o What is Thermodynamics?

bankercordMechanics

Oct 27, 2013 (3 years and 9 months ago)

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o What is Thermodynamics?

"Thermodynamics is defined as the study of energy, its forms
and transformations, and the interactions of energy with
matter." [1, p.5]

Energy can exist in a number of forms, electrical energy,
chemical energy, potential energy,
kinetic energy, PV energy,
mechanical energy, etc. In order to apply the laws of
thermodynamics mathematically, which is the only way to
"prove" anything, you must have a definition of energy that is
consistent with the laws of Thermo. The Laws of Thermo
d
escribe the Laws by which transformations in energy must
abide. They have never been shown false, and they have been
demonstrated so thoroughly, that they are not considered
theories, but laws. The field of engineering is based largely on
these laws, and i
n most fields of engineering*, proposed
processes must first be shown to satisfy these laws to merit
furthur consideration. In chemical engineering, they are
necessary criteria for chemical reaction equilibria.

* At Auburn University, all departments in th
e Engineering
School except Computer Science require their students to take
EGR 201
-

Introductory Thermodynamics. Applications of the
Laws of Thermodynamics can be demonstrated in every field of
engineering represented, including Agricultural Engineering.

W : Work
-

Energy transfer due to a force acting against a
resistance. In most general sense, work can be described as
change.

Q : Heat Transfer
-

one of three different types of energy
transfer:

1) Conduction

2) Convection

3) Radiation

Enthalpy is a conv
enient property defined in terms of internal
energy, pressure, and volume:


H = U + PV



H : enthalpy


U : internal energy


P : pressure


V : volume

Gibbs energy is a measure of the amount of energy available
to do work (ie. to effect a change)
in chemical processes. To
determine Gibbs energy we take the enthalpy, and subtract
out the disordered energy, ie. the energy that is not available
to do work:


G = H
-

TS



G : Gibbs free energy


S : entropy


T : temperature

So, what is entropy?

Entropy is a measure of the disorder of the energy of a
system. Ordered energy is available to do work, disordered
energy is not. So, mathematically we see that
entropy*temperature is the amount of disordered energy at
that temperature.

One criterion for c
hemical equilibruim in a closed system is
that the total Gibbs free enery must be at a minimum, which
means that entropy must be at a maximum.

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o What are the First and Second Laws of Therm
odynamics?

The first law is generally stated in terms of a closed system,
also called a control mass. So an auxiliary law is the
conservation of mass:

THE LAW OF THE CONSERVATION OF MASS

"The mass of a control mass never changes." [1, p.120]

THE FIRST LAW
OF THERMODYNAMICS

For a Closed system (control mass)

"A change of the total energy (kinetic, potential, and internal)
is equal to the work done on the control mass plus the heat
transfer to the control mass." [1, p.121]

"Although energy assumes many forms,

the total quantity of
energy is constant, and when energy dissapears in one form, it
appears simultaneously in other forms." [2, p.22]

E2
-

E1 = 1Q2 + 1W2

E2 : energy of the system at state 1

E1 : energy of the system at state 2


1Q2: the net heat trans
fer into the system in going from


state 1 to state 2



1W2: the net work done on the system in going from state 1


to state 2

[1, p.121]

"Equililibrium is a word denoting a static condition, the
absence of change. In thermodynamics it is taken to me
an not
only the absence of change, but the absence of any tendency
toward change on a macroscopic scale. Thus a system at
equilibrium is one which exists under such conditions that
there is no tendency for a change in state to occur." [2, p.37]

For an Open

system: (control volume)

rate of change of energy = energy flow rate in
-

energy flow
rate out

Control Volume : A system fixed in space which permits mass
to cross the system boundaries. [1, p.130]

. . . .

d~Ecv/d~t = Ein
-

Eout + Qcv + W


d~

: partial derivative


Ecv : Total energy of the control volume

. .

Ein, Eout : Energy flow rate in and out, across crossing boundaries
(Energy

. flux at crossing boundaries)

Qcv : Net rate of heat transfer into system across inside boudaries.

(heat

. flux)

W : Net rate of work done on system by surroundings (power)


[1, p.133]

Steady State: Implies that conditions at all points in the
[system]* are constant with time. For this to be the case, all
rates must be constant, and there
must be no accumulation of
material or energy within the [system] over the period of time
considered. [2, 30]

* lit. apparatus. For a control volume analysis, each
component of the apparatus is considered an open system
(control volume). The apparatus as a

whole is actually a closed
system. A point in the system would be represented by a
particular location in the apparatus.

