JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV BASICS OF THERMODYNAMICS Previous Question Paper (July/August - 2022 - R16)

The macroscopic approach, also known as classical thermodynamics, analyzes the behavior of matter by considering the aggregate effect of molecules without regard for individual molecular activity. It focuses on time-averaged properties such as pressure, temperature, and volume, which can be measured directly by instruments. This approach is mathematically simpler and sufficient for most engineering applications.

The microscopic approach, or statistical thermodynamics, is based on the kinetic theory of gases and considers the behavior of individual molecules. It analyzes the statistical average of molecular properties like position, velocity, and energy to predict the macroscopic state of the system. While more fundamental and detailed, it requires complex mathematical modeling and is typically reserved for specialized research or extreme conditions where continuum assumptions fail.

A quasi-static process is an idealized thermodynamic process that occurs infinitely slowly, such that the system remains in thermodynamic equilibrium at every instant. During this process, the state variables are uniform throughout the system, and the path of the process can be clearly defined on a property diagram.

A classic example is the slow compression of a gas in a cylinder by adding infinitesimal weights to a piston, allowing the pressure to remain uniform throughout the gas at every stage. In a non-quasi-static process, rapid movement would create pressure gradients and turbulence, making the state undefined.

To estimate work transfer (W) for a quasi-static process, we use the integral of pressure with respect to volume:

This represents the area under the process curve on a P-V diagram. Heat transfer (Q) is estimated using the First Law of Thermodynamics for a closed system:

where ΔU is the change in internal energy. For a quasi-static process, if the process is reversible, heat transfer can also be related to entropy change (dS) by the relation dQ = T dS, where T is the absolute temperature.