ICSE Class 9 Physics Chapter 04 Pressure in Fluids and Atmospheric Pressure

ATMOSPHERIC PRESSURE

  • The Earth is surrounded by an envelope of air, called the atmosphere, up to approximately 300 km from its surface.

  • The weight of the air column exerts thrust on the Earth's surface, which is defined as atmospheric pressure.

  • Atmospheric pressure = thrust exerted per unit area due to the weight of the air column.

  • The average atmospheric pressure at the Earth's surface is about 1 kgf/cm² or 10⁵ N/m², corresponding to about 2 x 10⁵ N acting on every m².

  • Despite this pressure, humans do not feel it because blood pressure slightly exceeds atmospheric pressure.

  • At high altitudes, atmospheric pressure decreases, which can lead to conditions like nosebleeds due to increased blood pressure.

DEMONSTRATIONS OF ATMOSPHERIC PRESSURE

Collapsing Tin Can Experiment

  • A small quantity of water is boiled in a tin can.

  • After boiling, steam fills the can and expels air.

  • The stopper is replaced and the can is cooled by pouring cold water over it.

  • As the steam condenses into water, the pressure inside the can drops, causing atmospheric pressure outside to crush the can inward.

Sucking a Drink with a Straw

  • When a drink is sucked through a straw, air goes into the lungs, decreasing the pressure inside the straw.

  • The atmospheric pressure acting on the liquid forces it up the straw.

Filling a Syringe

  • When the plunger of a syringe is pulled up, the pressure inside becomes less than the atmospheric pressure.

  • Atmospheric pressure forces liquid into the syringe.

Action of a Siphon

  • Water is transferred from a higher to a lower level using a siphon system.

  • The atmospheric pressure acting above the water in the higher vessel forces the water up into the siphon, causing it to flow downward to the lower vessel.

MEASUREMENT OF ATMOSPHERIC PRESSURE

Types of Barometers

  1. Simple Barometer: First designed by Torricelli in 1643.

    • Consists of a glass tube filled with mercury, open at one end and inverted in a mercury trough.

    • The height of the mercury column (barometric height) indicates the atmospheric pressure.

    • Normal barometric height at sea level is 0.76 m (760 mm) of mercury.

  2. Fortin Barometer: A modified form of the simple barometer used for accurate measurement in labs.

    • Features an adjustable mercury level in a glass vessel using a leather cup and screw mechanism.

  3. Aneroid Barometer: A portable barometer with no liquid, measures atmospheric pressure using a corrugated metal box that expands or contracts with changes in pressure.

VARIATION OF ATMOSPHERIC PRESSURE WITH ALTITUDE

  • Atmospheric pressure decreases non-linearly with an increase in altitude due to:

    1. Decrease in height of the air column.

    2. Decrease in density of air.

  • The atmosphere can be visualized as multiple layers; fewer air layers are above as altitude increases.

WEATHER FORECASTING WITH BAROMETERS

  • Changes in barometric height help forecast weather:

    1. Sudden fall indicates storms or cyclones.

    2. Gradual fall suggests increasing moisture and potential rainfall.

    3. Gradual rise indicates decreasing moisture and potentially dry weather.

    4. Sudden rises signify flows from low pressure, indicating dry weather.

    5. No significant change indicates stable weather.

ALTIMETERS

  • An altimeter measures altitude and works on the principle that atmospheric pressure decreases with increasing height.

  • It can be calibrated to reflect altitude directly based on barometric pressure readings.

EXAMPLES AND PROBLEMS

  • Atmospheric pressure can be expressed in terms of equivalent heights of mercury or other fluids, using density differences to derive measurements.

  • Examples include calculating barometric heights under varying conditions and converting atmospheric pressure units.