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Thermodynamics & Heat
Engines

Basic Concepts

Thermodynamics
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Thermodynamics= therme + dynamis
Latin word therme means = heat
Dynamis means = power or forces causing motion so, overall meaning of thermodynamics is heat–power or force interaction between system and surrounding. for example

It is based upon general observation and those may be formulated in form of thermodynamic law as –
Zeroth law of thermodynamics
First law of thermodynamics
Second law of thermodynamics

• Application areas of thermodynamics
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Steam power plant
I.C.Engine
Refrigerator and air conditioning
Gas turbine
Compressor etc.

Schematic of a Carnot refrigerator

• Microscopic and macroscopic view of thermodynamics
• Macro- Large scale
• Micro – small scale

Macroscopic
Attention is focused on certain quantity of matter without considering the activity occurred at molecular level
A few properties are required to describe the system such as P,V ,T etc and these can be perceived by senses and measured by available instruments. Example Expansion of gases in a I.C. engine
Requires Simple mathematical formulae to analyse the system

Microscopic
Matter consituting the system is considered to comprise a large no. of discrete particles called molecules.
Large no. of variables are required to describe the system such as position, KE,
Velocity, P,V, T etc. It is very difficult to measure these quantities with help of available instruments.Example KTG
Requires Advanced statistical and mathematical formulae to analyse the system Known as statistical thermodynamics

Known as Classical thermodynamics

SYSTEM- Definite region in space on which attention is concentrated for investigation of the thermodynamic problems i.e. heat, work transfer, etc. It may be classified on the basis of transfer of mass & energy as indicated in tableTYPES OF THERMODYNAMIC SYSTEM
System
Mass Energy Example
Closed System ×
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gas filled in a cylinder
Open System
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√ compressor, turbine, or nozzle gas filled in a cylinder but with
Isolated System ×
×
insulation

• Homogeneous System
• Quantity of matter is homogeneous throughout in chemical and physical structure i.e. system in a single phase

• Pure substance
• Substance that is homogeneous and invariable in chemical composition i.e. combustion product, atmospheric air

Thermodynamic Properties, Processes and Cycles
• Properties
• Characteristics by which physical condition of any system can easily be defined , is known as property.
• Two types• Intensive ( Independent of mass example pressure, temperature, density, composition, viscosity, thermal conductivity) • Extensive ( depends on mass examples- energy, enthalpy , entropy, volume etc.)
• Check for a propertydP= Mdx + Ndy would be a thermodynamic property if its differential is exact i.e.

• Specific quantity = Absolute / Mass and denoted by small letters.Applicable for quantities depending upon the mass like, internal energy, enthalpy, heat, work, volume etc.

• State
• If any system have definite values of properties , it is known as definite state . Properties are the state variables of any system

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Change in state
• Any change in property will lead to change in state.

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Path
• Locus of all change of states is known as path.

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Process
• When path is completely defined , it becomes one process
• Process may be reversible or irreversible in nature.
• Reversible: it is possible to attain the initial states by eliminating the effects. For example quasi static process ( reversible process)

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Cycle
• Final state of any process is identical with the initial state , it becomes one cycle.

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State, change in states, path, process, and cycles can be described on a diagram that is drawn between property vs property as shown

Quasi Static Vs Non Quasi Static
Quasi- Almost slow, or infinitely slow
Quasi static
1. Infinitely slowness is the characteristic of process and all the intermediate change in states are equilibrium with each other. 2. Path (1-2) of process can easily be

Non Quasi Static
1. Nature of process is very fast and there is no equilibrium with intermediate

be drawn on graph paper with firm line.
3. Processes are reversible in nature. It

change of states.
2. Path of process (1-2) can not be easily defined due to existence of non equilibrium change in states, hence can be drawn on graph paper with dotted line. 3. Processes are irreversible in natute.

means it is possible to attain the initial

It means it is not possible to attain the

states by eliminating the effect.
4. Example: Expansion of gases behind the pistion against infinitely small weigthts. initial states by eliminating the effect
4. Example: Expansion of gases behind the pistion against a single weigtht. defined due to all the change in states are in equilibrium , hence process can

Example : Compression process in piston –cylinder arrangement

Reversible & Irreversible process
Reversible Process

Irreversible Process

1. It is possible to attain the initial states 1. It is not possible to attain the initial after eliminating the effects introduced to states after eliminating the effects obtain the final state. introduced to obtain the final state.
Initial state will always be different in reverse process
2. All the quasi static processes are
2. All the non quasi static processes reversible in nature . are reversible in nature .
3.Process will become reversible by
3. Causes of irrversibility: (a) Internal eliminating the causes of irrversibility i.e. friction between molecules (b) Free resisted expansion of gases, no internal expansion of gases ( c) Paddle wheel molecular friction or external friction work- Braking action causing the conversion of mechanical work in form of heat., it is not possible to aobtain the motion of wheel by supplying the same
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∮ amount of heat to wheel.
4. Clausius inequality dQ/T=0 for cyclic 4. Clausius inequality dQ/T< 0 for process or no change in entropy for cyclic process or change in entropy for reversible process( ds =0) irreversible process( ds ≠0)

Thermodynamics Equilibrium
• No spontaneous change in macroscopic property (
i.e. isolated system)
• Conditions for thermodynamic equilibrium
• Mechanical equilibrium ( No pressure gradient within the system and also between system & surroundings
i.e.δΡ=0, or no unbalance force)
• Chemical equilibrium (No transfer of mass by any chemical process across the boundary of system i.e. diffusion and no unbalanced chemical reaction within the system)
• Thermal equilibrium ( No transfer of heat across the boundary of system when it is separated from universe by means of Diathermic wall- that allows the heat or δT=0) Concept of Continuum

• In concept of continuum matter within the system is assumed to be continuous and distributed uniformly.
• Importance- Used for defining the pressure and density

Pressure
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Definition : P = Normal Force / Cross sectional area
Units: Height of liquid ( 760 mm of Hg), N/m2, Pascal, Bar, Torr etc.
One atmospheric pressure= 1.01325 N/m2

Pascal’s Law
The pressure is the same at all points on a horizontal plane in a given fluid regardless of geometry, provided that the points are interconnected by the same fluid. (see figure below)

Measurement- Pressure
• U tube manometer- Used for measurement of pressure.
• For same liquid equation of pressure can be written very easily as: take +ve sign if it is desired to obtain the pressure at lower level as shown in diagram below.

Example

Solution:

Review Questions & Problems
• Book Engineering thermodynamics by P K Nag , (Ed.
Third )P. No. 15 Review questions section Q. No.1.1 ,
1.4 to 1.17
• Problems( P.No. 16, Q.No. 1.5, 1.6, 1.8, 1.9)
• Questions/ Problems given in assignment no. 1