Internal Energy

In thermodynamics, "Energy is defined as a quantity that can be stored within the system and transferred between the system and the surroundings". Energy may exist in different forms in physics like- kinetic energy, potential energy, mechanical energy, rotational energy, chemical energy and vibrational energy etc. Of all these types, kinetic energy and potential energy are mostly observed. 

While "Internal Energy", on the other hand, is a well-known and important kind of energy in thermodynamics.
Energy is divided into two broad categories for thermodynamic analysis: Macroscopic and Microscopic energies.
Internal energy falling into the microscopic category. And microscopic forms of energies are those that belong to the system at a molecule or atomic level. In this article, we will learn about the concept of internal energy.


Internal Energy 
It is defined as, "The sum of all the forms of microscopic energy (the energy that the system possesses at the molecular or atomic level) of a system is called internal energy". It is denoted by a symbol 'U'
(On the other hand, internal energy is a measure of the energies associated with the molecules, atoms or subatomic particles of the system)
"Unlike potential energy and kinetic energy, internal energy is related to the energy within the system itself".
The molecular potential and kinetic energies contribute to the internal energy of the system. The addition of heat to the system results in the increase in its molecular kinetic energy and thus increase in the internal energy of the system. 
Let us now see what are the energies possessed by a system, at the molecular level or atomic level. Consider a system of polyatomic gas (consists of two or more than two atoms such as CO₂). Because the molecules or atoms present in gases move from one place to another with irregular motion. They have the following types of energies; Translational energy, Rotational energy, Vibrational energy, chemical or bond energy, latent heat, electronic translational/rotational/vibrational energies, etc. 
Thus, 
"Internal energy is the total heat content of atoms, molecules, electrons, and protons, in short everything present inside specific matter".

Let Us Now Look At These Types Of Energies One By One;
  1. Movement of molecules within the system from one place to another is called translation and the energy associated with this transfer of molecules is called "Translational Energy". 
  2. As the gas molecules translate within the system, they also rotate about their centre of mass and the energy associated with it, is called "Rotational Energy".
  3. The molecules of a poly-atomic gas vibrate at their respective positions. The energy associated with them due to this vibration is called "Vibrational Energy". 
  4. On a subatomic scale, the electrons of an atoms also possesses these all three types of energy. When electrons translate from one place to another, it has translational energy and when electrons spin about their own axis, it has rotational energy and when these electrons are vibrated, then the energy associated with it is called vibrational energy.
  5. The internal energy associated with atomic bonds in a molecule is called Chemical Or Bond Energy.
  6. The internal energy associated with the phase of a system is called Latent Heat.
The sum of the translational, vibrational, rotational and subatomic energies is the kinetic energy of molecules and also a fraction of the system's internal energy called the "Sensible Energy" while the other fractions of internal energy are "Latent Energy, Chemical Energy And Nuclear Energy". Thus we can say that, 
    Internal ＝  Sensible +  Latent  +  Chemical
    Energy         Energy      Energy       /nuclear  
                                                                Energy

Mathematical Form Of Internal Energy
We can also define internal energy according to the equation of first law of thermodynamics. The first law equation shows how the heat supplied to a system is related to the internal energy and the work done by the system for non-cyclic process. 
It is defined as "The heat and work involved in a non cyclic process are not equal. Consequently, some energy get stored in the system or some stored energy get removed from the system during a non-cyclic process. These changes in the stored energy are measured as the change in the internal energy of the system".
➩ [ ∆U = Q –W ]

Notable Points
☛ A given system under a given set of conditions has a definite internal energy.
☛ Internal energy is a thermodynamic property of a system and its solely determined by the thermodynamics state & not by the path which that state was arrived at.
☛ The internal energy of a system after a series of changes remain the same as it was before in a cyclic process.
☛ Sensible energy is defined as the form of energy which is required to change the temperature of the system while Latent energy is defined as the form of energy which is required to change the phase of a system. Phase change occurs when the binding force between the molecules, atoms or subatomic particles are broken. Chemical energy is the energy stored in the bonds of chemical compounds and this energy is released during a chemical reaction.
  Sensible ＝ molecular＋molecular＋molecular
  Energy       translation     rotation      vibration
                  ＋  electron   ＋ electron ＋ nuclear  
                     translation       spin              spin 

☛ Internal energy is associated with the temperature of the object.
• If the temperature is higher, then the internal energy is more.
• If the temperature is lower, then the internal energy is less.
Remember
!!!!A SYSTEM DOES NOT CONTAIN HEAT. BUT IT CONTAINS INTERNAL ENERGY!!!!


FAQs
What Is Internal Energy??
Internal energy is the sum of all kinds of energies in a system. It is the total energy contained in a system. It is due to the translational, vibrational, and rotational motion of atoms in a molecule. It is also due to interactions between the molecules.
It is represented by either E or U.
In A Constant Volume Process, The Internal Energy Change Is Equal To?
At constant volume, the change in internal energy is equal to the amount of heat absorbed in a system.
From the 1st law of thermodynamics,
➩ dQ = dU + dW
(dQ)v = (dU)v



Hope you have found this article helpful!!
Let me know what you think about INTERNAL ENERGY. Feel free to comment if you have any queries.!!

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