Notes : Define Atmospheric Pressure ? - Physicskund

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Learn Atmospheric Pressure Class 11 Physics with complete NCERT-based notes, definition, formula, SI unit, 1 atm conversions, mercury barometer, FAQs, MCQs, true/false, fill in the blanks, and short & long questions

Atmospheric Pressure 

The Earth is surrounded by a thick layer of air called the atmosphere. Air has mass, and due to the Earth's gravitational force, it has weight. The weight of this air exerts pressure on every object present on the Earth's surface. This pressure is known as atmospheric pressure.

Definition of Atmospheric Pressure

Atmospheric pressure is the pressure exerted by the weight of the Earth's atmosphere on any surface due to gravity. It acts equally in all directions.

In simple words, atmospheric pressure is the force per unit area produced by the weight of the air surrounding the Earth.

Why Does Atmospheric Pressure Exist?

Atmospheric pressure exists because:

  • Air has mass.
  • Gravity pulls the air towards the Earth.
  • The weight of the air presses on every object and surface.

Therefore, every object on the Earth experiences atmospheric pressure from all directions.

SI Unit of Atmospheric Pressure

The SI unit of pressure is Pascal (Pa).

\[ 1\ \text{Pa}=1\ \text{N/m}^{2} \]

It means that when a force of one newton acts normally on an area of one square metre, the pressure produced is one pascal.

Standard Atmospheric Pressure (1 atm)

The average atmospheric pressure at sea level is called standard atmospheric pressure.

Its value is:

\[ \boxed{ 1\ \text{atm} = 101325\ \text{Pa} = 1.01325\times10^{5}\ \text{Pa} = 101.325\ \text{kPa} = 1.01325\ \text{bar} = 760\ \text{mmHg} = 76\ \text{cmHg} = 0.76\ \text{mHg} = 760\ \text{Torr} = 14.696\ \text{psi} = 101325\ \text{N/m}^{2} } \]

Formula of Atmospheric Pressure

Atmospheric pressure is calculated using the hydrostatic pressure equation:

\[ P=\rho gh \]

where,

  • \(P\) = Pressure (Pa)
  • \(\rho\) = Density of mercury (kg/m³)
  • \(g\) = Acceleration due to gravity (m/s²)
  • \(h\) = Height of the mercury column (m)

Calculation of Standard Atmospheric Pressure

For mercury barometer,

  • \(\rho=13600\ \text{kg/m}^{3}\)
  • \(g=9.8\ \text{m/s}^{2}\)
  • \(h=0.76\ \text{m}\)

Substituting the values,

\[ P=\rho gh \]

\[ P=13600\times9.8\times0.76 \]

\[ P=101292.8\ \text{Pa} \]

\[ P\approx1.013\times10^{5}\ \text{Pa} \]

The exact standard value is

\[ 101325\ \text{Pa} \]

The slight difference occurs because the value of \(g\) is rounded to 9.8 m/s².

Characteristics of Atmospheric Pressure

  • Atmospheric pressure acts equally in all directions.
  • It is produced due to the weight of air.
  • It is maximum at sea level.
  • It decreases with increasing altitude.
  • It changes with weather conditions.
  • It can support a mercury column of 76 cm in a mercury barometer.

Factors Affecting Atmospheric Pressure

1. Altitude

As altitude increases, the amount of air above decreases. Therefore, the weight of the air column becomes smaller and atmospheric pressure decreases.

2. Temperature

Warm air expands and becomes less dense, producing lower atmospheric pressure. Cold air is denser and produces higher atmospheric pressure.

3. Humidity

Humid air contains more water vapour, which is lighter than dry air. Therefore, humid air generally has lower atmospheric pressure than dry air.

Measurement of Atmospheric Pressure

Atmospheric pressure is measured using a Mercury Barometer.

At sea level:

  • Height of mercury column = 76 cm
  • Pressure = 1 atm

If atmospheric pressure increases, the mercury level rises. If atmospheric pressure decreases, the mercury level falls.

Importance of Atmospheric Pressure

  • Helps living organisms breathe.
  • Responsible for weather changes.
  • Used in mercury barometers.
  • Helps in forecasting weather.
  • Essential for the working of syringes, droppers and drinking straws.
  • Affects the boiling point of liquids.

Applications of Atmospheric Pressure

  • Drinking liquids through a straw.
  • Working of syringes.
  • Operation of droppers.
  • Suction cups sticking to smooth surfaces.
  • Mercury barometers.
  • Weather forecasting instruments.

