube of a vehicles. 
4. Atmospheric pressure is used to lift water by using a water pump. 
5. Atmospheric pressure is used to draw soft drinks through a straw. 
6. Atmospheric pressure is used in vacuum cleaners to clean dust and dirt. 

Units of Atmospheric Pressure

The SI units of atmospheric pressure is N m6^-2 or Pascal (Pa).

Atmospheric pressure is generally expressed in millimeter of mercury column. 

At sea level, 1atm = 760 millimeters of mercury (mmHg) = 1.013*10^5 Pa

Measurement of atmospheric Pressure

The instruments which are used to measure atmospheric pressure are called barometers. The commonly used barometers are mercury barometer , Fortin's barometer and Aneroid barometer.

Mercury barometer

Mercury barometer consists of mercury. A simple mercury barometer can be made with a  clean, dry, thick-walled glass tube of about one meter long. The tube is sealed at one end and filled with mercury. The tube is inverted over a trough with mercury. This tube is fixed with the help of a scale. A graduated scale is made on the tube to measure atmospheric pressure. 

At sea level, the mercury level in the tube drops until it is about 760 mm or 76 cm vertically above the mercury level in the trough. This height of the mercury column in the tube drops when the barometer is carried to higher altitudes due to decrease in the atmospheric pressure.

Standard Atmospheric Pressure

The pressure exerted by the atmosphere at sea level is called standard atmospheric pressure. It is equal to the pressure exerted by the mercury column of height 760 mm (76cm) .This pressure is taken as 1 atmosphere (1 atm). Atmospheric pressure is described as a pressure of 760 millimeters of mercury.

Variation of Atmospheric Pressure with Altitude

Atmospheric pressure changes with the altitude and the weather. It decreases with increase in altitude. At Mt. Everest atmospheric pressure is about 30% of the pressure at sea level.

Effect of gravity is less at a high altitude, so there is thin atmosphere. This reduces the density of the air as well as the atmospheric pressure. Aircrafts fly at a high altitude in a low pressure zone. In case  of the high altitude , the air pressure inside aero planes and jet planes is maintained for comfortable breathing of  the passengers.

Applications of the Atmospheric Pressure

a. Syringe

A syringe is a medical instrument having a barrel with a nozzle and piston for sucking and ejecting machines, blood and other liquids. It is mainly used in the hospital for injection medicine in the blood through veins or for ejecting blood.

Construction

A syringe has an airtight piston, storage cylinder (barrel) and needle. A hollow needle is attached to the storage cylinder. Its piston slides smoothly inside the cylinder. when the plunger handle is pulled , fluids are drawn into the storage cylinder. This fluid is forced out through the needle when the handle is pushed.

Working 

To fill medicine in a syringe, its needle is inserted into a medicine pot. when piston is pulled outward, the air pressure in the syringe barrel is reduced. As a result , there occurs formation of a partial vacuum in the barrel. Liquid flows from the region of high pressure to the low pressure. Therefore, the medicine in a pot or the  blood in the veins of our body is filled into the storage cylinder  (barrel).

b. Hand pump or water pump

Water pump is a manual suction pump that lifts water to a level of a spout. It is widely used in the rural plain area to lift underground water. Muscular power is used to lift water by using the water pump. There are different types of hand pumps. Among them, the suction and lift hand pumps are widely used.

In a handpump, suction lift is the vertical distance between the pump and the delivery point. Suction lift in a hand pump is limited by atmospheric pressure to an operating depth which is about 10 meters. So, a water pump is based on the principle that air exerts pressure.

Construction 

A lift pump has  a cylindrical barrel with a spout near  the top. an air tight piston moves up and down the barrel with the help of a handle. A  valve V1 on the piston is called piston value . It moves up and down with the help of a handle as shown in the given figure. The value V2 is at the  bottom of the barrel, called foot  valve. It is made up of rubber of leather flap.

Working 

Up stroke : When we push the handle down, the piston moves upward. There is increase in the gap between the piston valve and foot valve. It creates a vacuum in between  the two valves in the barrel. Due to this valve V2 opens and the valve V1 remains closed. In such conditions, water enters into the  barrel due to the atmospheric pressure.

Down stroke: when we pull the handle up, the piston moves downward . There is decrease in the gap between the piston valve and foot valve. It creates a high pressure in between the two valves in the barrel. Due to this, the valve V1 opens and the valve V2 remains closed. Water is transferred into the upper part of the  barrel . In next upstroke water comes out through the nozzle.

c. Air Pump or Bicycle Pump

An air pump is a device designed to trap a certain volume of air into a storage cylinder which is then pressed into a discharge pipe. They are commonly used for filling air in tubes inside bicycle tires,  balloons and football or volleyball bladders.

Construction : It consists of three main parts. They are: 

i.  A metal or a plastic barrel : air is stored in the barrel.

ii. Piston : The piston inside the barrel is connected to a handle . Piston compresses the air when the handle is pushed down. It is also referred to as the compressor. A leather or rubber cap is fixed on the piston with the help of a nut and bolt arrangement. 

iii. Nozzle: There is a nozzle on the other side of the barrel. 

There are two valves in a bicycle pump. They are inlet valve (V1)  and outlet valve (V2 ) .When one valve opens, another remains closed.

Working 

Up stroke : When we  pull the handle up, there occurs formation of  partial vacuum in between the two valves. Due to the high atmospheric pressure outside, the air enters into the barrel through a hole at the top of the barrel. It opens inlet valve (V1) and air is stored into the barrel . A t the same time the outlet valve (V2) remains closed. 

Down Stroke :  When we push the handle down, the air is compressed and air pressure increases. It opens the outlet valve (V2). Air goes out through the nozzle  to fill the bicycle tubes, footballs bladders , etc. 

In next upstroke , the outlet valve gets closed. It prevents returning  of the air from tubes to the barrel.