LESSON - 1 THERMODYNAMIC, THERMODYNAMIC SYSTEMS AND SURROUNDINGS, DIFFERENT APPROACHES IN THE STUDY OF THERMODYNAMICS AND DEFINITIONS OF SOME MEASURABLE PARAMETERS

INTRODUCTION

The word “thermo-dynamic,” originates from the Greek words Therme (heat) and Dynamics (power). 

1.1.  THERMODYNAMICS

Thermodynamics is the division of science that deals with HEAT and WORK interactions and those properties of matter that bear a relation to heat and work.  OR

 It deals with energy interactions between material systems. OR

 It is a science that deals with energy transactions in the form of work & heat in a given system.

1.1.1.  Laws of Thermodynamics:

The science of thermodynamics is based on the following four Laws of Thermodynamics:

  • The Zeroth law of thermodynamics deals with thermal equilibrium between systems and provides a means of measuring temperature.

  • The First law of thermodynamics deals with the conservation of energy and introduces the concept of internal energy.

  • The Second law of thermodynamics tells the limits on the conversion of heat into work and provides the yard stick to measure the performance of various processes involving work and heat interactions. It also tells whether a particular process is feasible or not and specifies the direction in which a process will proceed. As a consequence it also introduces the concept of entropy.

  • The Third law defines the absolute zero of entropy.

The above four laws of thermodynamics were postulated based on experimental observations and have no mathematical proof.

1.1.2.  Why should we study thermodynamics?

The use of thermodynamics is very widespread. Its principles are used in

  • analyzing and designing energy converting devices to meet human needs.

  • improving designs of existing energy converting devices for their better output.

1.1.3. Some application areas of thermodynamics areas below:

1.1.3.1  Thermal power plant: Fig. 1.1 shows energy transactions in the form of work & heat from different components of a thermal power plant which gives out net mechanical/electrical  energy as output from fuel as input.

 

 

Fig. 1.1. Thermal power plant

 

  • Boiler/Steam Generator: Water is converted into high pressure steam by adding heat using a nuclear reactor or by products of combustion of fuel oil/coal.

  • Turbine: High pressure steam from boiler/steam generator expands to produce useful mechanical work which in turn runs an electric generator giving electricity as output.

  • Condenser: Low pressure steam exhausted from turbine is completely condensed into water by rejecting heat into cooling water and then to atmosphere through cooling towers.

  • Pump: Water from condenser is pumped back into boiler/steam generator by doing work on pump.

1.1.3.2.   Refrigerator:  Fig. 1.2 shows energy transactions in the form of work & heat from different components of a refrigerator, giving cooling effect as output by absorbing work as input.

Fig. 1.2. Refrigerator

 

  • Evaporator: Heat from the space to be cooled is absorbed by the low temperature, low pressure liquid refrigerant, converting itself into vapor.

  • Compressor: Low temperature, low pressure vapor refrigerant from evaporator is compressed into high temperature, high pressure vapor refrigerant by work input on compressor.

  • Condenser: High pressure, high temperature refrigerant vapor rejects heat in the condenser, thus converting itself into high pressure liquid.

  • Expansion valve or capillary tube: High pressure liquid refrigerant from condenser is reduced to low pressure liquid refrigerant without any work input/output.

1.2. THERMODYNAMIC SYSTEM AND SURROUNDINGS

Thermodynamic System: Whatever we want to study or a region in space or quantity of matter upon which attention is focused for the study.

Surroundings: Everything external to the system that is affected by the changes taking place in the system.

System boundary: The boundary which separates the system from the surrounding. The system boundary may either be fixed or movable, real or imaginary. System boundary is represented by dotted line.

Universe: System and surroundings together makes the universe.

1.2.1.  Some examples of system and surroundings are given below:

1.2.1.1.  Room as a System: Fig. 1.3,

  • The fluid, generally air inside the room represents the system.

  • The walls, floor and roof of this room represent the system boundary.

  • The air conditioner and atmospheric air represent the surroundings.

  • The room, air conditioner and outside air of room together represent the universe.

  • Energy in the form of heat is flowing outside the system into the surrounding.

  • The work from the surrounding on the system is zero. Because air conditioner is not a part of the system, the work input on the air conditioner is work within the surrounding.

 

 

                                 Fig. 1.3. Room as a system

1.2.1.2.   Room and Air-conditioner as a system:  Fig. 1.4,

  • As compared to the previous example, the room air conditioner is also part of the system in addition to inside space of the room.

  • The walls of room and air conditioner represent the system boundary.

  • The atmosphere air represents the surroundings.

  • The room, air conditioner and outside air of room together represent the universe.

  • The heat is flowing outside the system into the surrounding.

  • Thework input on air conditioner represents the work from surrounding on the system.

  

Fig. 1.4. Room and air conditioner as a system

1.2.1.3.   Locomotive engine of a train as a system: Refer Fig. 1.5,

  • Only Locomotive engine of a train represents the system.

  • Body of engine represents the system boundary.

  • The compartments C1, C2, C3 and C4 of the train and atmosphere air represent the surroundings.

  • The whole train and atmosphere air represent the universe.

  • Energy in the form of heat is flowing outside the system into the surrounding.

  • Energy in the form of work is done by the system on the surrounding.


Fig. 1.5. Locomotive engine of a train as a system

1.2.1.4.  Whole train as a system: Refer Fig. 1.6,

  • Whole train represents the system.

  • Body of whole train represents the system boundary.

  • The atmosphere air represents the surroundings.

  • The whole train and atmosphere air represent the universe.

