THERMODYNAMICS

Definition of Thermodynamics LECTURE-01 Definition of Thermodynamics Thermodynamics deals with mutual conversion of different types of energy, the direction of physical and chemical processes and of equilibria. It also studies systems composed of many parts. System we consider any region of space separated from its surroundings.

Isolated – do not exchange matter or energy with According to the interaction of the system with its surroundings we discriminate systems: Isolated – do not exchange matter or energy with surroundings Closed – exchange only energy with surroundings, not matter Open – exchange both matter and energy with

Extensive parameters – characterize thermodynamic Thermodynamics studies two types of parameters: Extensive parameters – characterize thermodynamic system as a whole (mass,volume, total electric charge) Intensive parameters – they have different values in different parts of the system (concentration of chemical components,temperature, electrical potential) The studies of the relationship between extensive and intensive parameters create the basis for the formulation of thermodynamic laws.

Macroscopic and Microscopic Approach Concept of Continuum Macroscopic and Microscopic Approach In Macroscopic approach of thermodynamics the substance is considered to be continuous whereas every matter actually comprises of myriads of molecules with intermolecular spacing amongst them. For analyzing a substance in aggregate it shall be desired to use laws of motion for individual molecules and study at molecular level be put together statistically to get the influence upon aggregate.

In statistical thermodynamics this microscopic approach is followed, although it is often too cumbersome for practical calculations. In engineering thermodynamics where focus lies upon the gross behavior of the system and substance in it, the statistical approach is to be kept aside and classical thermodynamics approach be followed. In classical thermodynamics, for analysis the atomic structure of substance is considered to be continuous. For facilitating the analysis this concept of continuum is used in which the substance is treated free from any kind of discontinuity.

As this is an assumed state of continuum in substance so the order of analysis or scale of analysis becomes very important. Thus, in case the scale of analysis is large enough and the discontinuities are of the order of intermolecular spacing or mean free path then due to relative order of discontinuity being negligible it may be treated continuous. In the situations when scale of analysis is too small such that even the intermolecular spacing or mean free path are not negligible i.e. the mean free path is of comparable size with smallest significant dimension in analysis then it can not be considered continuous and the microscopic approach for analysis should be followed.

. For example, whenever one deals with highly rarefied gases such as in rocket flight at very high altitudes or electron tubes, the concept of continuum of classical thermodynamics should be dropped and statistical thermodynamics using microscopic approach should be followed. Thus, in general it can be said that the assumption of continuum is well suited for macroscopic approach where discontinuity at molecular level can be easily ignored as the scale of analysis is quite large. The concept of continuum is thus a convenient fiction which remains valid for most of engineering problems where only macroscopic or phenomenological information are desired.

Conclusion This approach is used in macroscopic approach. When: a)  the dimension of individual particles  constituting the system is negligible w.r.t overall dimensions of system b) when the numbers of individual particles is very large When both the above stated points are satisfied ,we can apply macroscopic approach.