11P12
Thermodynamics
Learning Objectives
Basic terminology of thermodynamics
Laws of thermodynamics
Heat Engine
11P12
Introduction
Heat
Thermodynamics is the branch of physics that deals with the concepts of heat and temperature and the interconversion of heat and other forms of energy.
Thermodynamics
Thermo
Dynamics
+
Force involved in conversion
Work done by our palms gets converted into heat.
Why ?
Work and Heat are interconvertible
Heat provided by steam is converted into work.
Why ?
11P12.1
Basic Terminology and Zeroth Law
Leaning Objectives
Basic Terminology
Temperature and Zeroth Law
11P12.1
CV 1
Basic Terminology
System :
A region in space in which our entire study is focused.
Note : System can be changed based on our priority.
System
4-Cylinder 4-Stroke Engine
System
Surroundings :
Everything other than system is known as surroundings .
System
Surroundings
Boundary :
It is a separating medium which distinguishes the system and surroundings .
System
Surroundings
Boundary
Features of Boundary :
Fixed
Movable
Real
Imaginary
For avoiding energy losses within the boundary.
Universe :
It is the combination of system and surroundings.
System
Surroundings
Boundary
Universe
Types of System
Closed System
Open System
Isolated System
Closed System :
i) Heat transfer takes place.
ii) No mass transfer takes place.
Eg :
Pressure Cooker (Without Whistling)
Open System :
i) Heat transfer takes place.
ii) Mass transfer takes place.
Eg :
Tea in a cup
Isolated System :
i) No heat transfer takes place.
ii) No mass transfer takes place.
Eg :
Thermoflask
Isolated system is a special case of closed system.
Properties of a System :
Properties are the characteristics which specify the system.
Types of Properties
Intensive Properties
Extensive Properties
Independent of quantity of the system.
Depends upon the quantity of the system.
Eg: Temperature, Pressure.
Eg: Mass, Volume, Energy.
i)They are distinguishable.
ii)They are observable.
Taking one more ice cube
Intensive Properties :
Temperature is independent of quantity, so it is an intensive property.
+
Extensive Properties :
Mass of the system depends upon the quantity, so it is an extensive property.
ConcepTest
Ready for Challenge
Q . Separate extensive and intensive properties.
Density, Pressure, Temperature, Viscosity, Volume, Specific
volume, Mass.
Pause the Video
(Time Duration : 2 Minutes)
Sol .
Extensive Properties : Volume, Mass.
Intensive Properties : Density, Pressure, Temperature,
Viscosity, Specific volume.
Macroscopic and Microscopic Approach :
Macroscopic approach deals with the study of bulk of particles but not the behaviour of individual particles. This branch is also known as Classical Thermodynamics .
The behaviour of individual constituent particles is studied in microscopic approach. It is also known as Statistical Thermodynamics
Microscopic
Macroscopic
Study of all the particles at once.
Study of single particles.
State 1
State 2
General meaning of State, Process and Path :
State 3
Path
Path
Path
Process
Process
Process
State of a System :
State is defined as the condition of a system described with the help of its properties like pressure, volume and temperature .
Thermodynamic properties
Thermodynamic State
Process :
It is the change of state from one equilibrium state to another.
Equilibrium States
Process
Types of Process :
I). Reversible Process :
It is defined as a process that can be reversed without leaving any trace on the surroundings.
It means both system and surroundings are returned to their initial states at the end of the reverse process.
Reversible Process
Backward and Forward paths are same.
Eg :
A
Irreversible Process :
B
It is a process that cannot return to their original conditions with same path, if reversed.
Backward and Forward paths are not same.
Irreversible Process
Eg :
Path :
It is the locus of all the intermediate equilibrium states of a system
during a process.
Processes :
Cycle :
A cycle is a collection of processes whose initial and final states are
the same.
Note : A minimum of two processes are required to from a cycle.
Processes :
State Function :
Those function which depends only upon the end states of a system known are as state function.
Eg: Energy.
Note : They are independent of the path taken by the system.
A
B
C
Path Function :
Those function which depends upon the path taken by the system during the process are known as path function .
Eg: Work, Heat.
A
B
C
11P12.1
CV 2
Temperature and Zeroth Law
Hotness and coldness are relative terms.
Temperature :
Temperature is a measure of the average kinetic energy of the particles in a system.
