THERMODYNAMIC SYSTEMS, PARAMETERS & POTENTIALS CLASS NOTES

THERMODYNAMIC SYSTEMS,

PARAMETERS & POTENTIALS

CLASS NOTES

A thermodynamic system is a

3-dimensional visible space in

the universe. It has one or more thermodynamic process (or

processes) of energy conversion

from one form to other. It is

mostly conversion of heat to

work in all power producing

devices. These devices are petrol,

diesel, steam engines, gas, steam

& water turbines. In a water

turbine, it is conversion of

potential energy into kinetic

energy. It includes conversion

of work to heat in all power

consuming devices. These are

refrigerating and air conditioning

machinery, compressors, pumps,

fans and blowers. Pumps and

blowers do not convert work

into heat. It is difficult to

understand thermodynamics

without learning the thermodynamic

potentials and thermodynamic

parameters.

 

SURROUNDINGS

3- dimensional visible space outside (external ) to the thermodynamic system is surroundings.

BOUNDARY

The separating border between the system and the surroundings is boundary. Boundary is the demarcation where system ends and surroundings starts.

UNIVERSE

  UNIVERSE = SYSTEM + BOUNDARY + SURROUNDINGS

All these are 3-dimensional spaces.

TYPES OF THERMODYNAMIC SYSTEM

Three types

(i)  Open System

(ii) Closed System

(iii) Isolated System

Open System

There is mass transfer & energy transfer across the boundary. Open Systems are
(i) Engine of any vehicle
(ii) Cooling tower
(iii) Desert cooler
(iv) Water heated in an open container
(v) only one component like compressor or condenser or evaporator.
(vi) Water pump

 Closed System

Only energy transfer
No mass transfer
Examples of Closed Systems
(i) A refrigerator as one unit
(ii) An air conditioner
(iii) Human body
(iv) Water heated in a closed vessel
(v) An electric motor
(vi) Mixer
(vii) Grinder

 Isolated System

No energy transfer and No mass transfer
There is no isolated system in actual practice. It is just a theoretical concept to make certain studies (equations) easier.

EXTENSIVE PROPERTIES

Which depend on mass.
Examples are

(i) Total volume

(ii) Total entropy

(iii) Total enthalpy

(iv) Total Internal energy

(v) Total humidity

INTENSIVE PROPERTIES

Which do not depend on mass.
Examples: Density, temperature, pressure

THERMODYNAMIC PARAMETERS AND POTENTIALS

 THERMODYNAMIC PARAMETERS

The parameters are extremely important in deep understanding of thermodynamics. It enables to apply the fundamentals in real situations & makes further improvements in a system. Followings is the list of various thermodynamic parameters.

  1. Principle of Maximum Entropy and that of Minimum Energy are equivalent.

  2. Laws of Thermodynamics

  3. Datum in Thermodynamics

  4. Thermodynamic Temperature

  5. Thermodynamic Scale

  6.  Properties Thermodynamic

  7. Transport Properties

  8. Temperature Entropy Chart

  9. Pressure Enthalpy Chart

  10. Availability, Unavailability And Ir-reversibility

 Thermodynamic potentials-Introduction

 Pierre Durham introduced the concept of thermodynamic potentials in 1886. Josiah Willard Gibbs called these as fundamental functions. Derive these potentials from  potential energy.  Potential energy is the capacity to do work. Thermodynamic potential is a scalar quantity. These are not measurable. It represents the thermodynamic state of a system. Thermodynamic potentials are useful in the thermodynamics of chemical reactions. These potentials describe the non-cyclic processes. There are four thermodynamic potentials. 

 Chemical reactions use thermodynamic potentials. They are four thermodynamic potentials. These are internal energy, the enthalpy, the Helmholtz free energy and the Gibbs free energy. All the thermodynamic potentials are energy terms. These potentials are not  measurable as temperature or pressure. Find these from the measurable variables like pressure, volume and temperature. These involve the terms +pV and –TS. Thermodynamic properties are the partial derivatives of the thermodynamic potentials.

 (∂U/∂T) at (V=C )=T

(∂F/∂P) at (T=C )=  -P

(∂G/∂V) at (T=C )=V

  Useful work done under constant temperature (T) and volume( V)=ΔF

  Useful work done at constant temperature (T) and constant pressure (P) conditions=ΔG

 

TABLE: THERMODYNAMIC POTENTIALS AND THEIR FORMULAS

Sr.

No.

Name

Symbol

Formula

1.
Internal Energy
U
Q—pV
2.
Enthalpy
H
U + pV
3.
Helmholtz free energy
F
U—TS
4.
Gibbs Free Energy
G
U + pV—TS
TABLE: THERMODYNAMIC POTENTIALS AND NATURAL VARIABLES
Sr. No.
Thermodynamic potential         
   Natural variables kept constant in the process
1.
U
S, V
2.
H
S,p
3.
G
T,p
4.
F
T, V
 
TABLE: THERMODYNAMIC POTENTIALS, FUNDAMENTAL EQUATION AND EQUILIBRIUM CONDITION
Sr. No.
Thermodynamic potential
Fundamental equation
Equilibrium condition
1.
S (U, V, N)
Entropy fundamental equation
Maximum
2.
U (S, V, N)
Internal Energy fundamental equation
Minimum
3.
H (S, P, N)
Enthalpy fundamental equation
Minimum
4.
F (T, V, N)
Helmholtz free energy fundamental equation
Minimum
5.
G (T, P, N)
Gibbs free energy fundamental equation
Minimum
N = Number of particles related to molecular weight
Obtain thermodynamic potentials from thermodynamic parameters. Their estimation is indirect. It is noticeable that thermodynamic potentials have multiple independent variables. There is a important feature of all the multiple variable functions.  Mixed partial derivatives are independent of the order.
2f/∂x ∂y = 2f/∂y ∂x are same
It is Euler’ Reciprocity Law of thermodynamics.

REFERENCES

  1. Chemical Thermodynamics, D.J.G. Ives, University Chemistry, MacDonald Technical and Scientific, 1971

  2. Elements of Statistical Thermodynamics (2nd Edition), L.K. Nash, Addison-Wesley, 1974

  3. Thermal Physics (2nd Edition), Kittel, Charles & Kroemer, Herbert (1980).

  4. Encyclopedia of Physics (2nd Edition, W. H. Freeman Company. McGraw Hill,”), C.B. Parker, 1994

  5. Thermodynamics – an Engineering Approach , Cengel, Yunus A., & Boles, Michael A, McGraw Hill, 2002

  6. Statistical Physics (2nd Edition), F. Mandl, Manchester Physics, John Wiley & Sons, 2008

  7. Thermodynamics, From Concepts to Applications (2nd Edition), A. Shavit, C. Gutfinger, CRC Press (Taylor and Francis Group, USA), 2009