Thermodynamic Views

When a system has been chosen, the first step is to describe it in terms of quantities which will be helpful in discussing the behavior of the system or its interactions with the surroundings. An investigation into the behavior of a system might be undertaken from either a microscopic or macroscopic point of view.

Macroscopic Point of View

• The description of a system involves the specification of a few fundamental measurable properties of a system like pressure, volume, temperature, composition, etc.

• As an example, if we consider the fuel in a petrol engine cylinder as a system the chemical analysis shows that there is mixture of petrol vapour and air before explosion and the product of explosions, which can be described by chemical analysis. The volume occupied at any moment and the temperature of the mixture at any instant can be measured. Hence the contents in the engine cylinder i.e. system can be described by specifying four quantities like composition, pressure, volume and temperature. This type of description is called the Classical or Macroscopic description.

Microscopic Point of View

• We would consider matter to be composed of molecules having a certain position, velocity and energy at a given instant. These change frequently as a result of collision for each molecule. The behavior of the system could be described by summing up the behavior of each molecule. Such a study is known as Statistical Mechanics or Microscopic description.

Comparison

 Macroscopic Microscopic Without taking into account the events occurring at molecular level a certain quantity of matter is considered and is known as classical thermodynamics. This approach is concerned directly with the structure of the matter. This is known as statistical thermodynamics. The analysis requires simple mathematical formulae. Behavior of the system is found by using advanced statistical and mathematical methods as the number of molecules is very large. The values of the properties of the system are their average values and can be measured easily by instruments like pressure, temperature, etc. The properties cannot be easily measured by instruments like velocity, momentum, impulse, etc. In order to describe a system only a few properties are needed and the approach is simple. To describe a system large numbers of variables are needed and the approach is complicated.