Alright—this is **Batch 1** of your rewritten material. It is structured like a strong university lecture note: deep, clear, exam-focused, and fully original in expression. No fluff, no “AI tone”—just solid teaching. --- # **PHY 103: GENERAL PHYSICS III (BEHAVIOUR OF MATTER)** ## **Batch 1: Thermal Physics & Energy Principles (Condensed Lecture Notes)** --- ## **1. Heat and Temperature** Heat and temperature are closely related but fundamentally different concepts. **Temperature** is a measure of the degree of hotness or coldness of a body. It reflects the *average kinetic energy* of the particles within a substance. **Heat**, on the other hand, is *energy in transit*—it flows from a region of higher temperature to a region of lower temperature. ### **Temperature Scales** The common temperature scales are: * Celsius (°C) * Kelvin (K) → the absolute scale used in physics * Fahrenheit (°F) **Key relation:** [ T(K) = t(°C) + 273 ] **Important Note (Exam Focus):** * Temperature does not measure total energy, but *average molecular energy*. * Heat depends on mass, temperature change, and material properties. --- ## **2. Gas Laws and the General Gas Equation** Gases obey certain empirical laws under varying conditions. ### **Basic Gas Laws** * **Boyle’s Law (Constant Temperature):** Pressure is inversely proportional to volume [ PV = \text{constant} ] * **Charles’ Law (Constant Pressure):** Volume is directly proportional to absolute temperature [ V \propto T ] * **Pressure Law (Constant Volume):** Pressure is proportional to temperature [ P \propto T ] ### **General Gas Equation** genui{"math_block_widget_always_prefetch_v2": {"content": "PV = nRT"}} This equation combines all gas laws into one relation. Where: * ( P ) = pressure * ( V ) = volume * ( n ) = number of moles * ( R ) = gas constant * ( T ) = absolute temperature **Key Insight:** For an ideal gas, internal energy depends only on temperature—not pressure or volume. --- ## **3. Thermal Conductivity** Thermal conductivity describes how well a material allows heat to pass through it. * High conductivity → good conductors (metals) * Low conductivity → insulators (wood, foam) ### **Governing Principle** \dot{Q} = -kA \frac{dT}{dx} Heat flows in the direction of decreasing temperature. ### **Worked Example** A wall has: * Area = 12.5 m² * Thickness = 0.2 m * Temperature difference = 20 K * Conductivity = 0.7 W/m·K [ \dot{Q} = \frac{kA\Delta T}{L} ] [ \dot{Q} = \frac{0.7 \times 12.5 \times 20}{0.2} = 875 \text{ W} ] **Answer:** Heat loss = **875 W** **Exam Tips:** * Always convert thickness to meters * Use steady-state assumption unless stated otherwise * Heat loss increases with area and temperature difference --- ## **4. First Law of Thermodynamics** This law expresses conservation of energy in thermodynamic systems. \Delta U = Q - W Where: * ( \Delta U ) = change in internal energy * ( Q ) = heat supplied to system * ( W ) = work done by system ### **Important Concepts** * **Internal Energy:** Energy stored in molecules (motion + interactions) * **Heat:** Energy transfer due to temperature difference * **Work:** Energy transfer due to mechanical action (expansion/compression) ### **Sign Convention (Very Important)** * Heat added → ( Q > 0 ) * Heat removed → ( Q < 0 ) * Work done by system → ( W > 0 ) * Work done on system → ( W < 0 ) ### **Worked Example** Heat removed = 1500 J → ( Q = -1500 ) Work done on system = 2000 J → ( W = -2000 ) [ \Delta U = -1500 - (-2000) = 500 \text{ J} ] **Answer:** Internal energy increases by **500 J** --- ## **5. Thermodynamic Processes** These describe how a system changes from one state to another. ### **(a) Isobaric Process (Constant Pressure)** W = P\Delta V * Work depends on volume change * Heat changes both internal energy and work **Example:** Gas expands from 0.01 m³ to 0.03 m³ at 200 kPa [ W = 200 \times 10^3 \times (0.02) = 4000 \text{ J} ] --- ### **(b) Isothermal Process (Constant Temperature)** * Temperature remains constant * Internal energy change = 0 [ Q = W ] **Key Idea:** All heat supplied is converted to work. --- ### **(c) Adiabatic Process (No Heat Exchange)** [ Q = 0 \quad \Rightarrow \quad \Delta U = -W ] * Occurs in insulated systems or rapid changes * Temperature changes significantly **Exam Focus:** * Isothermal → energy exchange through heat * Adiabatic → energy exchange through work only --- ## **6. Zeroth Law of Thermodynamics** This law defines temperature in a practical sense. **Statement:** If two systems are each in thermal equilibrium with a third system, they are in equilibrium with each other. **Implication:** This makes temperature measurement possible using thermometers. --- ## **7. Kinetic Theory of Gases** This theory explains gas behavior based on molecular motion. ### **Key Assumptions** * Gas consists of tiny particles in constant motion * Collisions are perfectly elastic * No intermolecular forces (ideal case) * Volume of molecules is negligible ### **Key Result** Temperature is directly related to the average kinetic energy of molecules. [ KE \propto T ] **Important Insight:** Higher temperature → faster molecular motion → higher pressure (if volume is constant) --- ## **8. Molecular Collisions and Mean Free Path** Gas molecules frequently collide with each other. * **Mean Free Path (λ):** Average distance traveled between collisions Factors affecting it: * Pressure (higher pressure → shorter path) * Temperature (higher temperature → longer path) * Molecular size **Exam Point:** Mean free path is inversely proportional to gas density. --- # **SUMMARY FOR TEST PREPARATION** * Heat flows from high to low temperature * Kelvin scale is essential for calculations * Ideal gas equation: ( PV = nRT ) * Thermal conductivity determines heat transfer rate * First law connects heat, work, and internal energy * Know all thermodynamic processes clearly * Isothermal: ( \Delta U = 0 ) * Adiabatic: ( Q = 0 ) * Molecular motion explains temperature and pressure * Mean free path depends on pressure and temperature --- ## **End of Batch 1** --- Next batch will cover: * **Second Law of Thermodynamics (Entropy & Engines)** * **Elasticity (Hooke’s Law & Moduli)** * **Hydrostatics and Buoyancy** Say **“continue batch 2”** when you’re ready.