For a cyclical process in a closed system, a point in the system
would be represented by a periodic time in the cycle.

Pseudo steady s
tate is a condition in which it is convenient to
assume steady state for portions of a non
-
steady state system.

THE SECOND LAW OF THERMODYNAMICS:

"The entropy S, an extensive* equilibrium property, must
always increase or remain constant for an isolated
system**.
[1, p.187]

dSi >= 0

dSi : change in entropy of an isolated system**

* The units of extensive entropy are energy divided by
temperature. The units of the intensive property would be
energy divided by temperature and mass, or divided by
temperature

and moles.

In SI units:


S (the extensive property) has units: J/K


s (the intensive property) has units: J/(kg K) or J/(kg mole K)

** An isolated system is one in which there is no mass or
energy transfer across system boundaries. (see How do
the
Laws...Apply to Various Systems)

In terms of a non
-
isolated system:


dSsys + dSsur >= 0 or alternately: dSu >= 0



dSsys : change in entropy of system


dSsur : change in entropy of surroundings


dSu : change in entropy of universe

Entropy ge
neration within a system is due to friction. In the
absence of any friction, then the net entropy change is 0.
Friction here includes things like electical resistance, resistance
to heat transfer, resistance to chemical reactions (inverse of
rate constant)
, mechanical friction, air resistance, etc.

For any real process, there is friction. Entropy is transferred
with heat transfer, and the direction of entropy transfer is the
same as that of heat transfer. So, any time heat is transferred
into the system, th
e entropy of the system increases. Any time
heat is transferred out of the system, the entropy of the
system decreases. Heat transfer out is the only means of
decreasing system entropy of a closed system. For an open
system, entropy can be transferred out
with energy transfer
out, and with mass transfer out.

So, any real process increases the entropy of the system +
surroundings, but whether the entropy of the system itself
increases or decreases is dependent on the heat transfer.

Since for an isolated syst
em, there is no heat transfer, there is
no means of reducing the entropy of the system. Which means
that for any real process in an isolated system, the entropy of
the system increases, and the energy available to do work
decreases.

Equilibrium, the state
in which all properties stop changing, is
defined by the second law as is the state of maximum entropy,
that is, there is no more energy available to do work, and no
capacity for change.

In terms of cyclic process:

"It is impossible by a cyclic process to
convert the heat
absorbed by a system completely into work." [2, p.139]

"The word cyclic requires that the system be restored
periodically to its original state." [2, p.139]

In terms of a cyclic process, there are two implications here:

1) Some of the heat

in the system is unavailable to do work in
restoring the system to its original state.

2) In order to achieve a steady state cycle, there must
necessarily be some energy lost, ie. not available to do work.
This would mean a continuous input and output of
energy is
necessary to drive a continuous cycle.

Attempts have been made to invent devices called perpetual
motion machines. There are two classes of perpetual motion
machines called PMM1 and PMM2.

PMM1
-

"A perpetual motion machine of the first kind is a
continuously operating device that produces a continuous
supply of energy without receiving energy input." [1, p.159]

PMM2
-

"An engine that, operating continously, will produce no
effect other than the extraction of heat from a single reservoir,
and the p
erformance of an equivalent amount of work." [1,
p.246]

A PMM1 violates the First Law because it has more energy
output than input.

A PMM2 violates the Second Law, because it allows a complete
conversion of heat energy into work. In order to operate, an
en
gine must have a heat sink. In a steady state cycle, entropy
generation must be offset by heat transfer to the heat sink.
This is why it is impossible to convert all of the heat input into
work
-

some of that heat must necessarily be lost to the heat
sink.

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o How are the Laws of Thermodynamics applied to various
systems?

System
-

that part of the universe set apart for examination.
[1, p.33]

Surroundings
-

that part of the universe which str
ongly
interacts with the system under study. [1, p.33] Universe
-

the
totality of matter that exists. [1, p.33]

This definition does not include energy that exists apart from
matter. For example, radiation in space. Pure energy, apart
from matter, is not m
easureable. This is somewhat like the
uncertainty principle. In order to measure energy, you must
first allow it to interact with matter, and then you measure the
effect on the matter. But once you have done that, it is no
longer pure energy, so you can't
be certain that what you are
measuring is accurate for pure energy. For example, the speed
of light. We must depend on the interaction of light with
matter to measure the speed, but in so doing, we are no
longer measuring light *apart from* matter, but rat
her
measuring the effect of light *on* matter. By this definition of
the universe, pure energy exists in another dimension
-

outside of the space
-
time continuum that we call the universe.
It is only when energy interacts with matter that it enters the
univ
erse. This gives us a clue about the nature of entropy
-

anytime energy interacts with matter, some of that energy is
transferred to the matter, and some of that energy becomes
disordered.

