CBSE Physics Experiment Atmospheric Pressure Using Fortin’s Barometer

Study of atmospheric pressure carries an essential significance for us. Atmospheric pressure helps us to estimate the suitable weather conditions prevailing over an area – a low-pressure system leads to precipitation, cloudiness and wind, whereas a high-pressure system leads to calm and fair weather. A small change in atmospheric pressure leads to a change in health conditions among humans, such as change in blood pressure and uneasiness. Following is a simple experiment, intended to give the reader an insight of a simple apparatus to measure atmospheric pressure.

Table of Contents

• Aim

• Theory

• Procedure

• Observations

• Result

Aim

To measure the atmospheric pressure using Fortin’s barometer.

Apparatus Required

• Fortin’s barometer

• Thin safety gloves

• Thermometer

• A paper and pencil

Theory

Fortin’s barometer consists of a vertical glass tube of about a meter, opened at the downward end into a beaker labelled C. The tube is filled with mercury up to a certain limit put into the beaker containing mercury. Leather patch L closes the side and a small ivory pin P touches the level of mercury in the beaker at its tip for adjusting zero of the scale in accordance with the mercury level. A graduated scale is engraved on the outer tube.

There exists a vacuum at the top as some mercury spills down from the tube into the beaker due to gravity. Mercury level stabilizes when the pressure exerted by the mercury in the tube is equal to the atmospheric pressure exerted in the beaker. At this point, from the graduated scale, we can determine the atmospheric pressure by noting the mercury level and the formula

\[P = h\rho g\]

where P = Pressure exerted by the mercury column under stable condition = Atmospheric pressure

h = Height of the mercury column

\[\rho \]= Density of mercury =\[13600\;kg/{m^3}\]

g = Acceleration due to gravity =\[9.8\;m/{s^2}\]

By noting three observations at different temperatures, the atmospheric pressure can be determined.

Barometer

Procedure

1. Hang the barometer vertically on a wall using a plumb line.

2. Check all the screws and least count of the engraved scale.

3. Adjust the level of mercury in the tube. Ensure that the tip of the ivory pointer P exactly touches the surface of mercury in the beaker (see below figure).

Mercury Surface in the Reservoir.

Procedure of Seeing the Level of Meniscus.

4. Adjust the smaller scale such that its zero exactly touches the convex meniscus of the mercury in the tube. The convex meniscus should be the level to be observed. 

5. Allow the mercury level to stabilize for 5 minutes. Note the readings on both scales. Also note the temperature, date and time.

6. Repeat the same procedure two more times (in the interval of some days) such that the temperature changes. Tabulate the data.

Observations

Divisions on main scale = ..........

Least count of main scale = ..........

Divisions on vernier scale = ..........

Least count (LC) of vernier scale = .......... 

Room temperature = ..........\[^\circ C\]

Observation Table

Result

1. At temperature\[{T_1}\], atmospheric pressure\[{P_1}\]= ............ \[(N/{m^2})\]

2. At temperature\[{T_2}\], atmospheric pressure\[{P_2}\]= ............ \[(N/{m^2})\]

3. At temperature\[{T_3}\], atmospheric pressure\[{P_3}\]= ............ \[(N/{m^2})\]

Precautions

1. Handle the barometer very carefully while hanging on the wall.

2. The wall mount should be rigid.

3. Note the least count and readings of both the scales very carefully.

4. Screws should be handled with due care.

5. Temperature differences should be finite but not very small/ very large.

Lab Manual Questions

1. Why is the barometer required to be adjusted every time before use?

Ans: Various pressures can vary the accuracy of mobile signals and bounce-off walls. They also changed due to altitude. To remove this residual pressure error, the barometer requires to be adjusted every time before use.

2. What will happen to the readings if the ivory pin is not adjusted properly?

Ans: Ivory pin adjusts the zero of the scale at the same level as that of mercury in the beaker. If not adjusted properly, the zero of the scale will not be adjusted and we may record an incorrect height of the mercury column.

3. Why should the tube be perfectly erect and not tilted?

Ans: If the mercury tube gets tilted, the pressure exerted by the mercury column in the tube will not be the actual pressure, but only a component of it depending upon the angle of tilt. So, the observer will not be able to measure correct atmospheric pressure from the graduated scale.

4. Can we use water in place of mercury? Why/ Why not?

Ans: From a theoretical point of view, any liquid can be used in a barometer. But practically, mercury is much denser than water, leading the tube height in a convenient zone. By contrast, the tube height in the case of water would be over 10 meters.

Viva Questions

1. What is atmospheric pressure?

Ans: Atmospheric pressure refers to the weight per unit area exerted by the air column on the surfaces beneath it due to the action of gravity. This is because air particles have a finite mass and hence get attracted by the gravitational force of the earth.

