Pressure in Fluids and Atmospheric Pressure Class 9 ICSE—Ch4

Pressure in fluids and atmospheric pressure class 9 ICSE is Chapter 4 of your Physics book. It begins with thrust and pressure, then shows how a liquid column produces pressure through P = h\rho g. Later sections cover Pascal’s law, the three hydraulic machines, and how a barometer measures atmospheric pressure. Each worked example below uses fresh numbers so you can follow every step.

Download Pressure in Fluids and Atmospheric Pressure Class 9 ICSE Physics Chapter 4 PDF

Download the Pressure in Fluids and Atmospheric Pressure Class 9 ICSE Physics chapter PDF to revise offline or print for practice. It is the full chapter as printed in the book — 20 pages, 14 sections, 89 questions.

What is inside Chapter 4

20 pages · 14 sections · 1 worked examples · 89 exercise questions

• EXERCISE 4 (A): 32 questions
• Multiple Choice Type :: 18 questions
• EXERCISE 4 (B): 31 questions
• Multiple choice type :: 8 questions

4.1 THRUST AND PRESSURE

Thrust is a force acting normally (perpendicular) to a surface. A body resting on a surface exerts a thrust equal to its own weight. Thrust is a vector quantity, with the newton (N) as its S.I. unit and the dyne in the C.G.S. system, where 1\ \text{N} = 10^5 dyne.

Pressure is the thrust per unit area of surface:

P = \frac{F}{A}

Pressure is a scalar. Its S.I. unit is the pascal (Pa), with 1\ \text{Pa} = 1\ \text{N m}^{-2}. Other units are the bar, where 1\ \text{bar} = 10^5\ \text{N m}^{-2}, the atmosphere, and the torr, where 1\ \text{torr} = 1\ \text{mm} of Hg.

Two factors control pressure: the size of the thrust and the area it acts on. For a fixed thrust, pressure rises when area shrinks. That is why nail tips are pointed and knife edges are sharp. For a fixed area, pressure rises when thrust grows. That is why railway tracks rest on wide wooden sleepers.

Worked pattern: A block measuring 40 cm by 20 cm by 10 cm weighs 6 kgf. Its largest face has area 40 \times 20 = 800\ \text{cm}^2, so pressure there is 6 / 800 = 0.0075\ \text{kgf cm}^{-2}. Its smallest face has area 20 \times 10 = 200\ \text{cm}^2, giving 6 / 200 = 0.03\ \text{kgf cm}^{-2}. The smaller face feels four times the pressure of the larger one.

A common mistake is confusing thrust with pressure. Thrust is a force, so it is a vector. Pressure is the ratio of thrust to area, so it is a scalar. This builds on the forces you met in the laws of motion chapter.

4.2 PRESSURE IN FLUIDS

A fluid is any substance that can flow. All liquids and gases are fluids.

A solid presses only downwards, on the surface that supports it. A fluid behaves differently. Because it can flow, it presses on the bottom of its container, on the walls, and in every direction at once.

You can see this with a vessel full of water. Make several small holes in the wall below the water surface. Water spurts out of every hole, which shows the liquid presses on the wall at each point. If you cover a hole with your finger, you feel the thrust. The deeper the hole, the farther the jet travels before landing. Liquid pressure grows with depth below the free surface.

That observation leads to the liquid pressure formula at the heart of this ICSE chapter on pressure: P = h\rho g.

4.3 PRESSURE EXERTED BY A LIQUID COLUMN (P = h rho g)

The pressure a liquid exerts at a depth h is:

P = h\rho g

Here \rho is the liquid’s density and g is the acceleration due to gravity.

The proof uses the weight of the column above a point. Consider a horizontal surface of area A at depth h. The liquid column above it has volume A \times h and weight A h \rho g. The pressure on the surface is:

P = \frac{A h \rho g}{A} = h \rho g

The atmosphere pushes on the free surface of any liquid. So the total pressure at depth h is the sum of atmospheric pressure P_0 and the liquid’s own pressure:

P_{\text{total}} = P_0 + h \rho g

Pressure in a liquid depends on three factors: depth h, density \rho, and g. It does not depend on the shape or size of the vessel, nor on the area of the surface.

Worked pattern: At a depth of 2 m in water of density 10^3\ \text{kg m}^{-3}, with g = 10\ \text{m s}^{-2}, the liquid pressure is 2 \times 10^3 \times 10 = 2 \times 10^4\ \text{N m}^{-2}.

