Chapter Five: Pressure & States of Matter
Pressure · Density · Atmospheric Pressure · Torricelli's Experiment · Liquid Pressure · Buoyancy · Archimedes' Principle · Floatation · Pascal's Law · Elasticity · Stress-Strain · Hooke's Law · Molecular Kinetic Theory · Plasma — Complete Guide.
📋 Chapter 5 — Complete Content Structure
5.1 Pressure
Pressure is a fundamental concept in physics. It describes how force is distributed over a surface. Understanding pressure helps explain why sharp knives cut better, why nails penetrate wood, and why heavy vehicles use wide tyres.
🔬 Definition & Formula
Pressure (P) is the force (F) acting normally per unit area (A) of a surface.
SI Unit: Pascal (Pa) or N·m⁻²
1 Pa = 1 N·m⁻²
Dimension: [ML⁻¹T⁻²]
✅ Greater Force → Greater Pressure
P ∝ F when area is constant. Pushing harder on a surface increases pressure.
⚠️ Smaller Area → Greater Pressure
P ∝ 1/A when force is constant. Sharp knives, nails, and high heels create high pressure due to small contact area.
🌍 Everyday Applications
Broad tyres reduce pressure on ground. Camel's feet prevent sinking in sand. Track shoes provide grip through spikes.
🧮 NCTB-Type Numerical
A person of mass 50 kg stands on shoes with total area 200 cm². Find pressure on ground.
A = 200 cm² = 0.02 m²
P = F/A = 490/0.02 = 24,500 Pa
5.2 Density
Density is the mass per unit volume of a substance. It determines whether objects float or sink and plays a crucial role in convection and everyday phenomena.
🔬 Formula
SI Unit: kg·m⁻³ or gm/cc
Dimension: [ML⁻³]
📊 Common Density Values
| Substance | Density (kg/m³) |
|---|---|
| Air | 1.29 |
| Cork | 250 |
| Ice | 920 |
| Water | 1000 |
| Glycerin | 1260 |
| Iron | 7800 |
| Gold | 19300 |
🌊 Density & Floating
Object density < liquid density → floats.
Object density > liquid density → sinks.
Object density = liquid density → suspended.
Examples: Cork floats on water, iron sinks, rotten egg floats.
🧮 Density Numerical
Liquid mass = 2000 kg, volume = 2 m³. Find density.
5.3 Atmospheric Pressure
The Earth's atmosphere exerts pressure on all surfaces. Torricelli's famous experiment demonstrated this pressure using a mercury column.
🔬 Definition & Standard Value
= 76 cm Hg = 760 mm Hg
🧪 Torricelli's Experiment
A glass tube filled with mercury is inverted into a mercury reservoir. The column settles at ~76 cm. The empty space above is called Torricelli's vacuum.
📊 Barometer
An instrument used to measure atmospheric pressure. Based on Torricelli's method.
5.4 Altitude and Atmospheric Pressure
Atmospheric pressure decreases with increasing altitude because the weight of the air column above becomes smaller.
📉 Key Relationship
Altitude ↑ → Atmospheric Pressure ↓
Breathing at high altitude is difficult due to lower oxygen pressure. Aircraft cabins are pressurized for passenger comfort.
5.5 Atmospheric Pressure and Weather
Changes in atmospheric pressure help predict weather conditions using a barometer.
| Mercury Level | Weather Indication |
|---|---|
| Gradually falling | Increasing moisture, possibility of rain |
| Rapidly falling | Stormy weather possible |
| Rising | Dry and stable weather |
5.6 Pressure in Liquids
Liquid pressure depends on depth, density of the liquid, and gravitational acceleration.
Where: h = depth, ρ = density, g = acceleration due to gravity
🧮 NCTB Numerical
Density = 800 kg/m³, depth = 75 cm = 0.75 m, g = 9.8 m/s².
Buoyancy, Archimedes' Principle & Floatation
When an object is immersed in a fluid, it experiences an upward buoyant force. This principle explains why ships float and why objects feel lighter in water.
Apparent Weight: W_app = W - F_B
Sinking
W > F_B → object sinks.
Equilibrium
W = F_B → object floats fully submerged.
Rising
W < F_B → object rises until equilibrium.
🚢 Why Ships Float
Ships are hollow, displacing enough water to produce buoyant force equal to their weight. Overloaded ships sink when weight exceeds safe displacement.
Pascal's Law & Hydraulic Machines
Pascal's law states that pressure applied to an enclosed fluid is transmitted equally in all directions. This principle enables force multiplication in hydraulic systems.
F₁/A₁ = F₂/A₂
F₂ = F₁ × (A₂/A₁)
🧮 Hydraulic Lift
F₁ = 50 N, A₁ = 0.01 m², A₂ = 0.5 m².
Elasticity, Stress, Strain & Hooke's Law
Elasticity is the property of a body to regain its original shape after deformation. Stress is the restoring force per unit area, and strain is the fractional change in dimension.
Strain = ΔL/L (No unit)
Hooke's Law: Stress ∝ Strain (within elastic limit)
Molecular Kinetic Theory & Fourth State of Matter (Plasma)
Matter is composed of tiny particles in continuous motion. Under high-energy conditions, matter exists as plasma — an ionized gas.
📊 Three States Comparison
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Molecular distance | Very small | Moderate | Large |
| Attraction | Very strong | Moderate | Very weak |
| Shape | Fixed | Container | Container |
| Compressibility | Very low | Low | High |
⚡ Plasma — Fourth State
Plasma is an ionized gas containing free electrons and positive ions. It conducts electricity and is affected by magnetic fields.
Examples: Sun, stars, lightning, neon lights, fusion reactors.
Interactive MCQ Bank — 50 Questions
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Board Question Analysis (2016–2025)
Pressure and States of Matter is a heavily tested chapter in SSC Physics.
🏆 Highest Priority Topics
| Topic | Priority |
|---|---|
| Pressure (P=F/A) | ⭐⭐⭐⭐⭐ |
| Density (ρ=m/V) | ⭐⭐⭐⭐⭐ |
| Atmospheric pressure & Torricelli | ⭐⭐⭐⭐⭐ |
| Liquid pressure (P=hρg) | ⭐⭐⭐⭐⭐ |
| Buoyancy & Archimedes' principle | ⭐⭐⭐⭐⭐ |
| Floatation & immersion conditions | ⭐⭐⭐⭐⭐ |
| Pascal's law & hydraulic machines | ⭐⭐⭐⭐⭐ |
| Elasticity, stress-strain, Hooke's law | ⭐⭐⭐⭐⭐ |
| Molecular kinetic theory | ⭐⭐⭐⭐ |
| Plasma (fourth state) | ⭐⭐⭐⭐⭐ |
Mathematical Problems with Solutions
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Important Short Questions (SQ)
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Creative Questions (CQ) with Solutions
Key CQs for board preparation.
Chapter 5 Formula Sheet
Density: ρ = m/V
Liquid Pressure: P = hρg
Buoyant Force: F_B = ρVg
Apparent Weight: W_app = W - F_B
Pascal's Law: F₁/A₁ = F₂/A₂
Stress = F/A | Strain = ΔL/L
Hooke's Law: Stress ∝ Strain
1 atm = 1.013×10⁵ Pa = 76 cm Hg = 760 mm Hg
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Md. Mominul Islam