Transcript Chapter 1 - All Made Easy

Concepts of
Thermodynamics and
Properties of Gases
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
Chapter 1
Thermodynamics is a branch of science and engineering that
deals with interaction of energy mainly in the forms of heat
and work.
There are different forms of energy:
•
all the energy cannot be used as a work.
•
the convertibility of energy into work depends on its
availability.
Thermodynamics is studied in two forms:
•
Classical
•
Statistical
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
Introduction
•Statistical thermodynamics is concerned with the
microstructure of the matter and addresses
behaviour of individual molecules of the matter.
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
•Classical thermodynamics is concerned with the
macrostructure of matter and addresses the major
characteristics of large aggregations of molecules and
not the behaviour of individual molecules.
Thermodynamics: It is the field of thermal engineering
that studies the properties of systems that have a
temperature and involve the laws that
o
govern the conversion of energy from one form to
another.
o
the direction in which heat will flow.
o
the availability of energy to do work.
System: System is the fixed quantity of matter and/or the
region that can be separated from everything else by a
well-defined boundary/surface.
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
Important Terminologies
State: At any instant of time, the condition of a system is
called state. The state at a given instant of time is defined
by the properties of the system such as pressure, volume,
temperature, etc.
Property: It is any quantity whose numerical value depends
on the state but not on the history of the system. There are
two types of properties: extensive and intensive.
o
Extensive properties depend on the size or extent of the
system. Volume, mass, energy and entropy are the
examples of extensive properties.
o
Intensive properties are independent of the size or
extent of the system. Pressure and temperature are the
examples of intensive properties.
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
Important Terminologies (Cont…)
Change in State
o Thermodynamic
system
undergoes changes due to
flow of mass and energy.
The mode in which the
changes in the state of a
system takes place is known
as process such as isobaric
(constant pressure) process,
isochoric (constant volume)
process,
isothermal
(constant
temperature)
process, adiabatic (constant
entropy) process, etc.
o The path is loci of series of state changes from initial state
to final state during a process.
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
•
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
Process: Two states are identical if, and only if, the
properties of the two states are same. When any property of
a system changes in value there is a change in state and
the system is said to undergo a process.
Cycle: When a system from a given initial state goes into a
sequence of processes and finally returns to its initial state,
it is said to have undergone a cycle.
Phase: Phase refers to a quantity of matter that is
homogeneous throughout in its chemical composition and
physical structure.
o
A system can contain one or more phases.
o
A pure substance is one that is uniform and invariable
in chemical composition.
o
A pure substance can exist in more than one phase,
but its chemical composition must be the same in
each phase.
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
Equilibrium: In thermodynamics, the concept of equilibrium
includes not only a balance of forces but also a balance of other
influencing factors, such as thermal equilibrium, pressure
equilibrium, phase equilibrium, etc.
Zeroth law of thermodynamics is law of thermal equilibrium,
which states that if a system A is in thermal equilibrium with
systems B and C, then systems B and C will be in thermal
equilibrium as well.
Quasi-static Process: When a process proceeds in such a way
that the system remains infinitesimally close to an equilibrium
state at all times, it is called a quasi-static process.
o A quasi-static process can be understood as sufficiently
slow process that allows the system to adjust internally so
that properties in one part of the system do not change
faster than those at other parts.
°C = °K − 273.15
°R = 1.8°K
°F = °R − 459.67
°F = 1.8°C + 32
•
Internal Energy: The Internal Energy (U) of a system is
the total energy content of the system.
o It is the sum of the kinetic, potential, chemical,
electrical, and all other forms of energy possessed by
the atoms and molecules of the system.
o The Internal Energy (U) is path independent and
depends only on temperature for an ideal gas.
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
Temperature: Temperature is a property of a substance by
which it can be differentiated from other substance in terms
of degree of hot or cold.
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
Work: Work in thermodynamics may be defined as any
quantity of energy that flows across the boundary between
the system and surroundings which can be used to change
the height of a mass in the surroundings.
Heat: Heat is defined as the quantity of energy that flows
across the boundary between the system and surroundings
because of a temperature difference between system and
surroundings. The characteristics of heat are as follows:
o
Heat is transitory and appears during a change in state
of the system and surroundings. It is not a point
function.
o
The net effect of heat is to change the internal energy
of the system and surroundings in accordance to first
law.
o
If heat is transferred to the system, it is positive and if
it is transferred from the system it is negative.
