Class 11 Physics: Thermodynamics

• Basic Concepts & Systems

  • Thermodynamic System: Quantity of matter or region under study
    • Open System: Exchanges both mass and energy
    • Closed System: Exchanges energy only, not mass
    • Isolated System: No exchange of mass or energy
  • Surroundings: Everything external to the system
  • Thermodynamic Variables
    • Intensive Variables: Independent of size/mass (e.g., Pressure, Temperature)
    • Extensive Variables: Depend on size/mass (e.g., Volume, Internal Energy)
  • Thermal Equilibrium: Equal temperature throughout system and surroundings

• Zeroth Law of Thermodynamics

  • Statement: If systems A and B are each in thermal equilibrium with C, then A and B are in thermal equilibrium with each other
  • Significance: Introduces and defines the concept of Temperature

• First Law of Thermodynamics (FLOT)

  • Concept: Law of Conservation of Energy applied to thermodynamic processes
  • Equation: ΔQ = ΔU + W
    • ΔQ: Heat energy supplied to the system
    • ΔU: Change in Internal Energy (State function)
    • W: Work done by the system (W = ∫P dV)
  • Sign Conventions
    • Heat: Added to system (+ve) | Removed from system (-ve)
    • Work: Done by system (+ve) | Done on system (-ve)
    • Internal Energy: Increase in temperature (+ve) | Decrease in temperature (-ve)

• Thermodynamic Processes

  • Isothermal Process
    • Condition: Temperature remains constant (ΔT = 0, ΔU = 0)
    • Equation: PV = Constant (Boyle's Law)
    • Work Done: W = nRT ln(V₂ / V₁)
  • Adiabatic Process
    • Condition: No heat exchange with surroundings (ΔQ = 0)
    • Equation: PV^γ = Constant
    • Work Done: W = nR(T₁ - T₂) / (γ - 1)
  • Isobaric Process
    • Condition: Pressure remains constant (ΔP = 0)
    • Work Done: W = P(V₂ - V₁)
  • Isochoric Process
    • Condition: Volume remains constant (ΔV = 0, W = 0)
    • First Law Form: ΔQ = ΔU
  • Cyclic Process
    • System returns to initial state (ΔU = 0)
    • Net Work Done: W = Area enclosed on PV diagram

• Specific Heat Capacities of Gases

  • Molar Specific Heat at Constant Volume (Cv): Heat to raise 1 mole gas temperature by 1K at constant volume
  • Molar Specific Heat at Constant Pressure (Cp): Heat to raise 1 mole gas temperature by 1K at constant pressure
  • Mayer's Relation: Cp - Cv = R
  • Adiabatic Index (γ): Ratio γ = Cp / Cv

• Second Law of Thermodynamics

  • Kelvin-Planck Statement: Impossible to build an engine operating in a cycle that converts 100% heat into work
  • Clausius Statement: Heat cannot spontaneously flow from a colder body to a hotter body without external work
  • Reversibility
    • Reversible Process: Quasi-static, no dissipative forces
    • Irreversible Process: Natural processes, dissipative forces present

• Heat Engines & Refrigerators

  • Heat Engine
    • Absorbs heat Q₁ from hot reservoir (T₁), does work W, releases Q₂ to cold reservoir (T₂)
    • Efficiency (η): η = W / Q₁ = 1 - (Q₂ / Q₁)
  • Refrigerator / Heat Pump
    • Extract heat Q₂ from cold body using work W, reject Q₁ to hot body
    • Coefficient of Performance (β): β = Q₂ / W = Q₂ / (Q₁ - Q₂)

• Carnot Engine

  • Concept: Ideal, theoretical reversible heat engine operating between two temperatures
  • Carnot Cycle Steps
    1. Isothermal Expansion (at T₁)
    2. Adiabatic Expansion (from T₁ to T₂)
    3. Isothermal Compression (at T₂)
    4. Adiabatic Compression (from T₂ to T₁)
  • Carnot Efficiency: η = 1 - (T₂ / T₁)
  • Carnot Theorem: No engine operating between two given temperatures can be more efficient than a Carnot engine