Key Points to Remember

  • Air has mass.
  • Gravity gives air its weight.
  • The weight of air produces atmospheric pressure.
  • Atmospheric pressure acts in all directions.
  • Pressure decreases with altitude.
  • Standard atmospheric pressure is 1 atm.
  • 1 atm = 760 mmHg = 76 cmHg = \(1.013\times10^{5}\) Pa.

Summary

Atmospheric pressure is the pressure exerted by the weight of the Earth's atmosphere on any surface. It is produced due to gravity acting on air and acts equally in all directions. Atmospheric pressure is maximum at sea level and decreases with altitude. It is measured using a mercury barometer. The standard atmospheric pressure is:

\[ 1\ \text{atm} = 1.013\times10^{5}\ \text{Pa} = 101.325\ \text{kPa} = 760\ \text{mmHg} = 76\ \text{cmHg} \]

Frequently Asked Questions (FAQs)

1. What is atmospheric pressure?

Atmospheric pressure is the pressure exerted by the weight of the Earth's atmosphere on any surface due to gravity.

2. What is the SI unit of atmospheric pressure?

The SI unit of atmospheric pressure is Pascal (Pa).

3. What is the value of standard atmospheric pressure?

\[ 1\ \text{atm}=1.013\times10^{5}\ \text{Pa}=760\ \text{mmHg}=76\ \text{cmHg} \]

4. Which instrument is used to measure atmospheric pressure?

Atmospheric pressure is measured using a Mercury Barometer.

5. Why does atmospheric pressure decrease with altitude?

As altitude increases, the amount of air above decreases. Therefore, the weight of the air column decreases, resulting in lower atmospheric pressure.

6. Why is mercury used in a barometer?

Mercury has high density, low vapour pressure, and does not wet glass. Therefore, it is ideal for measuring atmospheric pressure.

7. What is the height of the mercury column at standard atmospheric pressure?

The height of the mercury column is 76 cm (760 mm).

8. Does atmospheric pressure act only downward?

No. Atmospheric pressure acts equally in all directions.

9. What causes atmospheric pressure?

Atmospheric pressure is caused by the weight of the air due to Earth's gravitational force.

10. What is the formula used to calculate atmospheric pressure in a mercury barometer?

\[ P=\rho gh \]

Multiple Choice Questions (MCQs)

1. Atmospheric pressure is produced due to the:

A. Heat of the Sun
B. Weight of air
C. Rotation of the Earth
D. Motion of clouds

Answer: B. Weight of air

2. The SI unit of pressure is:

A. Newton
B. Joule
C. Pascal
D. Watt

Answer: C. Pascal

3. Standard atmospheric pressure is equal to:

A. 760 Pa
B. 76 mmHg
C. 760 mmHg
D. 760 cmHg

Answer: C. 760 mmHg

4. Atmospheric pressure is measured using a:

A. Thermometer
B. Hydrometer
C. Mercury Barometer
D. Hygrometer

Answer: C. Mercury Barometer

5. Atmospheric pressure decreases with increasing:

A. Temperature
B. Altitude
C. Density
D. Gravity

Answer: B. Altitude

6. Mercury is used in a barometer because it has:

A. Low density
B. High density
C. High viscosity
D. High temperature

Answer: B. High density

7. Atmospheric pressure acts:

A. Upward only
B. Downward only
C. Horizontally only
D. In all directions

Answer: D. In all directions

8. One atmosphere is approximately:

A. \(10^{3}\) Pa
B. \(10^{4}\) Pa
C. \(10^{5}\) Pa
D. \(10^{6}\) Pa

Answer: C. \(10^{5}\) Pa

9. The height of mercury in a standard barometer is:

A. 7.6 cm
B. 76 cm
C. 760 cm
D. 7.6 m

Answer: B. 76 cm

10. Atmospheric pressure is maximum at:

A. Mountain tops
B. Sea level
C. Aeroplanes
D. Space

Answer: B. Sea level

True or False

1. Atmospheric pressure acts in all directions. (True)

2. Air has no weight. (False)

3. Atmospheric pressure decreases with increasing altitude. (True)

4. The SI unit of pressure is Newton. (False)

5. Mercury barometer measures atmospheric pressure. (True)

6. Standard atmospheric pressure is 760 mmHg. (True)

7. Atmospheric pressure is maximum at sea level. (True)

8. Mercury wets glass easily. (False)

9. Atmospheric pressure depends on gravity. (True)

10. One atmosphere is equal to 101325 Pa. (True)

Fill in the Blanks

1. Atmospheric pressure is produced by the __________ of air.

Answer: weight

2. The SI unit of pressure is __________.

Answer: Pascal (Pa)

