Fig. l.l(a) shows a cylinder having air. and fuel at definite volume, pressure and temperature.The piston is now pushed to level 2-2, and at the new situation there is definite new volume, pressure and temperature.
The volume, pressure and temperature can be measured in both the situations. The state or condition of the systems (Air+ Fuel mixture) can be determined by means of above characteristics or properties. This type of analysis is macroscopic analysis and is used in engineering thermodynamics.
However, if the analysis is made on the basis of the atomic or molecular behaviours of the substance, the analysis is called microscopic analysis. It is to be noted that macroscopic results can also be derived from microscopic analysis.