# The laws of thermodynamics

Mechanics

Oct 27, 2013 (4 years and 6 months ago)

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The
laws of thermodynamics
, in principle, describe the specifics for the transport
of
heat

and
work

in

thermodynamic processes
. Since their inception, however,
these
laws

have become s
ome of the most important in all of
physics

and other
types of
science

associated with
thermodynamics
.

It is wise to distinguish classical
thermodynamics
, which is focused on systems in
thermodynamic equilibrium
, from
non
-
equilibrium thermodynamics
. The present
article is focused on classical or thermodynamic equilibrium thermodynamics.

There are generally considered to be four principles (referred to as "laws"):

1.

The
zeroth law of thermodynamics
, which underlies the definition of temperature.

2.

The
first law of thermodynamics
, which mandates
conservation of energy
, and
states in particular that
heat

is a form of energy.

3.

The
second law of thermodynamics
, which states that the
entropy

of an isolated
macroscopic system never decreases,

or (equivalently) that
perpetual motion
machines

are impossible.

4.

The
third law of thermodynamics
, which concerns the entropy of a perfect crystal
at
absolute zero

temperature, and implies that it is impossible to cool a system

all
the way to exactly absolute zero.

During the last 80 years or so, occasionally, various writers have suggested
additional Laws, but none of them have become well accepted.

Contents

[
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]

1

Zeroth law

2

First law

o

2.1

Fundamental Thermodynamic Relation

3

Second law

4

Third law

5

Tentative fourth laws

or principles

6

History

7

8

References

9

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Zeroth law

Main article:
Zeroth law of thermodynamics

If two thermodynamic systems are each in thermal equilibrium with
a third, then they are in thermal equilibrium with each other.

When two syste
ms are put in contact with each other, there will be a net
exchange of
energy

between them unless or until they are in
thermal equilibrium
,
that is, they are at the same temperature. While this is a fundamental concept of
thermodynamics, the need to state it explicitly was not perceived until the first
third of the 20th century, long after the first three
use, hence the zero numbering. The Zeroth Law asserts that thermal equilibrium,
viewed as a
binary relation
, is a
transitive relation

(and since any system is
always in equilibrium with itself, it is furthermore an
equivalence relation
).

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First law

Main article:
First law of thermodynamics

Energy can neither be created nor destroyed. It can only change
forms.

In any process in an isolated system, the total energy remains the
same.

For a
thermodynamic cycle

the net
heat

supplied to the system
equals the net work done by the system.

The First Law states that energy cannot be created
or destroyed; rather, the
amount of energy lost in a steady state process cannot be greater than the
amount of energy gained. This is the statement of
conservati
on of energy

for a
thermodynamic system
. It refers to the two ways that a
closed syste
m

transfers
energy to and from its surroundings

by the process of heating (or cooling) and
the process of mechanical work. The rate of gain or loss in the stored energy of a
system is determined by the rates of these two processes. In open systems, the
flow of matter is another energy transfer mechanism, and extra terms must be
included in the expression of the first law.

The First Law clarifies the nature of energy. It is a stored quantity which is
independent of any particular process path, i.e., it is

independent of the system
history. If a system undergoes a
thermodynamic cycle
, whether it becomes
warmer, cooler, larger, or smaller, then it will have the same amo
unt of energy
each time it returns to a particular state. Mathematically speaking, energy is a
state function

and infinitesimal changes in the energy are
exact differentials
.

All laws of thermodynamics but the First are statistical and simply describe the
tendencies of macroscopic systems. For microscopic systems with few particles,
the varia
tions in the parameters become larger than the parameters themselves,
and the assumptions of thermodynamics become meaningless. The First Law,
i.e. the law of conservation, has become the most secure of all basic principles of
science. At present, it is un
questioned (although it is said to be criticized by
people who do not accept the idea that the potential to gain energy is a form of
actual energy).

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Fundamental Thermodynamic Relation

The first law can be expressed as the
Fundame
ntal Thermodynamic Relation
:

Heat supplied

=
internal energy

+
work done

I nternal energy

=
Heat supplied

-

work done

Here, E is
internal energy
, T is
temperature
, S is
entropy
, p is
pressure
, and V is
volume
. This is a statement of conservation of energy: The net change in internal
energy (dE) equals the heat energy that flows in (TdS), minus the energy that
flows out via the system performing work (pdV).

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Second law

Main article:
Second law of thermodynamics

The
entropy

of an
isolated system

consisting of two regions of
sp
ace, isolated from one another, each in
thermodynamic
equilibrium

in itself, but not in
equilibrium

with each other, will, when
the isolation that separates the two regions is broken, so that the
two regions become able to exchange matter or energy, tend to
increase over time, approaching a maximum value wh
en the jointly
communicating system reaches
thermodynamic equilibrium
.

In a simple manner, the second law states "energy systems have a tendency to
increa
se their entropy rather than decrease it." This can also be stated as "heat
can spontaneously flow from a higher
-
temperature region to a lower
-
temperature
region, but not the other way around." (Heat
can

flow from cold to hot, but not
spontaneously

-

for e
xample, when a refrigerator expends electrical power.)

A way of thinking about the second law for non
-
scientists is to consider entropy
as a measure of ignorance of the microscopic details of the system. So, for
example, one has less knowledge about the se
parate fragments of a broken cup
than about an intact one, because when the fragments are separated, one does
not know exactly whether they will fit together again, or whether perhaps there is
a missing shard. Solid
crystals
, the most regularly structured form of matter, have
very low entropy values; and
gases
, which are very disorganized, have high
entropy values. This is because th
e positions of the crystal atoms are more
predictable than are those of the gas atoms.

