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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 some 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 See also
8 References
9 Further reading
[edit] 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 systems 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 principles were already widely in
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).
[edit] 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 conservation of energy for a
thermodynamic system. It refers to the two ways that a closed system 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 amount 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 variations 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 unquestioned (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).
[edit] Fundamental Thermodynamic Relation
The first law can be expressed as the Fundamental Thermodynamic Relation:
Heat supplied = internal energy + work done
Internal 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).
[edit] Second law
Main article: Second law of thermodynamics
The entropy of an isolated system consisting of two regions of
space, 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 when the jointly
communicating system reaches thermodynamic equilibrium.
In a simple manner, the second law states "energy systems have a tendency to
increase 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 example, 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 separate 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 the 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 exhibit fluctuations of entropy opposite to that stated by
the Second Law (see Maxwell's demon and Fluctuation Theorem).
[edit] 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 zero.
[edit] Tentative fourth laws or principles
Over the years, various thermodynamic researchers have 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
thermodynamics 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 attempts 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 Contributions 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 laws 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
Because energy cannot be
Entropy—that is, the
The Third Law of
systems is in
created or destroyed (with the
disorder—of an isolated
thermodynamics states
equilibrium with a
special exception of nuclear system can never decrease.
that absolute zero
third, the first two
reactions) the amount of heat Therefore, when an isolated cannot be attained by
systems must be in transferred into a system plus
system achieves a
any procedure in a
equilibrium with each the amount of work done on configuration of maximum finite number of steps.
other. This shared
the system must result in a
entropy, it can no longer
Absolute zero can be
property of
corresponding increase of
undergo change (it has
approached arbitrarily
equilibrium is the
internal energy in the system.
reached equilibrium).
closely, but it can never
temperature. The Heat and work are mechanisms
Additionally, it is not
be reached
concept of
by which systems exchange enough to conserve energy
temperature is based energy with one another. This
and thus obey the First
on this Zeroth Law. First Law of thermodynamics Law. A machine that would
identifies caloric, or heat, as a deliver work while violating
form of energy.
the second law is called a
"perpetual-motion machine
of the second kind." In such
a system, energy could
then be continually drawn
from a cold environment to
do work in a hot
environment at no cost.
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
about 'energy production', or 'energy consumption' is probably ignorant about
the First Law). This means that the amount of energy in the universe is
constant.
So, the First Law tells us something about the 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 Law tells us about the direction of the universe and all
processes, namely towards a decreasing exergy content of the universe.
Processes that follows this general principle 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 system 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 same 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 depression increases as
the amount of paste diminishes, but the depression is not a negative paste.
(You can not take the depression and unbrush your teeth!)
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 exergy in the energy. 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.