Thermodynamics Equations depends on its laws. Here we will understand the concept of thermodynamics equation using its laws. First law of thermodynamics is also known as energy conservation law. According to this law, energy can neither be generated nor be destroyed bu it can be transformed from one state to other.
According to first law of thermodynamics, if amount of heat dQ is added to system then it may increases to its internal energy by amount dU, while remaining part may be used up external work dW one by system. So value of dQ , dU and dW all are in same states then it can be written as:
dQ = dU + dW. Now we will understand this concept with help of small example.
Let's consider a gas enclosed in a cylinder that provided frictionless and weightless piston. The initial state, pressure and volume of gas are denoted as ‘P' and ‘V' respectively. Further, we have a piston that moves thorough an infinitesimally small distance ‘dx' at constant pressure ‘P', so that its volume becomes V + dV. Then we will see that small work done by the object, dW is equal to the force on piston and dx. So, mathematically it can be written as:
dW = force on piston * dx.
If ‘a' is area of cross – section of piston, then force on piston will be equal to A a. So we can write it as:
dW = P a X dx = p x (a dx),
Now, a x dx = dV, small increase in volume. Therefore, we can say that:
dQ = dU + p DV,
Now we will see some application based on first law of Thermodynamics Equations. First we will see cyclic process.
Cyclic process: In cyclic process, system proceeds to its initial state at end of the cycle. A cyclic process in which system proceeds from state A to P along the path X and then returns back to initial state A along path Y. Obviously, the change in internal energy of system along the complete cyclic process is zero that is dU = 0. Therefore, for a cyclic process, First law of thermodynamics becomes,
dQ = 0 + P dV,
or dW = P dV, that is in cyclic process, heat transfer to gas is wholly converted into work. It can prove that area of cyclic method is numerically equal to the work done for period of the method.
Now we will discuss the boiling process based on thermodynamics laws:
Boiling process: When a liquid is supplied to heat, it starts boiling at a particular temperature is called its boiling point. At boiling point (here we have to take pressure constant), unit mass of liquid requires a definite amount of heat energy to change form liquid to vapour state. The amount of heat supplied is called latent heat of vaporization of the liquid. This is all about thermodynamics equation.