  • Energy in the form of heat is flowing outside the system into surrounding.

  • Energy in the form of work is done within the system but it is not done by the system on the surrounding.

 

Fig. 1.6. Locomotive engine and its compartments i.e. whole train as a system

1.2.2.  Classification of thermodynamic system

Thermodynamic system can further be classified as

  • closed system,

  • open system and

  • isolated system

 1.2.1.1.  Closed System (Control mass)

Closed System (Control mass) is one where matter does not cross the boundary of the system i.e. the quantity and number of molecules of matter within the boundary of the system is fixed. The boundaries of this system are determined by the space the matter occupies.

The closed system is shown by a hydraulic press having piston cylinder arrangement in Fig. 1.7.

  Characteristics:

  1. Boundaries can move.

  2. Energy in the form of heat andwork may cross the system boundaries.

  3. Matter (gas) is fixed.


 

Fig. 1.7. Hydraulic press: a closed system with moving boundary

1.2.2.2.  Open System (Control volume)

Open System (Control volume) is one where matter crosses the boundary of the system and the quantity and number of molecules of the matter within the boundary of the system is not fixed. The air compressor with one inlet and one exit, shown in Fig. 1.8, is an example of open system. The boundaries of control volume are known as control surface.

Characteristics:

  1. Boundary is usually fixed but may move in certain cases.

  2. Energy in the form of work and heat may cross the system boundaries.

  3. Matter (gas) is not fixed.

  

 

Fig. 1.8. Air compressor: an open system (control volume)

1.2.2.3. Isolated System

Isolated System is one which is completely uninfluenced by the surroundings i.e. there is no transfer of mass and energy (heat and work) from and into the system. The thermos bottle shown in Fig. 1.9 is an example of isolated system.

Characteristics:

  1. Boundary is fixed

  2. Energy (work and heat) do not cross the system boundaries.

  3. Matter (gas) is fixed.

 

Fig. 1.9: Thermos bottle: an isolated  system

 1.3.  DIFFERENT APPROACHES IN THE STUDY OF THERMODYNAMICS

Thermodynamics can be studied through two different approaches:

(a) Macroscopic Approach   and (b) Microscopic Approach

1.3.1.  Microscopic Approach

When the molecules of matter in the system are studied for their position, velocity and energy individually and then the behavior of each molecule is summed up to know the overall behavior of the system, then this type of study is called microscopic approach.

This type of approach is adopted when

  • as a result of collision, the position, velocity and energy of the molecules of matter in the system change very frequently.

  • the matter is not uniformly distributed throughout the system as in the case of high vacuum.

Analysis in such approach is carried out under the subject of Statistical Thermodynamics.

1.3.2.  Macroscopic Approach

On the other hand, when the collective effect of action of many molecules are studied to know the effect of all the molecules of the system as a whole, so that the overall behavior of the system can be studied, then this type of study is called macroscopic approach.

The collective effect of action of many molecules can be perceived by human senses and measured by instruments.

For example, the pressure exerted by a gas is one such useful and relevant measurable parameter. Here we are not concerned with the actions of individual molecules but with force on a given area measured by a pressure gauge, even pressure results from the change in momentum of the molecules. In addition to pressure, the temperature, density etc. are the other useful and relevant measurable parameters which are used in this approach to study the behavior of the system.

The Macroscopic approach is adopted when

  • as a result of collision, the position, velocity and energy of the molecules of matter in the system does not change very frequently.

  • the matter is continuous i.e. the matter is seen as being distributed through space and not localized.

       Such analysis is carried out under the subject of Classical Thermodynamics.

1.4.   DEFINITIONS OF SOME MEASURABLE PARAMETERS

1.4.1.  Density,  ρ (kg/m3)

The density of a gas in a system is the ratio of total mass of matter in the system to the total volume occupied by the same mass. 

 1.4.2.  Specific volume, v (m3/kg)

The specific volume of a gas in a system is the volume occupied by matter per unit mass of it.  It is the reciprocal of the density.

 1.4.3  Pressure, p (N/m2)

It is the force acting normal to unit area.

  • Pressure in excess of atmospheric pressure is called (+ve) gauge pressure. This is the pressure measured by pressure gauge in actual practice.

  • Deficit of pressure from the atmospheric pressure is called vacuum or (-ve) gauge pressure.

So gauge pressure and vacuum can be given by the following equations:

 PAbsolute more than atmospheric pressure = PAtmospheric + PGauge

 PAbsolute less than atmospheric pressure   = PAtmospheric - PVacuum

 Graphically it is shown in Fig. 1.10.

 Fig. 1.10: Pressure relations

hips

1.5. UNIT CONVENTIONS

The fundamental quantities and their SI units are as follows:

     Fundamental quantities

 

SI units

Mass

:

kg (kilogram)

Length

:

m (meter)

Time

:

s (second)

The derived quantities and their SI units are as follows:

 Derived quantities

 

SI units

Conversions

Force

:

N (Newton)

 

 

Velocity

:

m/s

 

Acceleration

:

m/s2

 

Pressure

:

N/m2

1 N/m2 = 1 Pascal (Pa)

1 atmospheric pressure = 76 cm of Hg = 1.01325 bar

1 bar = 105 N/m2

1 kgf/cm2  = 1 × 9.8 × 104  N/m2  ≈ 1 bar

 

Work/Heat

:

J or kJ (Joule or kilo Joule)

1 kJ = 1000 J

 

Power

:

W (Watt)

1 W = 1 J/s
Last modified: Tuesday, 18 March 2014, 5:10 AM