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# Thermodynamics

Thermodynamics PDF Notes, Important Questions And Synopsis

## Thermodynamics PDF Notes, Important Questions and Synopsis

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.

• 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

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how we find work done
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A P-T graph is shown for a cyclic process. Select correct statement regarding this:- (1) During process CD, work done by gas is negative. (2) During process AB, work done by the gas is positive. (3) During process BC internal energy of system increases. (4) During process BC internal energy of the system decreases
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