Energy and its types in Thermodynamics

Before defining energy, let us define work. In physics, "work is defined as the product of force and displacement" according to Newton's second law of motion. Let us now define energy. When we study physics, we define the term energy as "energy is the ability to do work". And when we study thermodynamics, energy is defined as a quantity that can be stored within the system and transferred between the system and the surroundings. 

In general, the former definition of energy is the most well-known and accepted.

"Energy is a conserved quantity that means energy can neither be created nor destroyed. It can only be transferred from one form to another. And this is known as the law of conservation of energy". 

Energy is the quantitative property that must be transferred to a physical system in order to do work on that system or heat it. 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 for the purpose of thermodynamics analysis, energy is classified into two broad categories namely "macroscopic energies and microscopic energies". Microscopic is related to molecular or atomic level whereas macroscopic is not related to molecular or atomic level.

Microscopic forms of energies are those that belong to the system at a molecule or atomic level. These are of many types such as translational energy, vibrational energy, rotational energy etc. but we conveniently grouped them into a single category referred to as internal energy.

Macroscopic forms of energy are those possessed by a complete system with respect to a fixed external reference. These include only two types of energies, kinetic energy and gravitational/potential energy.

Let us now read about them in detail one by one;

(1) Kinetic Energy

"When an object is in a state of motion, an energy is also associated with that object, that energy is called kinetic energy". We represent kinetic energy by the symbol "KE". The kinetic energy includes another forms of energies like thermal energy, radiant energy and sound energy (as sound travels from one place to another and due to this motion, an energy is associated with it which is called as kinetic energy of sound or simply sound energy). "It is generally based on external velocity reference".

For example-

(a) A speeding bullet, Initially the bullet has no velocity. But when that bullet leaves the gun, due to its motion, there is an energy associated with it, the same is called kinetic energy for the bullet.

(b) A running person associated with an energy, called kinetic energy.

(c) A wreaking ball

(d) Electromagnetic radiations like light etc. all have kinetic energy.

(e) A moving cars, bus, trains etc.

So we can say that if an object is in motion, it must have kinetic energy associated with it. The kinetic energy of an object is given by a formula,

              ➩ { KE ＝ mu² / 2 }

Here,

       m represent the mass of an object

       u represent the velocity of an object

The unit of kinetic energy,

                         SI unit = Joule or N-m

                    MKS unit = kg-m²/sec²

                      CGS unit = g-cm²/sec²

    While, in metric engineering systems;

               Unit of kinetic energy = kgf-m

While change in kinetic energy of an object is given by a formula;

Where,

         u₁&u₂ are the initial and final velocity

         m represent the mass of an object

(2) Potential Energy

"When an object is in a stationary state, the energy associated with it is called potential energy". There are two forms of potential energy in any object namely "Elastic potential energy" and "Gravitational potential energy". 

        First, "Elastic potential energy is a form of energy that is stored in a deformed body"(A deformed body is a body on which if any external force is applied, its shape and volume changes and as soon as the external force is removed, it returns to its original shape and volume) such as an elastic solid or a spring. 

            While the second one, "Gravitational potential energy is the energy possessed by a system due to its elevation with respect to any reference in the gravitational field". In most cases, a reasonable reference is the ground surface because it is a convenient origin. 

        

➛ Elastic potential energy is of less importance in thermodynamics whereas gravitational potential energy is widely used in thermodynamics. That's why we usually abbreviate gravitational potential energy as potential energy. The potential energy of an object is given by a formula,

          ➩ { PE = m×g×h }

Here,

      m represent the mass of an object

      g represent the acceleration due to gravity

      h represent the height of an object with 

          respect to a reference

➛ If a body of mass 'm' is raised from an initial elevation 'z₁' to a final elevation 'z₂' then the change in potential energy is given as,

        ➩ ∆PE = PE₂ - PE₁ 

                      = m×g×z₂ - m×g×z₁

                      = mg (z₂-z₁)

       ➩ { ∆PE = mg (∆z) }

Here, 

     PE₂ is the potential energy at z₂ elevation

     PE₁ is the potential energy at z₁ elevation

     g is the acceleration due to gravity

➛ The unit of potential energy is the same as the unit of kinetic energy as both are forms of energy. Hence, the unit of potential energy is

               SI unit = Joule or N-m

          MKS unit = kg-m²/sec²

           CGS unit = g-cm²/sec²

While, in metric engineering systems

       Unit of potential energy = kgf-m

(3) Internal energy

As we have seen above,"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".

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. So they have the following types of energies,

(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 polyatomic 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

☆ Internal energy has no concise thermodynamic definition and cannot be measured directly. But the change in internal energy can be measured.

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