The
entropy

of an isolated macroscopic system never decreases. However, a
microscopic system may exhibi
t fluctuations of entropy opposite to that stated by
the Second Law (see
Maxwell's demon

and
Fluctuation Theorem
).

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Third law

Main article:
Third law of thermodynamics

As temperature approaches
absolute zero
, the
entropy

of a system
approaches a constant minimum.

Briefly, this postulates that entropy is temperature dependent and results in the
formulation of the idea of
absolute zer
o
.

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]

Tentative fourth laws or principles

Over the years, various thermodynamic researchers h
ave come forward to
ascribe to or to postulate potential fourth laws of thermodynamics (either
suggesting that a widely
-
accepted principle should be called the fourth law, or
proposing entirely new laws); in some cases, even fifth or sixth laws of
thermody
namics are proposed
[1]
. Most fourth law statements, however, are
speculative and controversial.

The most commonly proposed Fourth Law is the
Onsager reciprocal relations
,
which give a quantitative relation between the parameters of a system in which
heat and matter are simultaneously flowing.

Other tentative fourth law statements are attem
pts to apply
thermodynamics

to
evolution
. During the late 19th century, thermodynamicist
Ludwig Boltzmann

argued that the fundamental object of contention in the life
-
struggle in the
evolution of the organic world is 'available energy'. Another example is the
maximum power principle

as put forward initially by biologist
Alfred Lotka

in his
1922 article
Contr
ibutions to the Energetics of Evolution
.
[2]

Most variations of
hypothetical fourth laws (or principles) have to do with the environmental
sciences, biological evolution, or galactic phenomena.
[3]

The field of thermodynamics studies the behavior
of
energy

flow in natural systems. From this
study, a number

of physical laws have been
established. The
laws of thermodynamics

describe some of the fundamental truths of
thermodynamics observed in our
Universe
.
Understanding these la
ws is important to students
of Physical Geography because many of the
processes studied involve the flow of energy.

Zeroth Law

First Law

Second Law

Third Law

When each of two
systems is in
equilibrium with a
third, the first two
systems must be in
e
quilibrium with each
other. This shared
property of
equilibrium is the
temperature. The
concept of
temperature is based
on this Zeroth Law.

Because energy cannot be
created or destroyed (with the
special exception of nuclear
reactions) the amount of heat
transferred into a system plus
the amount of work done on
the system must result in a
corresponding increase of
internal energy in the system.
Heat and work are mechanisms
by which systems exchange
energy with one another. This
First Law of thermodynamics
identifies caloric, or heat, as a
form of energy.

Entropy

that is, the
disorder

of an isolated
system can never decrease.
Therefore, when an isolated
system achieves a
configuration of maximum
entropy, it can no longer
undergo change (it has
reached equil
ibrium).
enough to conserve energy
and thus obey the First
Law. A machine that would
deliver work while violating
the second law is called a
"perpetual
-
motion machine
of the second kind." In such
a system, energy could
then be conti
nually drawn
from a cold environment to
do work in a hot
environment at no cost.

The Third Law of
thermodynamics states
that absolute zero
cannot be attained by
any procedure in a
finite number of steps.
Absolute zero can be
approached arbitrarily
closely
, but it can never
be reached

These are Natural Laws, i.e. they are fundamental and can not be negotiated. On
the other hand, if somebody find out something that might falsify them, they will
cease to be fundamental.

The
First Law

tells us that energy can be neither created nor destoyed.

(The

production or consumption of energy is impossible. Anyone who speaks
the First Law). This means that the amount of energy in the universe is
constant.

So, the First Law tells us

state

of the universe and all
processes in it.

The
Second Law

tells us that the
quality

of a particular amount of energy

i.e.
the amount of work, or action, that it can do, diminishes for each time this
energy is used. This is true for all instances of energy use, physical
, metbolic,
interactive, and so on.

This means that the quality of energy in the universe as a whole, is
constantly diminishing. All real processes are irreversible, since the quality of
the energy driving them
is lowered for all times
.

Thus, the Second La
direction

of the universe and all
processes, namely towards a decreasing exergy content of the universe.
Processes that follows this general principl
e will be preferred.

Some people seem to think that this law should be revoked...

But perhaps
they are misled by their notion of entropy.

The usable energy in a sy
stem is called
exergy
, and can be measured as
the total of the free energies in the system.

Unlike energy, exergy can be

consumed.

To more easily understand the concept of
exergy
, you can consider this
picture as an analogy: You buy is the (toothpaste) tube. But you have to
squeeze it to get at what you really need, the toothpaste. When the tube is
empty of paste (exergy) the tube is still there, the sam
e amount as when you
bought it.

In theese circumstances, the word
entropy

often comes up. In the picture this
is represented as the depression in the tube. The depressio
n increases as
the amount of paste diminishes, but the depression is
not

a negative paste.
(You can not take the depression and
un

Entropy is
not

negative exergy, but another description of the system.
Furthermore, it is not defined in

far
-
from
-
equilibrium systems
, as living
systems and other organised systems.

Life
-
processes consume the e
x
ergy in the e
n
ergy. After the energy is used,
it contains a lower amount of exergy. The extracted energy is
low
-
exergy
energy
,
not entropy.

Exergy consumption by living systems

Life
-
processes are more efficient in consuming exergy than non
-
living
processes. Therefore, when living systems appears, they offer a faster route
of exergy
consumption, and hence a better way of 'obeying' the Second Law.