Thermodynamic system – first variable “Volume”

When we do any experiment, we focus our attention to a specific part or a system with boundaries.

Now in our purpose of learning thermodynamics, we mostly take the system of some air or water with certain boundaries.

For our convenience we made a word for that specific system with its boundaries and that is so called Thermodynamic System.

The first independent variable in a Thermodynamic system to notice by a common man is Volume.

Volume is the measure of amount of space occupied by the boundary of the system, and for gases and liquids the volume of the container is considered to be the volume of the system.

Besides, its measurement is very simple and done using our everyday measurements such as length, breadth and width. This is the reason why most of our thermodynamics study involves none other than other shapes but cylinder, being one of the simplest shapes.  

Since length, breadth and height are measured in unit lengths, the volume will be measured in cube of the power to that unit length. 

Unit length is yours to choose. In general for most of the purpose, meter is used as the unit length.

All the information about the volume of the system is thus possible to obtain just using our standard measurement scales. As a result, now it is quantitatively analyzed.  

Note that,

We never cared about the internal structure of the particles or the forces between them or how this volume was created by this particles or etc. in this thermodynamic system.

We don’t even know whether it is atoms are inside this volume.

You may ask the question,

We don’t know anything about the internal properties but we just define the external properties to understand the thermodynamic system. Doesn’t it meaningless?

It is like explaining an object by just seeing its outside shadow and not caring about its internal structure.

It is true and the doubt is quite meaningful.

Moreover, “Everything depends on the purpose”. If we start to try and analyze the internal structure, we may not get a correct solution or we may get complicated and even we don’t know how to analyze it. There is no information about internal structures at those times.

But for the survival, there is a conditional purpose for the people and scientist to understand a thermodynamic system.

Hence, we forget to care about its internal structure and just put our concern on trying to explain it from the external behaviours.

That is why the thermodynamic relations doesn’t hold good at all situations. But it is a good estimated outline for basic understanding.

And studying the outline to understand its behaviour is not a wrong approach.

Whatever may be the internal structure, the outer structure should be the same for a same state.

It is like from the shadow, for example from a human shadow,

If you predict his behavior, color and etc. it would go wrong.

But the properties of the shadow will not change. From all these things, it is certain that it is a human shadow.

The internal properties may vary accordingly to the same external property.

When the information about the system is more accurately known, the internal behaviours are also predictable based upon those data. And it needn’t to be worried.

Therefore, the external properties are perfectly logical to define a thermodynamic system. But the relations between them needn’t to be an absolutely correct one.