We were discussing various basic concepts of thermodynamics such as thermodynamic state, path, process and cycles in our previous post. We have also discussed the concept of reversible and irreversible process in our recent post.

Today we will see here the very important term used in thermal engineering i.e. internal energy with the help of this post.

### Let us see, what is internal energy of a substance?

Each thermodynamic system, in an equilibrium state, will have one state variable which is termed as internal energy of the system.

Internal energy of a substance or a system is also termed as thermal energy of the system and it is basically defined as the summation of the kinetic energy and potential energy of the molecules of the system.

A system will have kinetic energy due to movement of individual molecule of the system. Movement of molecules might be vibrational, rotational or translational. Similarly system will have potential energy also due to the individual position of the molecules of the system.

So, a system will have internal energy which is also termed as system energy and will be given by following way.

#### U = K.E + P.E

Where, U is internal energy of the system and similarly K.E is kinetic energy and P.E is potential energy.

Internal energy is one extensive property of the system as it will be dependent over the size of the system. Hence we must have to note here that internal energy is an extensive property of the system.

We are never interested to determine the absolute internal energy or total internal energy of the system because it is not feasible and required to have information about the total internal energy or absolute internal energy of the system and therefore we are always concerned with the change in internal energy of the system for a given thermodynamic process.

Change in internal energy will be determined on the basis of quantity of energy exchanged between system and its surrounding for a given thermodynamic process in the field of thermal engineering. In simple way, we can say that change in internal energy of a system will be final energy of the system minus initial energy of the system during a process.
Let us recall the “first law of thermodynamic system
Let us consider a system undergoing in a non-cyclic process. Heat energy and work energy, both forms of energies are involved in the process i.e. work energy and heat energy are crossing the system boundary. Now we must have to note here that net energy will be accumulated within the system in the form of internal energy.
Figure showing a system, where Q is the heat energy supplied to the system from surrounding and work energy W is provided by the system to surrounding. What will be the net energy transferred to the system from surrounding?

Your answer will be (Q-W) i.e. net energy (Q-W) will be transferred to the system and it will be stored within the system and it will yield the change in internal energy of the system.
According to the joule’s law of internal energy, internal energy of a perfect gas will be a function of temperature only and it will not be affected by change in pressure or change in volume. Change in internal energy will always be dependent over the change in temperature only.

Do you have suggestions? Please write in comment box
We will discuss in our next post in the field of thermal engineering.

#### Reference:

Engineering thermodynamics by P.K. Nag
Engineering thermodynamics by R. K. Rajput