Heat, Work, Laws of Thermodynamics and Ideal gases. ( Part 2 )

Heat : Heat is defined as the form of energy that is transferred between two system or a system and surrounding due to temperature difference. It has unit Joule.

Work : Work is energy transfer associated with the force or any other medium acting through a distance on system. An energy transfer which is not caused by the temperature difference between a system and its surrounding is called as work.

Example : Piston moving inside the cylinder, a rotating shaft, …etc.

Power : Work done per unit time is called as power. Unit of power is KJ/sec or KW.

Enthalpy : Enthalpy is defined as the sum of internal energy and flow work. The unit of enthalpy is J/sec.

Different types of work transfer :

**Mechanical Work Transfer**

 

1.    Displacement work / PdV work / Reversible work :

                            

Displacement Work

     Consider a piston moving inside the cylinder. The diameter of cylinder is D and displacement is dL. Whatever the work is given by the piston by the displacement inside the cylinder is known as displacement work.

                                                  dw = F x dL

2.    Paddlewheel work or Stirrer work : 

      

    Input say force is transferred through paddlewheel in order to obtain work output.

3.    Electrical work : In an electrical field, electrons in a wire move under the effect of electromotive force to perform work.

The transfer of electron is done by the potential difference.

4.    Spring work :

    Length of spring changes by differential amount dx due to application of force, the work is done is given by,

                                         dw = F x dx

5.    Shaft work : 

     

            It refers to energy transferred across the boundary by the rotating shafts.

6.  Flow work : It is also known as flow energy, the energy required to maintain a flow of liquids.

Difference between Heat and Work :

Work :

•   Work is a mechanical energy obtain due to force, tension and displacement.
•   It is a microscopic process while,

Heat :

•   Heat is thermal energy. It flows due to temperature difference.
•   It is a microscopic process.

Zeroth law of thermodynamics : 

    The Zeroth law of thermodynamics states that “If two bodies are in thermal equilibrium with a third body, then all the three bodies are in thermal equilibrium with each other.”

                                              Like, A = B   and B = C

                                           Then, A = B = C

First law of thermodynamics : 

    The First law of thermodynamics states that “Energy can be neither created nor destroyed, but it can be converted from one form to another form.”

Joule`s law : Joule`s law states that “The internal energy of a perfect gas is a function of the absolute temperature only.”

                                                            U = f ( T )

Ideal Gas : The gases which do not change their phase easily and obey some set of governing laws is called as an ‘Ideal Gas.’ 

Governing Laws of Ideal Gas :

1.    Boyle`s Law : Boyle`s Law states that “At constant temperature, the volume of a given mass of gas is inversely proportional to its absolute pressure.”

   V α 1 / P

⸫ V = C / P           PV = Constant

2.    Charle`s Law : Charle`s Law states that “At constant pressure, the volume of a given mass of gas is directly proportional to its absolute temperature.”

   V α T       

⸫ V = CT          V / T = Constant        

3.    Gay-Lussac Law : Gay-Lussac Law states that “At constant volume, the pressure of a given mass of gas is directly proportional to its absolute temperature. 

           P α T

       ⸫ P = CT        P / C = Constant 

 

Characteristics Equation of Ideal Gas :

Consider constant pressure process 1 – 2

The governing equation is.

V / T = C

⸫ V₁ / T₁ = V₂ / T₂

⸫ V₂ = V₁T₂ / T₁

Now consider Isothermal process 2 – 3

PV = C

⸫ P₂V₂ = P₃V₃

⸫ V₂ = P₃V₃ / P₂

comparing the values of V₂

⸫ V₁T₂ / T₁ = P₃V₃ / P₂

But P₁ = P₂ as process 1 - 2 is constant pressure process

In addition, T₂ = T₃ as process is constant temperature process

⸫ V₁T₃ / T₁ = P₃V₃ / P₁

By rearranging the terms

V₁P₁ / T₁ = P₃V₃ / T₃

⸫ In general PV / T = Constant

Let R is a gas constant

⸫ PV / T = R

⸫ PV = RT

For m kg of an ideal gas

⸫ PV = mRT

This is called as Characteristics Equation of Ideal Gas

   

4.    Avogadro`s Law : Avogadro`s Law states that “Equal volume of all perfect gases at equal pressure and temperature contains an equal number of molecules (n).

Avogadro's Law

If   V_A = V_B = V_C

If   P_A = P_B = P_C

If   T_A = T_B = T_C

Then,  n_A = n_B = n_C

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