Two mistakes appear often:

• Forgetting to add atmospheric pressure P_0 when the question asks for total pressure at a depth.
• Using depth in centimetres inside h\rho g. Convert h to metres first.

4.4 LAWS OF LIQUID PRESSURE

Five laws describe how a stationary liquid presses on its container:

• Pressure inside a liquid increases with depth below the free surface.
• Pressure is the same at all points on the same horizontal plane.
• About any point, pressure is the same in all directions.
• At the same depth, different liquids give different pressures; pressure rises with density.
• A liquid seeks its own level.

Each law follows from P = h\rho g. The first law is that formula with h growing. The second holds because points on one horizontal plane sit at the same depth, so their pressures match. The fourth is the density term at work.

These laws matter for U-tubes and communicating vessels. The fifth explains why water in connected containers settles at one common level.

4.5 SOME CONSEQUENCES OF LIQUID PRESSURE P = h rho g

This section applies P = h\rho g to real situations.

Sea water is denser than river water. At the same depth, then, sea water exerts more pressure. This is the density term in the formula.

A dam wall is thicker at the bottom because pressure grows with depth. The lower wall must withstand the largest force, so builders add thickness where h is greatest. Water supply tanks stand high for the same reason. Raising the tank raises the pressure at your tap.

Deep-sea divers need protective suits because water pressure at depth can exceed blood pressure. The suit keeps the pressure inside at one atmosphere.

A gas bubble grows as it rises in a lake. At the bottom, total pressure on it is P_0 + h\rho g. As h falls, the pressure falls, and Boyle’s law says the bubble’s volume increases. At the surface, only P_0 remains, so the bubble is largest there.

You will meet the upthrust idea again in the next chapter, Archimedes’ principle and floatation.

4.6 TRANSMISSION OF PRESSURE IN LIQUIDS; PASCAL’S LAW

Pascal’s law states that pressure applied anywhere in a confined liquid is transmitted equally and undiminished in all directions throughout the liquid.

Why it holds: in a stationary liquid, the pressure difference between two points depends only on the vertical height between them. If you raise the pressure at one point, the other must rise by the same amount to keep that difference fixed. The change travels through the whole liquid.

The chapter demonstrates this with a glass flask fitted with narrow tubes at its sides and bottom. The flask is filled with water and sealed with a piston. When the piston is pushed down, water jets rise from every tube to the same height. Equal heights show equal pressure in all directions.

This single law powers every hydraulic machine in the next two sections.

4.7 APPLICATION OF PASCAL’S LAW

Hydraulic machines rest on one principle: a small force on a small piston produces a large force on a larger piston.

Two connected cylinders hold a liquid, with water-tight pistons of areas A_1 and A_2. A force F_1 on the smaller piston creates pressure P_1 = F_1 / A_1. Pascal’s law transmits this pressure to the larger piston, so P_2 = P_1. With P_2 = F_2 / A_2, we get:

\frac{F_2}{F_1} = \frac{A_2}{A_1}

Since A_2 is larger than A_1, the force F_2 is larger than F_1. The machine acts as a force multiplier.

Worked pattern: A small piston of area 5\ \text{cm}^2 pushes with a force of 12 N. A large piston of area 150\ \text{cm}^2 receives pressure through the liquid. The load it lifts is F_2 = 12 \times (150/5) = 360\ \text{N}. The force grew 30 times because the area grew 30 times.

4.8 EXAMPLES OF HYDRAULIC MACHINES

This section walks through three machines, each built on Pascal’s law.

Hydraulic press (Bramah press). Two cylinders connect through a pipe. The small cylinder holds the pump plunger, the large one holds the ram. A lever works the pump plunger. Pushing the plunger down forces liquid into the large cylinder, raising the ram against a fixed roof and compressing whatever sits on it. It is used for pressing cotton bales, extracting sugarcane juice, squeezing oil from seeds, and engraving monograms.

Hydraulic jack. A lever moves a piston in a narrow cylinder, pushing liquid into a wider cylinder whose piston carries a platform. The platform lifts cars and trucks at service stations. A valve stops the liquid flowing back once the load reaches its height.

Hydraulic brakes. A foot pedal presses a piston in the master cylinder. Liquid runs through pipes to wheel cylinders, whose wider pistons push brake shoes against the wheel rims. Because the wheel pistons have larger area, a small pedal force produces a large braking force on every wheel.