SI Unit : Kelvin (K)
Adiabatic Wall :
Diathermic Wall :
A wall which prevents thermal interaction is known as adiabatic wall and a system enclosed within an adiabatic wall is called thermally isolated.
If exchange of heat takes place between the system and surroundings through the boundary wall, the boundary is called diathermic wall.
Heat
Heat
Adiabatic
Wall
Diathermic
Wall
Two physical systems are in thermal equilibrium if there is no net flow of thermal energy between them when they are connected by a path permeable to heat.
Thermal Equilibrium :
A
B
Conducting wall
(Diathermic wall)
Insulation
Heat transfer stops.
Thermal equilibrium achieved.
ConcepTest
Ready for Challenge
B
Non conducting wall
(Adiabatic wall)
Q . If we use adiabatic wall in place of diathermic wall then is it
possible to achieve thermal equilibrium ?
Pause the Video
(Time Duration : 2 Minutes)
Sol .
Answer : No.
Explanation : If we use adiabatic wall then there will be no heat transfer between the systems and we know for achieving thermal equilibrium a permeable medium for heat transfer is required.
Zeroth Law of Thermodynamics :
The zeroth law of thermodynamics states that, “if two thermodynamic systems are each in thermal equilibrium with a third one, then they are in thermal equilibrium with each other.”
A
B
C
Adiabatic wall
Diathermic wall
After some time
Adiabatic wall
Diathermic wall
Zeroth law is a consequence of thermal equilibrium and allow us to conclude the temperature is a well defined physical quantity.
So on the basis of thermal equilibrium temperature can be defined as,
Temperature is that thermodynamic property whose restricted value defines the thermal equilibrium.
A
B
ConcepTest
Ready for Challenge
Pause the Video
(Time Duration : 2 Minutes)
Sol .
For Room B,
So, Heat will flow from room A to room B.
Summary :
Summary :
11P12.2
First Law of Thermodynamics
and
Thermodynamic Processes
Learning Objectives
Concept of Internal Energy, Heat and Work
First Law of Thermodynamics
Specific heat capacities and indicator diagram
Thermodynamic Processes
11P12.2
CV 1
Concept of Internal Energy, Heat and Work
Concept of Internal Energy (U) :
Internal energy is defined as the energy associated with the random, disordered motion of molecules.
All objects are made up of small particles and every particle has both kinetic and potential energy.
Internal energy can also be defined as the sum of the kinetic energies and potential energies of these small particles.
Unit : Joule (J)
Internal energy is a state function i.e. it does not depend upon the path followed by the process.
A
B
C
Internal energy is the energy in storage.
Internal energy depends upon temperature.
ConcepTest
Ready for a Challenge
Q. What is the change in internal energy for a process which
has final and initial states are exactly similar ?
Sol.
Pause the Video
(Time Duration : 2 Minutes)
A) Positive
B) Negative
C) Zero
D) Can’t say
Answer : C
Since internal energy is a state function, so change in internal energy for this process will be zero.
A
B
Explanation :
Heat (Q) :
Heat is energy in transfer to or from a thermodynamic system, by mechanisms other than thermodynamic work or transfer of matter.
High Temperature
Low Temperature
Heat Flow
When there is temperature difference or phase change then heat will flow from high temperature to low temperature.
Sign Convention :
(i) Heat supplied to the system is always taken as Positive (+)
(ii) Heat rejected by the system is always taken as Negative (-)
System
Heat supplied (+)
Heat rejected (-)
Note : Heat transfer is a path function i.e. if we change the path,
amount of heat transfer will change.
Latent Heat :
Types of Heat :
This heat is responsible for the change in phase of the system at constant temperature.
Sensible Heat :
It is the form of heat which is responsible for the changes in temperature of the system.
Sensible Heat
Sensible Heat
Latent Heat
Latent Heat
Thermodynamic Work (W) :
In thermodynamics, work performed by a system is the energy transferred by the system to its surroundings.
Work for Closed System (W) :
It is given by,
It is valid only when,
(i) System is a closed system.
(ii) Process must be reversible.
Work is a form of energy, but it is energy in transit.
Must cross the system boundary.
Sign Convention :
(ii) Work done by the system is always taken as Positive (+).
(i) Work done on the system is always taken as Negative (-).
