Pressure In Fluids and Atmospheric Pressure Chapter Notes | Physics Class 9 ICSE PDF Download

Pressure In Fluids And Its Transmission

Thrust And Pressure

• Thrust is the force applied perpendicular to a surface.
• It equals the weight of a body placed on a surface, regardless of its position.
• Thrust is a vector quantity, having both magnitude and direction.
• SI unit of thrust is newton (N); CGS unit is dyne (1 N = 10⁵ dyne).
• Gravitational units: 1 kgf = 9.8 N, 1 gf = 980 dyne.
• Pressure is the thrust per unit area, calculated as P = F/A.
• Pressure is a scalar quantity, having only magnitude.
• SI unit of pressure is pascal (Pa), where 1 Pa = 1 N m^-2.
• CGS unit of pressure is dyne cm^-2 (1 N m^-2 = 10 dyne cm^-2).
• Other pressure units: 1 bar = 10⁵ N m^-2, 1 millibar = 10² N m^-2.
• Atmospheric pressure is often measured in terms of mercury column height (1 atm = 0.76 m of Hg = 1.013 × 10⁵ Pa).
• 1 torr = 1 mm of Hg; 1 atm = 760 torr.
• Pressure depends on thrust and area: higher thrust increases pressure, larger area decreases pressure.
• Example: A brick (4 kgf) on a 50 cm² base exerts 0.08 kgf cm^-2 pressure, but on a 200 cm² base, it exerts 0.02 kgf cm^-2.
• Increasing pressure: Reduce surface area (e.g., sharp nail tips increase pressure for easier penetration).
• Decreasing pressure: Increase surface area (e.g., wide building foundations reduce ground pressure).

Pressure In Fluids

• Fluids (liquids and gases) can flow and exert pressure due to their weight.
• Unlike solids, fluids exert pressure in all directions, including on container walls and bottom.
• Fluid pressure acts at all points within the fluid.

Pressure Exerted By a Liquid Column (P = Hρg)

• Pressure in a liquid at depth h is given by P = hρg, where ρ is density and g is acceleration due to gravity.
• Proof: For a liquid column of height h and area A, thrust = weight = A h ρ g. Pressure = thrust/area = (A h ρ g)/A = h ρ g.
• Total pressure at depth h includes atmospheric pressure: P_total = P₀ + h ρ g, where P₀ is atmospheric pressure.
• Factors affecting liquid pressure: depth (h), density (ρ), and gravity (g).
• Pressure is directly proportional to depth and density but independent of vessel shape or surface area.

Laws of Liquid Pressure

• Pressure increases with depth below the free surface.
• Pressure is the same at all points on a horizontal plane in a stationary liquid.
• Pressure is equal in all directions at a point in a liquid.
• Pressure at the same depth varies with liquid density (higher density, higher pressure).
• A liquid seeks its own level in connected vessels.

Some Consequences of Liquid Pressure (P = Hρg)

• Sea water exerts more pressure than river water at the same depth due to higher density.
• Dam walls are thicker at the bottom to withstand higher pressure at greater depths.
• Water supply tanks are placed high to increase pressure in taps for better flow.
• Sea divers wear protective suits to counter high pressure in deep sea, maintaining internal pressure at 1 atm.
• Gas bubbles grow in size as they rise in water due to decreasing pressure (Boyle’s law).

Transmission of Pressure in Liquids; Pascal’s Law

• Pascal’s law: Pressure applied to a confined liquid is transmitted equally and undiminished in all directions.
• Pressure difference depends only on vertical height difference (Δh).
• Demonstration: In a flask with tubes, pushing a piston raises water equally in all tubes, showing equal pressure transmission.

Application Of Pascal’s Law

• Hydraulic machines use Pascal’s law to multiply force.
• Principle: A small force on a smaller piston creates pressure transmitted to a larger piston, producing a larger force.
• Formula: F₂/F₁ = A₂/A₁, where F is force and A is area.

Examples of Hydraulic Machines

Hydraulic Press:

• Uses Pascal’s law to compress materials.
• Construction: Two cylinders (small pump plunger, large ram) connected by a pipe, with valves and a reservoir.
• Working: Force on the pump plunger transmits pressure to the ram, compressing items like cotton bales.
• Uses: Pressing cotton, extracting juice, squeezing oil, engraving monograms.

