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Scope of thermodynamics

Thermodynamics is a study and a sub division in pure physical science. It deals with the change or transformations of all types of energy. Most of the time it deals with the relationship between heat and the work relationship with other forms of energy with respect to flow of these other form of energy. There are different forms of energy.

Energy can be found in everything. There are even forms of energy that are not even found in the three basic states of matter, so you can imagine how vast energy apply to everything. It cannot be denied that the relationship between energy has become quite the study. The learning of how the transformation of energy is done has; aided man in the creation of relatively all the mechanical age machines, It has also helped chemists to understand reactions better, it has aided efficiency in all field of physical sciences.

By expanding the fore mentioned use of energy transfer you would get the idea of how important thermodynamics is, it you have not, you can always research.

The axioms of thermodynamics

After extensive study of thermodynamics through time, some basic laws has been validated. These laws has been tested through vast experience and tests in the real practical works, with some calculations (though for study sake no one will admit it). These laws are expressed in mathematical forms for easier approach to answers, hence making measurement in thermodynamics more efficient. Studying thermodynamics is based on four main postulates and axioms from which a wide range of governing laws are established

First axiom. A form of energy known as internal energy (U) can be found in substances. For systems at equilibrium these form of energy is intrinsic property of the system functionality, which is related to measurable co-ordinates that describe the system. The down side of this is that the energy cannot be measured directly.

Second axiom. All the energy of a system and its environment is conserved (cannot be created or destroyed), this is the first law of thermodynamics and is stated mathematically as thus;

ΔUtotal = 0

Third axiom. In a system there is a property called Entropy (S). This property for systems at equilibrium, is related by function, to the measurable co-ordinates that describes the system. Like the first the downside is, entropy cannot be measured directly.

Fourth axiom. When there is an entropic change between the system and its environment it is worked on together. Entropic change that is resulted from any real life event or process is positive, and tends to a limiting value of zero as the process approaches reversibility. This is the second law of thermodynamics. This is stated as thus;

ΔStotal ≥ 0

Through combination of these axioms it can be deduced that

  • Energy can change forms i.e. it can transform from one form to another

  • The entropy of a separate, isolated system cannot change

  • Energy can be conserved

As you might have noted, the first and second axioms concentrated on the first law of thermodynamics. The third and fourth axioms (especially the fourth), emphasised on the second law of thermodynamics.

The System and its Environment

Universe: The universe in thermodynamics is the total environment where what is being studied can be found. (System + Surrounding + Boundary)

Boundary: The boundary in thermodynamics limit or edge of a system of a system in study, and the beginning of the surrounding.

Surrounding: After the boundary of a system, the environment the system interacts with is called the surrounding.

System: The system is the area of focus, where most of the work is done.

Terms in systems and surroundings

Open (flow): This is when there is a system boundary that does allow flow of heat and other type of energy to and from a system and its surrounding.

Closed (non-flow): This is when there is a system boundary that does not allow flow of heat and other type of energy to and from a system and its surrounding. Heat might be excluded from the forms of energy.

Isolated system: These are closed systems that are so insulated, that even heat cannot be transferred in or out of a system.

Process in thermodynamics

Isobaric or Isopiestic process: This is a process that has a constant pressure when it is being carried out. i.e. (Δp = 0)

= constant


Isothermal process: This is a process that has a constant temperature when it is being carried out. i.e. (ΔT = 0)

PV = Constant

P1 V1 = P2 V2

Isometric or Isochoric process: This is a process that has a constant volume when it is being carried out. i.e. (ΔV = 0)

Anabatic: The process change in heat is equal to zero.

PVr = Constant

P1 V1r = P2 V2r


Polytrophic process: In this process, there is no special condition attached, it is also subdivided, but I will not get into it here.


Work done (W) = PΔV

= PdV

Heat (Q) = RTinV


Enthalpy (H) = CpΔT

Heat at constant volume (during isometric process) = Internal energy (U)

Heat at constant pressure (during isobaric process) = Enthalpy (H)

The first law of thermodynamics



, ΔQ-ΔW= 0

Closed system: ΔU= ΔQ + ΔW

Work done: W= PΔV

The concept of Entropy

The combination of two energy terms and properties of systems and thermodynamics.

H= U + PV = Q


ΔW= H2-H1


ΔU= CRΔT or nCvΔT (for when dealing with non-ideal gases that has numbers of molecule) [n = number of moles]


ɣ= = constant


In ideal gas, PV=nRT


V1 =Initial volume

V2 = Final volume

P1 = Initial pressure

P2 = Final pressure

T1 = Initial Temperature

T2 = Final Temperature

V!r = Initial volume at gas constant

V2r = final volume at gas constant

Isothermal process (dT=0, P1 V1 = P2 V2)

From first law of thermodynamics

dV= dQ-dW

CvΔT = dQ- PdV

dT= 0

but PV= RT


Q= W = V

= RT


=RT (In V2 – In V1)

= RTIn (V2/V1)

But (V2/V1) = (P1= P2)

Isometric process (dV = 0,

From first law of thermodynamics

dV= dQ- dW

dW= PdV= 0

dQ= dU= CvΔT


Isobaric process (dP= 0,)

dU= dQ= dW

dH= dU + d(PV) =dU + d(RT)

dH= dU+ RdT


Divide through by ‘ΔT’

[Cp = Cr + R)

Divide through by ‘Cr

= ɣ-1

That means

By substituting

CP= + = = R= Rr –R =

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