ENGINEERING THERMODYNAMICS
Dr. M.R.SWAMINATHAN
Assistant Professor
Internal Combustion Engineering Division
Department of Mechanical Engineering
ANNA UNIVERSITY
CHENNAI-25 .

PROPERTIES- contd.
CYCLE
A process (or a series of connected processes) with identical end states is called a cycle.
A cycle composed of two processes,
A and B.
all other thermodynamic properties must also change

O ther thermodynamic properties must also change so that the pressure is a function of volume as described by these two processes
2
P Process
B
1
Process
A
V

A system is said to be in thermodynamic equilibrium if it maintains
 Thermal (Uniform Temperature)
 Mechanical (Uniform Pressure)
 Phase equilibrium
 Chemical equilibrium .

20
°
C 30
°
C
32
°
C 32
°
C
30
°
C
32
°
C
35
°
C 40
°
C
No thermal equilibrium
32
°
C 32
°
C
Thermal equilibrium

Process proceeds in such a way that the system remains infinitesimally close to an equilibrium state at all times, it is called a quasistatic, or quasi-equilibrium process

Quasi- equilibrium process can be viewed as a sufficiently slow process that allows the system to adjust itself internally so that properties in one part of the system do not change any faster than those at other parts

P
State 2 Process path
Intermediate states
20
State 1
20 pa 20 pa
20 pa
20 pa
20 pa
(a) Slow compression (quasi-equilibrium)
V

Zeroth law was first formulated and labeled by R. H. Fowler in 1931
• Two bodies are in thermal equilibrium if both have the same temperature even if they are not in contact .

• Heat is defined as the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference.
• Heat is energy in transition.

• Heat is energy in transition. It is recognized only as it crosses the boundary of a system
• The potato contains energy, but this energy is heat transfer only as it passes through the skin of the potato (the system boundary) to reach the air, as shown in figure.

• Energy possessed by the molecules by the virtue of its temperature
• Higher the temperature higher is the internal energy possessed by the medium ( solid / liquid/ gas )
• Internal energy is zero at absolute zero

.
• Internal energy is the sum of all microscopic forms of energy of a system.
• Internal energy will be highest for gas phase and minimum for the solid phase

• There are many thermodynamic processes in practice.
• In each of the processes we normally allow one of the properties to remain a constant during a process .

• Isobaric (P=c)
• Isochoric (V=c)
• Isothermal (T=c)
• Polytropic (PV n =c)
• Adiabatic (PV
γ
=c)
• Isentropic (PV
γ
=c)

• Work, like heat, is an energy interaction between a system and its surroundings.
• Energy can cross the boundary of a closed system in the form of heat or work.

• Work is the energy transfer associated between a system & the surrounding in the absence of any temperature difference.
• An example is the work done in expansion of a piston assuming there is no temperature difference between the cylinder and surroundings

• Displacement work (pdV work)
• Force exerted, F= p. A
• Work done dW= F.dL= p. A dL= p.dV
• If the piston moves through a finite distance say 1-2,Then work done has to be evaluated by integrating dW= ∫pdV

• Open systems involves flow of mass in and out of the system unlike closed system
• Energy associated with pushing mass or volume of a gas in or out of the system is called FLOW
WORK or FLOW ENERGY

• FLOW WORK is always associated with open systems or a stream of liquid or gas which is in motion
• Flow work is given by p
1 v
1
- p
2 v
2 or simply as pv

• Stretching of a wire
• Electrical Energy
• Work of a reversible chemical cell
• Work in stretching of a liquid surface
• Work done on elastic solids
• Work of polarization and magnetization

• All our efforts are oriented towards how to convert heat to work or vice versa:
• Heat to work Thermal power plant
• Work to heat Refrigeration

• Heat and work are directional quantities.

• Work done by a system or work given by a system is considered + ve e.g work supplied by turbine
• Work done on the system or work supplied to system is considered –ve e.g
compressor or pump

• Heat supplied to the system is considered + ve e.g heat supplied to boil water in a boiler
• Heat rejected by a system is considered – ve e.g condenser of a thermal power plan t

• Both heat and work are path functions
• They depend only on the path of travel and not on end states or end points
• BOTH HEAT AND WORK ARE PATH
FUNCTIONS (INEXACT DIFFERENTIAL)

• Properties like pressure , temperature depend only on end state
• They are POINT FUNCTIONS & are known as EXACT DIFFERENTIALS.
• All Exact differentials are property of a system

• Heat and work are energy transfer mechanisms between a system and its surroundings.
• Systems possess energy, but not heat or work.
• Both are recognised at the boundaries of a system as they cross the boundaries.
• Both are path functions

Consider the following example
An electric oven heat by a heating element

• Consider the same example
• An electric oven heat by a heating element