System
Work done on the system i.e. Compression (-)
Work done by the system i.e. Expansion (+)
Note : Work transfer is a path function i.e. if we change the path
amount of work transfer will change.
Unit : Joule (J)
Note : Heat and Work both are energy in transit.
Energy which is noticeable when it crosses the system boundary.
ConcepTest
Ready for a Challenge
Q. In which of the following systems there will be zero
thermodynamic work done ?
Systems are represented by dotted lines
Sol.
Pause the Video
(Time Duration : 2 Minutes)
Motor
Generator
Motor
Generator
Motor
Generator
Answer : C
A)
B)
C)
Explanation : According to definition of thermodynamic work,
work is energy in transit i.e. it must cross the boundary
11P12.2
CV 2
First Law of Thermodynamics
First Law of Thermodynamics :
The law of conservation of energy states that the energy can be transformed from one form to another, but neither be created nor destroyed.
The first law of thermodynamics is based on the law of conservation of energy, adapted for thermodynamic systems.
Conservation and Conversion of Energy in different forms
First law of thermodynamics states that,
“The change in the internal energy ΔU of a closed system is equal to the amount of heat Q supplied to the system, minus the amount of work W done by the system on its surroundings.”
System
According to first law of thermodynamics,
From first law of thermodynamics,
Rearranging the above formula,
Limitations of First Law of Thermodynamics :
(i) It does not tell us about the direction of flow of energy.
(ii) It does not tell us about anything whether the process is
spontaneous or not.
11P12.2
PSV 1
Sol.
Applying 1st law of thermodynamics
According to the question
11P12.2
PSV 2
Sol.
A
B
For thermodynamic cycle,
Applying 1st law of Thermodynamics
11P12.2
CV 3
Specific heat capacities
and
Indicator Diagram
Heat Capacity(S) :
System
Specific heat capacity(s) :
Specific heat capacity of water :
Old unit of heat : Calorie.
Now we use unit joule for energy, so specific heat of water,
C depends upon the following factors
a) Nature of substances.
b) Temperature of substances.
c) Condition under which heat is supplied.
Molar specific heat capacity(C) :
Types of molar specific heat
Vol.
Mass
Volume
Temp.
Heat
By 1st law of thermodynamics
For constant volume process
For constant pressure process
By 1st law of thermodynamics
For same change in internal energy, larger amount of heat required in a constant pressure process as compared to constant volume process.
ConcepTest
Ready for a Challenge
Q. Can the specific heat of gas be infinite ?
Sol.
Pause the Video
(Time Duration : 2 Minutes)
While the substance is undergoing a phase transition, such as melting or boiling, its specific heat is technically infinite, because the heat goes into changing its state rather than raising its temperature.
Answer : Yes
Explanation :
Indicator diagram :
Indicator diagrams are used to assess the performance of each unit of engine.
A pressure–volume diagram is used to describe corresponding changes in volume and pressure in a system known as indicator diagram.
It is based on the indicator diagram that the overall performance of the engine is assessed.
We know
Work done for closed system using indicator diagram :
If we project the curve on the volume axis of P-V diagram
Using indicator diagram (P-V diagram),
11P12.2
PSV 3
Sol.
From 1st law of Thermodynamics
Q. Calculate the heat absorbed by the system in going through one cycle for the cyclic process shown in figure.
Now work done is equal to area under this curve
11P12.2
CV 4
Thermodynamic Processes
Quasi-static Process
A quasi-static process is a thermodynamic process that happens slowly enough for the system to remain in internal equilibrium.
Quasi
Static
Almost
At Rest
Very Slow Process
Equilibrium States
Stopper
Quasi-Static Process
Thermodynamic Processes
It is the change of state from one equilibrium state to another.
Isobaric process
Isochoric process
Isothermal process
Adiabatic process
Isobaric Process :
Isobaric process is a thermodynamic process in which the pressure stays constant.
Indicator diagram
Eg : Boiling of water on the stove.
Work done during isobaric process :
Heat transfer during isobaric process :
Isochoric Process :
Indicator diagram
It is that thermodynamic process during which the volume of the closed system undergoing such a process remains constant.
Eg : Cooking in pressure cooker
Work done during isochoric process :
Heat transfer during isobaric process :
Isothermal Process :
Indicator diagram
An isothermal process is a change of a system, in which the temperature remains constant.
Eg : Evaporation of water
Heat transfer by isothermal process :
Work done by isothermal process :
In an adiabatic process, energy is transferred to the surroundings only as work.
