Explain the term atmospheric pressure along with its units. How is atmospheric pressure measured with a liquid barometer? Explain its construction and applications.

Atmospheric pressure is the pressure exerted by the weight of the Earth’s atmosphere on a given surface. It is caused by the molecules in the air colliding with surfaces, and it acts in all directions. The atmosphere exerts pressure on everything on Earth’s surface, including people, objects, and even the Earth’s own surface.

  • Unit of Atmospheric Pressure:
    • The standard unit for atmospheric pressure is pascals (Pa). 1 Pa = 1 N/m².
    • Atmospheric pressure at sea level is approximately 101,325 Pa or 101.3 kPa (kilopascals), which is also often referred to as 1 atmosphere (1 atm).

Measuring Atmospheric Pressure with a Liquid Barometer:

A liquid barometer is a device used to measure the atmospheric pressure by balancing the weight of a liquid column against the atmospheric pressure. It typically uses mercury or water as the liquid in the column.

Construction of a Liquid Barometer:

  1. Tube: The barometer consists of a long, vertical glass tube that is closed at one end.
  2. Mercury: The tube is filled with mercury (or sometimes water), and the open end of the tube is submerged in a pool of mercury contained in a dish.
  3. Vacuum: The upper part of the tube is evacuated to create a vacuum (no air inside).
  4. Mercury Column: The weight of the mercury inside the tube creates pressure that is balanced by the atmospheric pressure acting on the surface of the mercury in the dish.

Working Principle:

  • The atmospheric pressure forces the mercury up the tube, and the height of the mercury column is directly proportional to the atmospheric pressure.
  • When the atmospheric pressure increases, the mercury column rises. Conversely, if the pressure decreases, the mercury column falls.
  • The height of the mercury column is measured in millimeters (mm) or centimeters (cm), and this value represents the atmospheric pressure.

Applications of Liquid Barometer:

  1. Weather Prediction:

    • Barometers are used in meteorology to monitor changes in atmospheric pressure, which help predict weather patterns. A rising barometer indicates fair weather, while a falling barometer suggests that stormy weather or rain may be coming.

  2. Altitude Measurement:

    • Atmospheric pressure decreases with altitude. By measuring the atmospheric pressure at a given location and comparing it to standard sea-level pressure, the altitude of a location can be determined. This principle is used in altimeters, such as those in aircraft.

  3. Calibration of Instruments:

    • Barometers are used in laboratories to calibrate other measuring instruments that rely on atmospheric pressure.

Advantages and Disadvantages of Using Mercury Barometers:

  • Advantages:

    • High accuracy: Mercury is used because it is dense, so a small change in atmospheric pressure produces a large change in the height of the mercury column, making it easier to measure accurately.
    • Simple construction: The design of a liquid barometer is straightforward, and it provides a direct, observable measurement.

  • Disadvantages:

    • Mercury toxicity: Mercury is hazardous to health and the environment. Special care must be taken in handling mercury barometers.
    • Size: Mercury barometers require a relatively long column to measure typical atmospheric pressures, which can make them impractical for portable use.

Related Questions:

  1. Q.1 How does physics play an important role in our life?
  2. Q.2 Estimate your age in minutes and seconds.
  3. Q.3 What base quantities are involved in these derived physical quantities; force, pressure, power and charge.
  4. Q.4 Show that prefix micro is thousand times smaller than prefix milli.
  5. Q5. Justify that displacement is a vector quantity while energy is a scalar quantity.
  6. Q6. Screw gauge can give more precise length than vernier calipers. Briefly explain why?
  7. Q.7 Differentiate between Mechanical Stopwatch and Digital Stopwatch.
  8. Q.8 How measuring cylinder is used to measure the volume of an irregularly shaped stone?
  9. Q.9 What precautions should be kept in mind while taking measurements using a measuring cylinder?
  10. Q.10 Why do we need to consider significant digits in measurements?
  11. Q12. Differentiate between precision and accuracy.
  12. How are we able to break a metal wire by bending it repeatedly?
  13. A spring, having spring constant k when loaded with mass m, is cut into two equal parts. One of the parts is loaded with the same mass m again. What will be its new spring constant?
  14. Why do static fluids always exert a force perpendicular to the surface?
  15. How can a small car lifter lift a load heavier than itself?
  16. Define elasticity and elastic limit. Show that a force may produce a change in the size and shape of solids.
  17. What is Hooke’s law? Illustrate its applications. Also, define and calculate the spring constant.
  18. Draw and explain the force-extension graph for elastic solids.
  19. Define and explain pressure. What is the effect of area on pressure acting on a surface?
  20. Physical Quantities and Measurement