2. Give the atmospheric pressure formula. What is the unit of atmospheric pressure?

Ans: Atmospheric pressure is given by the formula

\[P = h\rho g\]

where 

P = Pressure exerted by the mercury column under stable condition = Atmospheric pressure

h = Height of the mercury column

\[\rho \]= Density of mercury =\[13600\;kg/{m^3}\]

g = Acceleration due to gravity =\[9.8\;m/{s^2}\]

The SI unit of atmospheric pressure is newtons per square meter (\[N{m^{ - 2}}\]) or Pascal (Pa).

3. Explain the three types of pressure.

Ans: The three types of pressure are:

1. Absolute pressure: Pressure having a reference of perfect vacuum is called absolute pressure.

2. Gauge pressure: Pressure having a reference of atmospheric pressure is called gauge pressure.

3. Vacuum pressure: Pressure below atmospheric pressure is known as vacuum pressure.

Absolute pressure is the sum of vacuum and gauge pressures.

4. State the relationship between different pressure units.

Ans: Relationship between different pressure units are as follows:

1. 1 atm = 14.6956 psi = 760 torr

2. 1 bar = 750.0623 torr = 0.98696 atm = 100000 pascal

3. 1 pascal = 10 \[dyne/c{m^2}\]

4. 1 \[kg/c{m^2}\] = 14.696 psi = 10000 mm \[{H_2}O\]

5. What is a manometer? Explain its working.

Ans: A manometer is an instrument used to measure differential pressure using a U-tube filled with liquid, whose both ends are connected to pipe/ pressure tapings. There exists a difference in heights of the liquid in both the tubes of the U-tube due to a difference in pressure, which is readily noted.

6. What is a strain gauge?

Ans: A strain gauge is a resistive element, whose shape changes on the application of pressure which is directly proportional to the pressure applied.

7. Name the most common type of pressure measuring device used in industries.

Ans: Most common type of pressure measuring device used in industries is capacitive-type pressure transmitter.

8. Name five important measuring devices used in industries.

Ans: Important devices used in industries are:

1. Pressure gauge

2. Manometer

3. Resistance type pressure measurement

4. Capacitive type pressure measurement

5. Inductive type pressure measurement

9. What causes pressure in the atmosphere?

Ans: Atmospheric pressure is created by various layers of atoms and molecules that make up the atmosphere. They constantly move in random directions and strike upon a surface to generate atmospheric pressure.

10. Give some useful atmospheric pressure examples.

Ans: Atmospheric pressure examples include:

1. Rubber sucker

2. Drinking straw

3. Syringe and dropper

Practical Based Questions

1. Divers wear special suits to get protection from:

1. Low pressure

2. Low temperature

3. High pressure

4. High temperature

Ans: (C) Divers wear special suits to get protection from combined high pressure due to atmosphere and water.

2. Air pressure is ............ as that at the sea lever at high altitudes.

1. Same

2. Less than

3. More than

4. Depends on region

Ans: (B) At higher altitudes, air pressure is less than the pressure at sea level.

3. The atmospheric pressure at the sea level is ............ atm.

1. 1

2. 10

3. 100

4. 1000

Ans: (A) The atmospheric pressure at the sea level is 1 atm.

4. Pressure increases with:

1. Increasing depth

2. Decreasing depth

3. Tangential to the depth

4. Depends on temperature

Ans: (A) Pressure increases with an increase in depth.

5. Atmospheric pressure is measured by an instrument called:

1. Seismograph

2. Compass

3. Anemometer

4. Barometer

Ans: (D) Atmospheric pressure is measured by an instrument called a barometer.

6. Atmospheric pressure is maximum at:

1. Top of a building

2. Sea level

3. About 1 km below the surface of sea level

4. None of these

Ans: (B) Atmospheric pressure is maximum at sea level because at the sea level, air molecules are compressed by weight above them at the most.

7. Why does a nail have a pointed tip?

1. Force acting on small area exerts small pressure

2. Force acting on large area exerts zero pressure

3. Force acting on small area exerts large pressure

4. Force acting on large area exerts large pressure

Ans: (C) A nail has a pointed tip so that the force acting on a small exerts a large pressure.

8. One atmospheric pressure is equal to:

1. 100 Pa

2. 1000 Pa

3. 100000 Pa

4. 10000000 Pa

Ans: (C) One atmospheric pressure is equal to 100000 Pa.

9. Gas pressure is caused due to:

1. Gas molecules colliding with one another

2. Gas molecules colliding with the walls of the container

3. Heating of gas molecules

4. Gas molecules reacting with each other

Ans (B) Gas pressure is caused due to the molecules colliding with the walls of the container.

10. What is the thrust on a unit area called?

1. Volume

2. Surface area

3. Mass density

4. Pressure

Ans: (D) The thrust on a unit area is called pressure.

Conclusion

From this experiment, we can conclude that the study of various kinds of pressures, particularly atmospheric pressure, is indispensable in the field of physics for us. A check on atmospheric pressure helps us to determine prevailing weather conditions over a region. Similarly, changing pressure conditions changes the health balance in human beings. Pressure is commercially essential in many of our daily services and a number of creative instruments exist to keep a check on pressure.