(note to RobD
-

Is matter the ten thousand things of which Lao
Tzu
spoke? And is pure energy the Tao? )

System Boundary
-

the surface that separates the system from
its surroundings. [1, p.34] Inside Boundary
-

the part of the
boundary impervious to mass flow. [1, p.35]

Crossing Boundary
-

the part of the boundary at whic
h mass
enters and leaves the system. [1, p.35]

Closed system, or Control Mass, which means that the mass of
the system is constant, and mass is not allowed to cross the
system boundaries.

Open System
-

a system in which mass is allowed to cross the
system
boundaries.

In actuality, there are very few truly closed systems. Take a
balloon
-

we know that the air inside is slowly diffusing through
the walls of the balloon, and given sufficient time the balloon
will deflate. What we have to do is make an approxim
ation. If
we are studying effects of heat transfer on the properties in
the balloon, then the rate of mass transfer across the
boundaries is negligible, and we can use a closed system as a
good approximation.

Take the earth
-

we know that gas molecules can

occasionally
escape into space, meteors occasionally shower down into the
atmosphere, and space missions leave the earth. So in the
strictest sense, the earth is an open system. But if we study
the rate of overall energy transfer, and compare that with th
e
rate of transfer due to matter entering and leaving, then for all
practical purposes the earth is currently a closed system,
because the effect on the overall energy balance is negligible.
If we take the upper reaches of the atmosphere as the system
boun
dary, then we can also say that the system has a fixed
boundary. In effect, we are really saying that a balloon is a
model of the earth
-

for the purpose of thermodynamic
analyses.

Two different forms of the Laws of Thermo are used for open
and for closed
systems. One consideration in an open system
is the fact that energy can be transferred across the system
boundaries due to intrinsic energy of the mass that is
transferred.

There are a number of different types of closed systems:

Diabatic
-

allowing heat
transfer across sytem boundaries
Adiabatic
-

not allowing heat transfer across system
boundaries. (insulated thermally)

Insulated System (electrically)
-

a System in which electrical
work cannot cross the system boundaries

Rigid System
-

a System in which
the boundaries are fixed, ie.
not allowing mechanical work to cross the system boundaries.

Isolated system
-

a closed system that allows neither mass nor
energy transfer across system boundaries.

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o Does Snowflake formation violate the Second Law of
Thermodynamics?

A forming snowflake is an open system. There is mass transfer
across the boundary. If snowflake formation causes a
redu
ction in the entropy of the snowflake, then, by the second
law, the entropy change of the surroundings must increase.

What about the order of the snowflake? A snowflake indeed
appears to have a high degree of order, but remember, we are
talking about order
ed energy. Ordered energy is energy that is
available to do work. Once a snowflake forms, it doesn't do
any work, it just slowly drifts down to the ground and then just
sits there, melts, or sublimates. A snowflake has a pattern, but
that pattern is static
, and is *not* ordered energy available to
do work. Snow has a remarkable ability to resist change.
Unmelted snowflakes will not readily bond to each other, snow
has a very poor ballistic coefficient which prevents snowflakes
from accumulating any signific
ant kinetic energy when they
fall. Snow does not readily absorb radiation or heat energy.
Snow is chemically inert compared to water in other states. At
the macroscopic level, since snowflakes are unique, we would
have to say that the pattern of snowflakes

is highly disordered
-

otherwise we should see large numbers of identical patterns.

[1] _Fundamentals_of_Engineering_Thermodynamics_, Howell
and Buckius, McGraw
-
Hill, 1987

[2]
_Introduction_to_Chemical_Engineering_Thermodynamics_,
_Fourth_Edition_, Smith
and van Ness, McGraw
-
Hill, 1987

[3] _Classical_Thermodynamics_of_Nonelectrolyte_Solutions_,
van Ness and Abbott, McGraw
-
Hill, 1982

James P. Henley Jr.

Chemical Engineering Dept.

Auburn University