Specific Heat at Constant Pressure (CP): The rate of
change of enthalpy with respect to absolute temperature
when pressure is constant is known as specific heat at
constant pressure (Cp).
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
Enthalpy: Enthalpy, h, of a substance is defined as h = u +
PV. It is intensive properties of a substance and measured
in terms of kJ/kg.
Specific Heat at Constant Volume (Cv): The rate of
change of internal energy with respect to absolute
temperature at constant volume is known as specific heat at
constant volume (Cv).
First Law of Thermodynamics: The first law of
thermodynamics is equivalent to law of conservation of
energy. It deals with the transformation of heat energy into
work and vice versa.
oWhen a small amount of work (dw) is supplied to a closed
system undergoing a cycle, the work supplied will be equal
to the heat transfer or heat produced (dQ) in the system.
o If Q amount of heat is given to a system undergoing a
change of state and W is work done by the system and
transferred during the process, the net energy (Q – W) will
be stored in the system named as internal energy or
simply energy of the system (∆U).
Q – W = ∆U
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
First Law of Thermodynamics
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
Sign Convention: The convention is adopted that Q
indicates the heat added to the system and W the work
done by it. Thus,
o
dQ > 0, heat added to system or system absorbs
heat.
o
dQ < 0, heat removed from system or system rejects
heat.
o
dW > 0, work is done by system.
o
dW < 0, work is done on the system.
o
∆U > 0, internal energy of system increases.
o
∆U < 0, internal energy of system decreases.
•Constant
•
•
Volume Process
Constant Pressure Process
Constant Temperature Process
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
Non-flow Processes
Adiabatic Process
• Polytropic Process
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
•
Steady Flow Process: In a steady flow process,
thermodynamic properties at any section remain constant
with respect to time; it can vary only with respect to space.
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
Application of First Law of Thermodynamics
in Steady Flow Process and Variable Flow
Process
•
Limitations of First Law of Thermodynamics
First law of thermodynamics does not tell about the
following:
o How much of the given quantity of heat is changed into
work?
o In which direction is changing take place (heat to work
or work to heat)?
o Under which condition the changing will take place?
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
Variable Flow Process: In some flow process, mass flow
rate is not steady but varies with respect to time. In such a
case, the difference in energy flow is stored in system as ∆Ev.
Kelvin–Planck
Statement:
It
is
impossible for any system to operate in a
thermodynamic cycle and deliver a net
amount of energy by work to its
surroundings while receiving energy by
heat transfer from a single thermal
reservoir.
Clausius Statement: It is impossible to
construct a device that operates in a
cycle and produces no effect other than
the transfer of heat from a lower
temperature body to higher temperature
body.
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
The Second Law of Thermodynamics
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
The Carnot Cycle
1–2 Reversible Isothermal
Expansion
2–3 Reversible Adiabatic
Expansion
3–4 Reversible Isothermal
Compression
4–1 Reversible Adiabatic
Compression
or
Entropy: Defining entropy in an exact word or line is
impossible. It can be viewed as a measure of molecular
disorder or molecular randomness. As a system becomes
more disordered, the positions of the molecules become
less predictable and the entropy increases. Thus, the
entropy of a substance is lowest in the solid phase and
highest in the gas phase.
• SGEN > 0 for an irreversible (real) process
• SGEN = 0 for a reversible (ideal) process
• SGEN < 0 for an impossible process
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
The Clausius Inequality
Third law of thermodynamics is law of entropy. It is a
statement about the ability to create an absolute
temperature scale, for which absolute zero is the point at
which the internal energy of a solid is zero. Third law of
thermodynamics states that it is impossible to reduce any
system to absolute zero in a finite series of operations.
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
Third Law of Thermodynamics
Boyle’s law is stated as ‘volume and pressure of a sample of
gas are inversely proportional to each other at constant
temperature’.
• Charle’s law can be stated as ‘volume of a sample of gas is
directly proportional to the absolute temperature when
pressure remains constant’.
• Gay–Lussac’s law states that the pressure of a sample of
gas is directly proportional to the absolute temperature
when volume remains constant’.
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
Gas Laws
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
•Combined
Gas Law
•Gas Constant
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Basic Mechanical Engineering, First Edition
by Dr Pravin Kumar
Copyright © 2013 Dorling Kindersley (India) Pvt. Ltd.
Thank You.