The most Important concept in thermodynamics is the concept of State. If we knew the details of mass, velocity, volume, position and modes of motion of each molecule of the system then that knowledge would help us describe the details of the system at a microscopic level, and is known as microscopic state. But as we know getting such detailed information is not possible thus we rely on state parameters which describe the combined state of all molecules i.e. the macroscopic state .
Thus,
Macroscopic State
- Has a small no of variables are used to describe the state of matter.
- These variables are easy to measure.
- Structure of matter is not taken into account.
Microscopic State
- It has a large no of variables.
- These variables can not be measured with current level of technology.
- The knowledge of the structure of matter is essential to analyze the behaviour of the constituent particles/molecules.
We consider here only Macroscopic state.
At first it might look like there a lot of variables which needs to be determined but in reality these variables are interdependent thus by fixing some of the variables, the values of rest of the variables get fixed. For a simple system with a given substance with a fixed composition we can fixed the system properties with just fixing two variables. These are known as Independent State Variables.
- The minimum number of variables required to describe the state of the system are called independent state variables.
We can choose any two properties/variables in case of gas, the variables most easy to control is Pressure and Temperature. Hence, let us take volume of a gas to be a fixed quantity and a function of Pressure and Temperature. Therefore,
V = V(P,T)
dV = (dV/dP)TdP + (dV/dT)PdT
Now, we can determine how volume changes with Pressure and Temperature according to the function V. Thus, here V is dependent Variable and P and T are independent variable, used to describe the macroscopic state of the system.
