Transcript Thermodynamics

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Thermodynamics
 Thermodynamics is the science of energy conversion
involving heat and other forms of energy, most
notably mechanical work. It studies and interrelates
the macroscopic variables (temperature, volume and
pressure).
 A thermodynamic system (a physical system) is a
precisely defined macroscopic region of the universe
that is studied.
Branches of Thermodynamics
 Classical thermodynamics is the description of the states of
thermodynamical systems at near-equilibrium, using
macroscopic, empirical properties directly measurable in the
laboratory. It is used to model exchanges of energy, work and
heat based on the laws of thermodynamics.
 Statistical mechanics (or statistical thermodynamics) gives
thermodynamics a molecular interpretation. This field relates
the microscopic properties of individual atoms and molecules
to the macroscopic or bulk properties of materials that can be
observed in everyday life, thereby explaining thermodynamics
as a natural result of statistics and mechanics (classical and
quantum) at the microscopic level.
 Chemical thermodynamics is the study of the interrelation of
energy with chemical reactions.
 Biological thermodynamics is the study of energy
transformation in the biological systems.
Equilibrium and non-equilibrium thermodynamics
 The word equilibrium implies a state of balance. In an
equilibrium state there are no unbalanced potentials, or
driving forces, within the system.
 Equilibrium thermodynamics is the systematic study
of transformations of matter and energy in systems as
they approach equilibrium.
 Most systems found in nature are not in
thermodynamic equilibrium because they are not in
stationary states, and are continuously and
discontinuously subject to flux of matter and energy to
and from other systems. Non-equilibrium
thermodynamics deals with such systems.
Thermodynamic system. Thermodynamic
parameters
A thermodynamic system (a physical system) is a
precisely defined macroscopic region of the universe that
is studied.
A thermodynamic system is characterized and defined by
a set of thermodynamic parameters
 An intensive property (parameter) is a physical
property of a system that does not depend on the system
size or the amount of material in the system
 extensive property (parameter) is one that is additive
for independent, noninteracting subsystems. It is directly
proportional to the amount of material in the system.
Thermodynamic system
 All space in the universe outside
the thermodynamic system is
known as the surroundings (the
environment, or a reservoir). A
system is separated from its
surroundings by a boundary
which may be notional or real,
but which by convention delimits
a finite volume.
 Systems are distinguished
depending on the kinds of
interaction they undergo and the
types of energy they exchange
with the surrounding
environment.
Thermodynamic systems
 Isolated systems are
completely isolated from
their environment. They
do not exchange heat,
work or matter with their
environment. An
example of an isolated
system is a completely
insulated rigid container,
such as a completely
insulated gas cylinder.
Thermodynamic systems
 Closed systems are able
to exchange energy (heat
and work) but not matter
with their environment.
A greenhouse is an
example of a closed
system exchanging heat
but not work with its
environment. Whether a
system exchanges heat,
work or both is usually
thought of as a property
of its boundary.
Thermodynamic systems
 Open systems may
exchange any form of
energy as well as matter
with their environment.
A boundary allowing
matter exchange is
called permeable. The
ocean would be an
example of an open
system.
Internal energy
 Internal energy is defined as the energy associated with the
random, disordered motion of molecules.
 It is separated in scale from the macroscopic ordered
energy associated with moving objects; it refers to the
invisible microscopic energy on the atomic and molecular
scale.
 The internal energy is the total energy contained in a
thermodynamic system. It is the energy necessary to
create the system, but excludes the energy associated
with a move as a whole, or due to external force fields.
Internal energy has two major components, kinetic
energy and potential energy.
 For an ideal monoatomic gas, this is just the translational
kinetic energy of the linear motion of the "hard sphere"
type atoms. However, for polyatomic gases there is
rotational and vibrational kinetic energy as well.
System Work
 When work is done by a
thermodynamic system, it is
ususlly a gas that is doing the work.
The work done by a gas at constant
pressure is:
A  pV
 For non-constant pressure, the
work can be visualized as the area
under the pressure-volume curve
which represents the process
taking place. The more general
expression for work done is:
A
V2
 pdV
V1
Heat Transfer
Heat is energy transferred from one body or
thermodynamic system to another due to thermal
contact when the systems are at different temperatures.
Heat Conduction
Conduction is heat transfer by means of
molecular agitation within a material without
any motion of the material as a whole. For
heat transfer between two plane surfaces,
such as heat loss through the wall of a house,
the rate of conduction heat transfer is:
Q kS (Thot  Tcold )

t
d
First law of thermodynamics
 The first law of thermodynamics is the application of
the conservation of energy principle to heat and
thermodynamic processes:
Q  U  A
Heat added to the thermodynamic system goes to
change the internal energy and to do the work by
the system.
First law of thermodynamics
The internal energy of a system can be changed by
heating the system or by doing work on it.
U  Q  A
 If the system is isolated, its internal energy cannot
change.
Entropy as a Measure of the Multiplicity of a
System
The probability of finding a system in a given state
depends upon the multiplicity of that state. That is to
say, it is proportional to the number of ways you can
produce that state. Here a "state" is defined by some
measurable property which would allow you to
distinguish it from other states. Entropy:
S  k ln W
where k is Boltzmann's constant, W is the number of
microstates The k is included as part of the historical
definition of entropy and gives the units J/K in the SI
system of units. The logarithm is used to make the
defined entropy of reasonable size. The multiplicity
for ordinary collections of matter are on the order of
Avogadro's number, so using the logarithm of the
multiplicity is convenient.
Entropy in Terms of Heat and Temperature
 A the change in entropy can be described as the heat
added per unit temperature
ΔS = Q/T
where S is the change in entropy,
Q is the heat flow into or out of a system, and T is the
absolute temperature in degrees Kelvin (K).
 This is often a sufficient definition of entropy if you don't
need to know about the microscopic details. It can be
integrated to calculate the change in entropy during a part
of an engine cycle.
 The concept of entropy (S) gives us a more quantitative
way to describe the tendency for energy to flow in a
particular direction.
Entropy
a state variable whose change is defined
Entropy: for a reversible process at T where Q is
the heat absorbed.
a measure of the amount of energy
Entropy:
which is unavailable to do work.
Entropy: a measure of the disorder of a system.
a measure of the multiplicity of a
Entropy:
system.
Enthalpy
 Enthalpy is a thermodynamic potential. It is a state
function since it is defined in terms of three other
precisely definable state variables, and it is an
extensive quantity. Enthalpy is a measure of the total
energy of a thermodynamic system. It includes the
internal energy U, which is the energy required to
create a system, and the amount of energy required to
make room for it by displacing its environment and
establishing its volume V and pressure P.
H = U + PV
 It is somewhat parallel to the first law of
thermodynamics for a constant pressure system
Q = ΔU + PΔV, since in this case Q=ΔH
The second law of thermodynamics
Clausius statement
 The second law of thermodynamics describes the flow of energy
in nature in processes which are irreversible.
 The second law of thermodynamics may be expressed in many
specific ways.
Second Law and Refrigerator
 It is not possible for heat to flow from a colder body to a
warmer body without any work having been done to
accomplish this flow. Energy will not flow spontaneously
from a low temperature object to a higher temperature
object.
 This precludes a perfect refrigerator.
 The statements about refrigerators apply to air conditioners and
heat pumps, which embody the same principles.
Kelvin-Planck statement
Second Law and Heat Engine
 It is impossible to extract an amount of heat from a
hot reservoir and use it all to do work. Some amount
of heat must be exhausted to a cold reservoir.
 It meams that the efficiency of a heat engine cycle is
never 100%.
 This precludes a perfect heat engine.
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