Adiabatic Process :
An adiabatic process occurs without transfer of heat or mass of substances between a thermodynamic system and its surroundings.
and
Adiabatic index
Indicator diagram :
Work done by adiabatic process:
ConcepTest
Ready for a Challenge
Q. Which process has larger slope on P-V diagram.
A) Adiabatic Process B) Isothermal Process
Sol.
Pause the Video
(Time Duration : 1 Minutes)
Answer : A
Explanation :
For isothermal Process
For Adiabatic Process
Differentiating above equation
Differentiating above equation
Adiabatic Process
Isothermal Process
Comparison of both isothermal and adiabatic curve :
Slope of adiabatic curve is always greater than the slope of isothermal curve
11P12.2
PSV 4
Sol.
Isobaric Process
Highest value of internal energy occurs at the end of constant volume process.
So,
Summary :
Summary :
Reference Questions
NCERT – 12.1, 12.2, 12.3, 12.4, 12.5, 12.6, 12.9
Workbook – 1, 4, 6, 7, 8, 9, 10, 12, 13, 15, 16, 17, 19
11P12.3
Heat Engine, Reverse Heat Engine
and
Second Law of Thermodynamics
Learning Objectives
Carnot engine
Second law of thermodynamics and Engines
Reverse heat Engines or refrigerator and heat pump
11P12.3
CV 1
Engines and their Performance
and
Second Law of Thermodynamics
Engine
An engine is a machine designed to convert one form of energy into mechanical energy.
Heat engines, like the internal combustion engine, burn a fuel to create heat which is then used to do work.
Heat Engine
External combustion engine
Internal combustion engine
Combustion of fuel takes place outside the engine.
Combustion of fuel takes place inside the engine.
Eg: Steam Engine
Eg: Petrol Engine
External combustion engine :
The fluid then, by expanding and acting on the mechanism of the engine, produces motion and usable work.
An external combustion engine is a heat engine where a working fluid, contained internally, is heated by combustion in an external source, through the engine wall or a heat exchanger.
Internal combustion engine :
An engine which generates motive power by the burning of petrol, oil, or other fuel with air inside the engine, the hot gases produced being used to drive a piston or do other work as they expand
Component of Heat Engine :
Thermal Reservoir
A thermal reservoir is the part of environment which can exchange heat energy with the system. Its temperature is not affected by the quantity of heat supplied to or from it.
Eg. Boiler
Eg. Atmospheric air
Heat Source
Heat Sink
Reservoir at high temperature
Reservoir at low temperature
A working fluid is a gas or liquid that primarily transfers force, motion, or mechanical energy.
Working Fluid
Eg.(i) Mixture of air and petrol used in small load engines.
Heat Engine
It does this by bringing a working substance from a higher state temperature to a lower state temperature.
In thermodynamics, a heat engine is a system that converts heat or thermal energy and chemical energy to mechanical energy, which can then be used to do mechanical work.
H.E
Heat Engine
Intake of Fuel
Working Fluid
Spark Plug
Continuous intake of air fuel mixture and exhaust of burnt gases produces a cycle
Heat Engine operates in a cycle
H.E
A heat engine absorbs heat energy from the high temperature heat
source, converting part of it to useful work and delivering the rest to the cold temperature heat sink.
The efficiency of a heat engine is defined as ratio of work output to the heat input.
Heat Engine
H.E
From first law of thermodynamics
Second Law of Thermodynamics :
The Kelvin–Planck statement states, “it is impossible to construct a cyclically operating heat engine, the effect of which is to absorb energy in the form of heat from a single thermal reservoir and to deliver an equivalent amount of work.”
H.E
H.E
With single reservoir a cycle can’t be produced.
Heat Engine
Heat Engine
H.P
Clausius statement states, “It is not possible to construct a device that operates in a cycle and whose sole effect is to transfer heat from a colder body to a hotter body.”
If no external work done then process not possible.
H.P
Heat
Pump
Heat
Pump
ConcepTest
Ready for a Challenge
Sol.
Pause the Video
(Time Duration : 2 Minutes)
Answer : No.
Explanation :
According to Kelvin–Planck statement it is impossible to construct a cyclically operating heat engine, the effect of which is to absorb energy in the form of heat from a single thermal reservoir and to deliver an equivalent amount of work.