In all three, effort is less than load, and the distance moved by effort exceeds the distance moved by load. Mechanical advantage and velocity ratio are both greater than 1.

Worked pattern: A hydraulic press has pistons of area 4\ \text{cm}^2 and 160\ \text{cm}^2. An effort of 8 N on the small piston gives load F_2 = 8 \times (160/4) = 320\ \text{N} on the ram. The press multiplies the force 40 times.

The rest of this Class 9 Physics book carries the same worked-numerical style through every chapter.

4.9 ATMOSPHERIC PRESSURE

Atmospheric pressure is the thrust exerted per unit area on the Earth’s surface by the column of air above it. Air surrounds the Earth to a height of about 300 km, and that air has weight.

The air column over 1\ \text{cm}^2 of ground weighs about 1 kgf. So atmospheric pressure is roughly 1\ \text{kgf cm}^{-2}, which equals about 10^5\ \text{N m}^{-2}. A typical human body carries about 2 \times 10^5\ \text{N} of thrust from the air.

We do not feel this load because blood pressure inside our bodies balances it. Blood pressure is slightly higher than atmospheric pressure. At high altitude, the air column is shorter, so atmospheric pressure drops. Blood pressure then exceeds it by a larger margin, which can cause nose bleeding. This is why this ICSE Class 9 chapter warns against high-altitude travel for people with blood pressure problems.

You can confirm the Class 9 Physics scope on the official CISCE website.

4.10 DEMONSTRATION OF ATMOSPHERIC PRESSURE

The collapsing tin can experiment shows atmospheric pressure directly.

Boil a little water in a thin tin can fitted with an airtight stopper. Steam fills the can and pushes the air out. Replace the stopper tightly and remove the flame. Now pour cold water over the can.

The cold water condenses the steam back into water. The vapour pressure inside the can drops far below the air pressure outside. The excess outside pressure pushes inward, and the can collapses.

The experiment works because the outside air was always pressing on the can. When the inside pressure fell, nothing balanced that outside thrust any more.

4.11 COMMON CONSEQUENCES OF THE ATMOSPHERIC PRESSURE

Everyday devices exploit the difference between atmospheric pressure and a lower pressure inside.

• Straw. Sucking removes air from the straw, lowering its pressure. The atmosphere on the drink’s surface then pushes the liquid up into your mouth.
• Syringe. Pulling the plunger up lowers the air pressure in the barrel. Atmospheric pressure on the liquid forces it up into the syringe. A water pump works the same way.
• Fountain pen. Pressing the rubber tube expels air as bubbles. Releasing it lowers the pressure inside, so atmospheric pressure on the ink pushes ink up into the tube.
• Rubber sucker. Pressing a sucker against a smooth wall squeezes air out, creating a vacuum. Outside atmospheric pressure then holds it firmly in place.
• Siphon. Sucking air out of a tube lowers its pressure. Atmospheric pressure on the higher vessel pushes water up and over the bend, and it flows down to the lower vessel.
• Oil can. A sealed can with one hole will not pour, because no air enters to push the oil. A second hole lets air in, and atmospheric pressure on the oil makes it flow out.

In each case, the same story repeats: lower the pressure inside, and the atmosphere does the pushing.

4.12 MEASUREMENT OF ATMOSPHERIC PRESSURE

A barometer measures atmospheric pressure, the topic of the second half of this chapter on pressure in fluids and atmospheric pressure class 9 ICSE. Three types appear in the book.

Simple barometer. Torricelli designed this in 1643. A glass tube about 1 m long, closed at one end, is filled with pure mercury and inverted into a trough of mercury. The mercury falls until its height above the trough surface is about 76 cm. This height is the barometric height. The empty space above the mercury is the torricellian vacuum. At normal temperature and pressure, the barometric height at sea level is 76 cm of mercury.

Fortin barometer. This refines the simple barometer for laboratory use. A screw adjusts the mercury level in the leather cup until it touches an ivory pointer at the zero mark. A vernier scale then reads the mercury height accurately.

Aneroid barometer. This has no liquid at all. A partially evacuated metal box with a springy corrugated top changes shape as pressure changes. A rod and gear move a pointer over a calibrated circular scale.

