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JEE Physics Thermodynamics

Thermodynamics PDF Notes, Important Questions and Synopsis



  • Thermodynamics deals with the exchange of heat between a body and the surrounding along with other processes accompanying it.
  • The nature of heat and its relationship to mechanical work was studied by Joule.
  • Thermal equilibrium implies that systems are at the same temperature.
  • Internal energy of a system is the sum of kinetic energies and potential energies of the molecular constituents of the system. It does not include the overall kinetic energy of the system.
  • Equilibrium states of a thermodynamic system are described by state variables. The value of a state variable depends only on the particular state and not on the path used to arrive at that state.
  • Examples of state variables are pressure (P), volume (V), temperature (T) and mass (m). Heat and work are not state variables.
  • Zeroth law of thermodynamics: Two systems in thermal equilibrium with a third system are in thermal equilibrium with each other.
  • The first law of thermodynamics is based on the principle of conservation of energy.
    ∆U = ∆Q − P∆V
  • Efficiency of a heat engine is the ratio of work done by the engine to input heat.

    begin mathsize 12px style text η= end text fraction numerator Work space done over denominator Input space heat end fraction equals straight W over straight Q subscript straight H end style
  • If all the input heat is converted entirely to heat, then the engine would have an efficiency of 1.
  • In a reversible process, both the system and its environment can be returned to their initial states.
  • Spontaneous processes of nature are irreversible. The idealised reversible process is a quasi-static process with no dissipative factors such as friction, viscosity etc.
  • A quasi-static process is an infinitely slow process such that the system remains in thermal and mechanical equilibrium with the surroundings throughout. In a quasi-static process, the pressure and temperature of the environment can differ from those of the system only infinitesimally.
  • A heat engine is a device in which a system undergoes a cyclic process resulting in conversion of heat to work.
  • The Carnot engine is a reversible engine operating between two temperatures T1 (source) and T2 (sink). The Carnot cycle consists of two isothermal processes connected by two adiabatic processes.

  • The efficiency of the Carnot engine is independent of the working substance of the engine. It only depends on the temperatures of the hot and cold reservoirs.
  • Efficiency of the Carnot engine is 𝜂 = 1 − TC/TH = 1 − (Temperature of cold reservoir/Temperature of hot reservoir).
  • The efficiency of an engine is never more than that of a Carnot engine.
    1. Heat lost by a hot body = heat gained by a cold body
    2. Heat can flow from cooler surroundings into a hotter body (e.g. coffee) to make it hotter.
  • Kelvin’s statement of the second law of thermodynamics:
    No heat engine can convert heat to work with 100% efficiency.
  • Clausius statement: No process is possible whose sole result is the transfer of heat from a colder body to a hotter body.
  • Kelvin’s statement: No process is possible whose sole result is the complete conversion of heat to work.
  • Coefficient of performance of a refrigerator is α = QC/W.
  • A heat pump is called so because it pumps heat from the cold outdoors (cold reservoir) into the warm house (hot reservoir).
  • When Q > 0, heat is added to the system.
    When Q < 0, heat is removed from the system.
    When W > 0, work is done by the system.
    When W < 0, work is done on the system.
  •  Heat engine

  • Work done by a heat engine