Which is violation of Kelvin-Planck statement because an engine can’t be made for working in a cycle with single thermal reservoir.
11P12.3
PSV 1
Sol.
We know,
H.E
And
11P12.3
CV 2
Reverse Heat Engine
(Refrigerator and Heat Pump)
It is a device that transfers energy from an object at a lower temperature to an object at a higher temperature by doing work on the system.
Reverse Heat Engine
Refrigerator
Heat Pump
Device which is used to cool down a space.
Device which is used to keep a space warm.
Reverse Heat Engine
Front side
Side of AC which is cooling the room.
Rear side
Side of AC which is heating the atmosphere.
If we reverse the heat engine than it can behave as both Refrigerator as well as Heat pump.
Working as Refrigerator
Working as Heat Pump
Refrigerator :
It is a mechanical device which is used to maintain the temperature
of a storage space lower than that of the surroundings.
H.P
Desired effect of refrigerator.
Coefficient of performance of refrigerator :
R
Removal of heat from storage space at lower temperature
From 1st law of thermodynamics
For ideal reversible refrigerator
11P12.3
PSV 2
Sol.
We know,
R
Heat Pump :
A heat pump is a mechanical device which helps in keeping the temperature of a system greater than that of the surroundings
H.P
Desired effect of heat pump.
Coefficient of performance of heat pump :
R
Addition of heat in the system at higher temperature
From 1st law of thermodynamics
For ideal reversible refrigerator
ConcepTest
Ready for a Challenge
Sol.
Pause the Video
(Time Duration : 2 Minutes)
We know
and
11P12.3
PSV 3
Sol.
We know,
H.P
Heat Pump
11P12.3
CV 3
Carnot Engine
It gives the estimate of the maximum possible efficiency of an engine.
Carnot Engine
A reversible heat engine operating between
two temperatures is called a Carnot engine.
Carnot Cycle :
We have taken ideal gas as an working substance.
Work done by the process is given by,
Here no heat transfer takes place and work is done by the system.
Carnot Cycle :
We have taken ideal gas as an working substance.
Carnot Cycle :
We have taken ideal gas as an working substance.
Here, the work done on the gas by the environment is given by:
Carnot Cycle :
We have taken ideal gas as an working substance.
Carnot Cycle :
We have taken ideal gas as an working substance.
ConcepTest
Ready for a Challenge
Sol.
Pause the Video
(Time Duration : 2 Minutes)
Note that W is positive since the work is done by the gas.
We know that work done by the gas in an isothermal expansion
Carnot Cycle :
We have taken ideal gas as an working substance.
Carnot Cycle :
We have taken ideal gas as an working substance.
Carnot Cycle :
We have taken ideal gas as an working substance.
We see Carnot efficiency depends only upon temperatures only not on the working substances.
Carnot Cycle :
We have taken ideal gas as an working substance.
Net work during Carnot cycle
Efficiency of Carnot cycle
Isothermal Expansion
Adiabatic Expansion
Adiabatic Compression
Isothermal
Compression
Carnot Theorem :
Heat Engine
Proof of Carnot Theorem :
H.E
We have to prove
We know
and
Proof of Carnot Theorem :
Now reversing the direction of reversible heat engine
H.E
Since we have assumed that
Proof of Carnot Theorem :
So Here we are getting continuous work with single thermal energy reservoir which is violation of Kelvin-Planck’s statement.
i.e. the reverse will be true.
This is the proof of Carnot’s Theorem.
H.E
Difficulties in Constructing A Carnot Engine
(ii) The Carnot cycle has alternating isothermal and adiabatic processes.
There are 2 reasons why it's not possible.
(i) The Carnot cycle is an ideal cycle. With ideal processes there is no waste of energy as heat and friction & thermal efficiency is 100% which obviously is never going to happen.
Isothermal is a very slow process.
Adiabatic is a very fast process.
Isothermal Process :
Adiabatic Process :
So it is very difficult to combine two different speed processes in one cycle.
11P12.3
PSV 4
Sol.
H.E
We know Carnot efficiency
Summary :
Summary :
Any engine operating between the same temperature limit will not have more efficiency than Carnot engine. This is known as Carnot Theorem.
Reference Questions
NCERT : 12.7, 12.8, 12.10.
Workbook : 3, 5, 11, 14, 18, 20.