Hydraulic Jack:

• Lifts heavy vehicles using Pascal’s law.
• Construction: Two cylinders connected by a tube with a valve, a lever, and a platform.
• Working: Pressing the lever opens the valve, transferring liquid to lift the platform.

Hydraulic Brakes:

• Slows vehicles by transmitting pressure to brake shoes.
• Construction: Master cylinder with a pedal, connected to wheel cylinders with pistons and brake shoes.
• Working: Pedal pressure transmits force to wheel cylinders, pushing brake shoes against wheels.
• Hydraulic machines act as force multipliers: Mechanical advantage (Load/Effort) > 1.

Atmospheric Pressure And Its Measurement

Atmospheric Pressure

• Atmospheric pressure is the thrust per unit area exerted by the air column on Earth’s surface.
• Approximate value: 10⁵ N m^-2 (1 kgf cm^-2).
• A human body (2 m²) experiences ~2 × 10⁵ N, balanced by blood pressure.
• At high altitudes, lower atmospheric pressure can cause nose bleeding due to excess blood pressure.

Demonstration of Atmospheric Pressure

• Collapsing Tin Can Experiment:
• Boil water in a can, seal it, and cool it with cold water.
• The can collapses as steam condenses, reducing internal pressure, and external atmospheric pressure crushes it.

Common Consequences of Atmospheric Pressure

• Sucking a Drink with a Straw: Sucking reduces straw pressure, allowing atmospheric pressure to push the drink up.
• Filling a Syringe: Pulling the plunger lowers barrel pressure, letting atmospheric pressure push liquid in.
• Filling a Fountain Pen: Squeezing the rubber tube expels air; releasing it allows atmospheric pressure to push ink in.
• Rubber Suckers: Pressing expels air, creating a vacuum; atmospheric pressure holds the sucker to the wall.
• Siphon System: Sucking air from a tube lowers pressure, allowing atmospheric pressure to push water from a higher to a lower vessel.
• Oil Can: Two holes are needed; one allows air to enter, increasing internal pressure to push oil out.

Measurement of Atmospheric Pressure

• A barometer measures atmospheric pressure.

Simple Barometer:

• Construction: A 1 m glass tube, closed at one end, filled with mercury, inverted in a mercury trough.
• Working: Mercury settles at ~76 cm, balancing atmospheric pressure; height indicates pressure.
• Barometric height at sea level: 0.76 m of Hg.
• Factors affecting height: Only atmospheric pressure changes the height; tube shape or tilt does not.
• Advantages of mercury: High density (shorter tube), low vapor pressure, doesn’t stick to glass, shiny surface.
• Disadvantages of water: Low density (10.4 m tube needed), high vapor pressure, sticks to glass, transparent.
• Demerits of simple barometer: Fragile, open trough, not portable, no fixed scale.

Fortin Barometer:

• Construction: Glass tube in a brass case, mercury in a leather cup, adjustable with a screw, vernier scale for accuracy.
• Measurement: Adjust mercury to touch an ivory pointer, read height with main and vernier scales.

Aneroid Barometer:

• Construction: Evacuated metallic box with a springy diaphragm, linked to a pointer on a calibrated scale.
• Working: Pressure changes move the diaphragm, rotating the pointer to indicate pressure.
• Advantages: Portable, no liquid, direct reading.
• Uses of barometers: Measure pressure, forecast weather, measure altitude (altimeter).

Variation of Atmospheric Pressure With Altitude

• Atmospheric pressure decreases with altitude due to:
• Decreased air column height (linear decrease).
• Decreased air density (non-linear, rapid near sea level, slower higher up).
• At Mount Everest, pressure is ~30% of sea-level pressure.
• Consequences: Difficulty breathing, nose bleeding, fountain pen leaks due to lower external pressure.

Weather Forecast by the Use of Barometer

• Atmospheric pressure changes with temperature and moisture, affecting air density.
• Sudden fall: Indicates a storm or cyclone.
• Gradual fall: Suggests increasing moisture, possible rain.
• Gradual rise: Indicates decreasing moisture, dry weather.
• Sudden rise: Suggests extremely dry weather.
• Stable height: Normal weather, no change.

Altimeter

• An altimeter is an aneroid barometer used in aircraft to measure altitude.
• Principle: Atmospheric pressure decreases with height; the scale is calibrated in height units.
• Scale increases leftward as pressure decreases with altitude.