Thermodynamics
What is Thermodynamics?
The field of science which deals with the energies possessed by gases and vapors is
known as Thermodynamics. It also comprises the conversion of these energies in terms
of heat and mechanical work and their relationship with properties of the system.
It is the science that deals with the relation between the properties of substances and
quantities of work and heat which cause a change of state.
Thermodynamics is based upon common observations of life which have been formulated into
thermodynamic laws. These laws govern the principle of energy conversion.
Applications of thermodynamics:
Steam Power Plants, Nuclear Power Plants
Refrigeration, Air conditioning
gas Turbine, Boiler
IC engines
chemical process plants etc.
Some Fundamental Terminology relating to thermodynamics
Thermodynamic System:
It is defined as a quantity of matter or a region in a space or a control volume upon which
we concentrate to analysis of a problem.
quantity of matter or a region in a space chosen for study.
Surroundings:
Everything external to the system is called the surroundings.
Boundary:
Real / Imaginary surface that separates the SYSTEM from SURROUNDINGS. The system is
separated from the surroundings by system boundary. The boundary may be moving or fixed.
Fixed/Movable
Shared by both system and surroundings
No thickness, No volume
Classification of thermodynamic system:
Open System: It is one in which mass and energy transfer across the system boundary. Most of
the engineering devices are open system. Boundary of open system is known as control surface.
And it is also known as control volume.
Example: Water Heater, Car Radiator, Turbine, Compressor etc.
An air compressor in which air enters at low pressure and leaves at high pressure and there is
also energy transfers across the system boundary.
Closed system: The system which permits only energy transfer and does not give any permeation
for mass transfer is called closed system and also known as control mass.
Example: A certain quantity of fluid in a cylinder bounded by a piston constitutes a closed
system.
cylinder contained gas
Isolated system: The system in which no mass and no energy transfer are occurred across the
system boundary (i.e No interaction between system and surroundings dQ=dW=0).
Example: An Insulated rigid container such as an insulated gas cylinder.
An Insulated gas cylinder
Control Volume: During analysis, attention is focused on a certain volume spaces surrounding
the system is called control volume and bounded by a surface called control surface.
Properties: They are defined as any characteristics by which a system can be defined.
Example: Pressure (P), Volume (V), Temperature (T) and mass (m),density etc.
Intensive property: Independent of the mass of the system. e.g pressure, temperature.
Extensive property: Dependent on mass of the system. e.g volume, enthalpy, internal energy.
When extensive properties are expressed as per unit mass, is called specific property. e.g. Sp.
Vol (v=V/m), Sp. Enthalpy (h=H/m), etc.
Why this law(zeroth law) has been named so?
Answer: Temperature is the base or concern of the 1st and 2nd law. Zeroth law provides the basis
for temperature measurement. But its significance is realized after half a century after formation
of First and Second Laws of Thermodynamics. Hence in order to maintain the sequence of the
law, it is named as Zeroth Law of Thermodynamics.
Quasi static process/ Quasi equilibrium process: Quasi means almost or nearly. A process can
be defined as quasi static process if it is nearly equilibrium.
The characteristics of quasi static process are stated below
1. Intermediate states are nearly equilibrium.
2. Process is reversible.
3. Process should be slow or happens infinitely slowly. (slow process)
Almost all the thermodynamic processes are quasi-static.
P2,V2,T2
P1,V1,T1
At the primary stage, the state is at 1.If we extract weight slowly , the pressure will decrease and
volume will increase. Again the removed weight is returned, the sate 1 will be obtained
reversibly. Now another weight is removed and state 2 is obtained. So we can say that process 12 is quasi-static.
Non-Flow process:
The process happening in closed system which do not permit the transfer of mass across their
boundaries, are known as Non-Flow process. It may be noted that in a non-flow process, the
energy transfer crosses the system in the form of heat and work but no mass flow in or out of
the system.
In non-flow process, the boundary can move.
Example:
Constant pressure process (Isobaric process i.e p=c)
Constant volume process (Isochoric process i.e V=c)
Adiabatic process ( Q=0 i.e 𝑝𝑉 𝛾 = 𝑐)
Constant Temperature process(Isothermal process i.e pV=C or T=c)
Polytropic process (𝑝𝑉 𝑛 = 𝑐)
Constant pressure process (Isobaric process i.e p=c):
𝑉2
𝑊1−2 = ∫ 𝑝𝑑𝑉 = 𝑝( 𝑉2 − 𝑉1 )
𝑉1
Constant volume process (Isochoric process i.e v=c)
𝑉2
𝑊1−2 = ∫ 𝑝𝑑𝑉 = 𝑝( 𝑉2 − 𝑉1 ) = 0
𝑖. 𝑒 𝑉1 𝑎𝑛𝑑 𝑉2 𝑎𝑟𝑒 𝑠𝑎𝑚𝑒.
𝑉1
Constant Temperature process (Isothermal process i.e pV=C or T=c):
Here,
𝑝𝑉 = 𝑐 and
𝑉2
𝑊1−2 = ∫ 𝑝𝑑𝑉
𝑉1
So we can write,
𝑝1 𝑉1 = 𝑝𝑉 = 𝑐
=> 𝑝 =
𝑝1 𝑉1
𝑉
Now,
𝑉2
𝑉2
𝑊1−2 = ∫ 𝑝𝑑𝑉 = 𝑝1 𝑉1 ∫
𝑉1
𝑉1
𝑑𝑉
𝑉2
𝑝1
= 𝑝1 𝑉1 𝑙𝑛 = 𝑝1 𝑉1 𝑙𝑛
𝑉
𝑉1
𝑝2
𝑖. 𝑒 𝑝1 𝑉1 = 𝑝2 𝑉2 =>
Polytropic process (𝒑𝑽𝒏 = 𝒄):
𝑝1 𝑉2
=
𝑝2 𝑉1
Flow process
The process happening in open system which permits the transfer of mass to and from the
system, are known as flow process. It may be noted that in flow process, the mass enters the
system and leaves the system after exchanging energy. The flow process may be
steady flow process (Turbine, Boiler, Compressor throttling devices etc)
non steady flow process.