Mercury suits a barometer because its high density keeps the column short, its vapour pressure is negligible, and it neither wets glass nor sticks to it. Water fails as a barometric liquid: its column would stand about 10.4 m high, its vapour spoils the vacuum, and it wets the tube.

A barometer serves three purposes: measuring atmospheric pressure, forecasting weather, and measuring height as an altimeter.

4.13 VARIATION OF ATMOSPHERIC PRESSURE WITH ALTITUDE

Atmospheric pressure falls as altitude rises, for two reasons.

First, the height of the air column above you shrinks. Higher up, less air sits above, so the weight pressing down is less. This causes a roughly linear decrease in pressure.

Second, air density falls with height. Lower air layers are compressed by the weight of the layers above them, so they are denser near the ground. The density drop is not linear. It is rapid near sea level and slow at high altitude. This makes the overall pressure drop non-linear.

At Mount Everest, atmospheric pressure is only about 30% of its sea-level value.

Barometer readings also forecast weather, because density changes with temperature and moisture:

• Sudden fall in barometric height — a storm or cyclone is coming.
• Gradual fall — moisture is rising, so rain is likely.
• Gradual rise — air is drying, so dry weather is coming.
• Sudden rise — dry air is flowing in, so very dry weather is ahead.
• No abrupt change — the weather will stay as it is.

4.15 ALTIMETER

An altimeter is an aneroid barometer used in aircraft to measure altitude. Since atmospheric pressure decreases with height above sea level, a barometer that reads pressure can be recalibrated to read height.

The scale is marked in height of ascent instead of pressure. Height increases towards the left on the dial because pressure decreases as altitude rises. The needle moves left as the pressure falls.

The altimeter gives pilots a direct height reading without separate pressure calculations. It is the same instrument as an aneroid barometer, just with a different scale.

What to practise from Pressure in Fluids and Atmospheric Pressure Class 9 ICSE

Four question blocks close the chapter.

• Exercise 4(A) — 32 questions covering sections 4.1 through 4.8.
• Multiple Choice Type — 18 questions after Exercise 4(A).
• Exercise 4(B) — 31 questions covering sections 4.9 through 4.15.
• Multiple choice type — 8 questions after Exercise 4(B).

Exercise 4(A) tests thrust, pressure, liquid pressure, Pascal’s law, and the three hydraulic machines. Exercise 4(B) tests atmospheric pressure, barometers, altitude variation, and the altimeter. The MCQ blocks check unit conversions and quick recall of the formulas P = F/A and P = h\rho g.

Pressure in Fluids and Atmospheric Pressure Class 9 ICSE frequently asked questions

What is the formula for pressure in fluids and atmospheric pressure class 9 ICSE?

Two formulas matter most. Pressure from a force is P = F/A, and pressure at a depth in a liquid is P = h\rho g. Total pressure at depth adds atmospheric pressure: P_0 + h\rho g.

Why is mercury used in a barometer instead of water?

Mercury’s high density keeps the barometric column short, about 76 cm instead of 10.4 m for water. Mercury also has negligible vapour pressure and does not wet the glass, so its reading stays accurate.

What are the five laws of liquid pressure in ICSE class 9?

Pressure increases with depth. Pressure is the same at all points on a horizontal plane. Pressure is the same in all directions about a point. Pressure at the same depth differs with liquid density. A liquid also seeks its own level.

Why does a gas bubble grow in size as it rises in a lake?

As the bubble rises, the depth h falls, so the liquid pressure on it falls. Lower pressure means larger volume, by Boyle’s law, so the bubble expands until it reaches the surface.

How does atmospheric pressure change with altitude in ICSE class 9 physics?

Atmospheric pressure decreases with altitude because the air column above is shorter and the air is less dense. The decrease is non-linear, and at Mount Everest the pressure is about 30% of its sea-level value.

The syllabus this chapter follows is set by the CISCE board.

Reference: ICSE Class 9 Physics textbook material, chapter 4: Pressure in Fluids and Atmospheric Pressure — CISCE-aligned.

More from this book

• Chapter 1: Measurements and Experimentation
• Chapter 2: Motion in One Dimension
• Chapter 3: Laws of Motion
• Chapter 5: Upthrust in Fluids Archimedes Principle and Floatation
• Chapter 6: Heat and Energy
• Chapter 7: Reflection of Light
• Chapter 8: Propagation of Sound Waves
• Chapter 9: Current Electricity
• Chapter 10: Magnetism