CBSE Class 11 Physics Thermodynamics Notes Set B

Class XI

Chapter 12: Thermodynamics

Key Learning

1. The nature of heat and its relationship to mechanical work was studied by Joule.

2. Thermal equilibrium implies that systems are at the same temperature.

3. 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 over – all kinetic energy of the system.

4. Equilibrium states of a thermodynamics system are described by state variables. The value of a state variable depends only on the particular state, not on the path used to arrive at that state.

5. Examples of state variables are pressure (P), volume (V), temperature (T) and mass (m). Heat and work are not state variables.

6. Zeroth law of thermodynamics states that two systems in thermal equilibrium with a third system, are in thermal equilibrium with each other.

7. The first law if thermodynamics is based on the principle of conservation of energy. It states that ΔU = ΔQ - PΔV

8. The efficiency of a heat engine is defined as the ratio of the work done by the engine to the input heat.

η= Work done/Input heat=W/Q_H

9. If all the input heat is converted entirely into heat, the engine would have an efficiency of 1.

10. In a reversible process both the system and its environment can be returned to their initial states.

11. Spontaneous processes of nature are irreversible. The idealized reversible process is a quasi – static process with no dissipative factors such as friction viscosity, etc.

12. 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.

13. Heat engine is a device in which a system undergoes a cyclic process resulting in conversion of heat into work.

14. 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.

15. 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.

16. Efficiency of Carnot engine is η = 1 – T_C /T_H = 1 – (Temperature of cold reservoir/Temperature of hot reservoir).

17. No engine can have efficiency more than that of a Carnot engine.

18. Implications of First law of thermodynamics:

i. Heat lost by hot body = heat gained by the cold body.

ii. Heat can flow from cooler surroundings into the hotter body like coffee to make it hotter.

19. Kelvin’s Statement of second law of thermodynamics: No heat engine can convert heat into work with 100 % efficiency.

20. Clausius’s Statement: No process is possible whose sole result is the transfer of heat from a colder to a hotter body.

21. Kelvin’s Statement: No process is possible whose sole result is the complete conversion of heat into work.

22. The co-efficient of performance of a refrigerator is α = QC/W.

23. A heat pump, is called so, because it pumps heat from the cold outdoors (cold reservoir) into the warm house (hot reservoir).

24. If Q > 0, heat is added to the system

If Q < 0, heat is removed to the system

If W > 0, Work is done by the system

If W < 0, Work is done on the system

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