3. Standard atmospheric pressure is __________ atm.

Answer: 1

4. One atmosphere is equal to __________ mmHg.

Answer: 760

5. Atmospheric pressure is measured using a __________.

Answer: Mercury Barometer

6. Atmospheric pressure decreases with increasing __________.

Answer: altitude

7. Mercury has a very high __________.

Answer: density

8. One atmosphere is equal to __________ Pa.

Answer: 101325

9. Atmospheric pressure acts in __________ directions.

Answer: all

10. The height of the mercury column in a standard barometer is __________ cm.

Answer: 76

Very Short Answer Questions

1. What is atmospheric pressure?

Atmospheric pressure is the pressure exerted by the weight of the Earth's atmosphere on any surface due to gravity.

2. What is the SI unit of atmospheric pressure?

The SI unit of atmospheric pressure is Pascal (Pa).

3. What is the value of standard atmospheric pressure?

\[ 1\ \text{atm}=1.013\times10^{5}\ \text{Pa} \]

4. What is 1 atm in mmHg?

\[ 1\ \text{atm}=760\ \text{mmHg} \]

5. What is 1 atm in cmHg?

\[ 1\ \text{atm}=76\ \text{cmHg} \]

6. Which instrument measures atmospheric pressure?

A Mercury Barometer.

7. Write the formula of atmospheric pressure.

\[ P=\rho gh \]

8. Why does atmospheric pressure decrease with altitude?

Because the amount and weight of air above decreases with increasing altitude.

Short Answer Questions

1. Define atmospheric pressure.

Atmospheric pressure is the pressure exerted by the weight of the Earth's atmosphere on any surface due to gravity. It acts equally in all directions and is maximum at sea level.

2. Explain why atmospheric pressure exists.

Atmospheric pressure exists because air has mass. Due to Earth's gravitational attraction, air has weight. The weight of the air column above any surface exerts pressure on that surface. This pressure is called atmospheric pressure.

3. Why does atmospheric pressure decrease with altitude?

As altitude increases, the thickness of the atmosphere above decreases. Therefore, the weight of the air column becomes smaller, resulting in lower atmospheric pressure.

4. Why is mercury used in a mercury barometer?

  • Mercury has a very high density.
  • It has low vapour pressure.
  • It does not wet glass.
  • It forms a sharp and clearly visible meniscus.
  • A short mercury column (76 cm) can balance atmospheric pressure.

5. Write four equivalent values of one atmosphere.

\[ 1\ \text{atm} = 1.013\times10^{5}\ \text{Pa} = 101.325\ \text{kPa} = 760\ \text{mmHg} = 76\ \text{cmHg} \]

6. Write any four applications of atmospheric pressure.

  • Drinking through a straw.
  • Working of syringes.
  • Operation of droppers.
  • Weather forecasting using barometers.

Long Answer Questions

1. Define atmospheric pressure and explain its characteristics.

Atmospheric pressure is the pressure exerted by the weight of the Earth's atmosphere on any surface due to gravity. Since air has mass, gravity pulls it towards the Earth, producing pressure. Atmospheric pressure acts equally in all directions. It is maximum at sea level and decreases with increasing altitude. Atmospheric pressure changes with weather conditions and is measured using a mercury barometer. At standard conditions, its value is:

\[ 1\ \text{atm} = 1.013\times10^{5}\ \text{Pa} = 760\ \text{mmHg} = 76\ \text{cmHg} \]

2. Explain the factors affecting atmospheric pressure.

The atmospheric pressure depends mainly on the following factors:

  • Altitude: Atmospheric pressure decreases as altitude increases because the amount of air above becomes less.
  • Temperature: Warm air expands and becomes less dense, producing lower pressure. Cold air is denser and produces higher pressure.
  • Humidity: Moist air is lighter than dry air. Therefore, humid air generally has lower atmospheric pressure.

Revision Points

  • Atmospheric pressure is produced by the weight of air.
  • Air has mass because gravity acts on it.
  • Atmospheric pressure acts equally in all directions.
  • The SI unit of pressure is Pascal (Pa).
  • Atmospheric pressure is measured by a mercury barometer.
  • Atmospheric pressure decreases with increasing altitude.
  • Standard atmospheric pressure is called one atmosphere (1 atm).
  • \[ 1\ \text{atm} = 101325\ \text{Pa} = 1.013\times10^{5}\ \text{Pa} = 760\ \text{mmHg} = 76\ \text{cmHg} \]

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