In
physics,
thermodynamics
(from the
Greek
θερμ-<θερμότης<></θερμότης<>em>,
therme, meaning "heat" and δυναμις,
dynamis, meaning "power")
is the study of the conversion of energy into work and heat and its
relation to macroscopic variables such
as temperature, volume and pressure.
Its progenitor, based on statistical
predictions of the collective motion of particles from their
microscopic behavior, is the field of statistical thermodynamics (or
statistical mechanics), a
branch of statistical
physics. Historically, thermodynamics developed out of
need to increase the efficiency of early steam engines.
Introduction
The starting point for most thermodynamic considerations are the
laws of thermodynamics, which
postulate that
energy can be exchanged
between physical systems as heat or
work. They also postulate the existence of a
quantity named
entropy, which can be defined
for any isolated system that is in
thermodynamic equilibrium. In
thermodynamics, interactions between large ensembles of objects are
studied and categorized. Central to this are the concepts of
system and
surroundings.
A system is composed of particles, whose average motions define its
properties, which in turn are related to one another through
equations of state. Properties can
be combined to express
internal
energy and
thermodynamic
potentials, which are useful for determining conditions for
equilibrium and
spontaneous processes.
With these tools, the usage of thermodynamics describes how systems
respond to changes in their surroundings. This can be applied to a
wide variety of topics in
science and
engineering, such as
engines,
phase
transitions,
chemical
reactions,
transport
phenomena, and even
black holes. The
results of thermodynamics are essential for other fields of
physics and for
chemistry,
chemical engineering,
aerospace engineering,
mechanical engineering,
cell biology,
biomedical engineering,
materials science, and
economics to name a few.
Developments
The history of thermodynamics as a scientific discipline generally
begins with
Otto von Guericke who,
in 1650, built and designed the world's first
vacuum pump and demonstrated a
vacuum using his
Magdeburg hemispheres. Guericke was
driven to make a vacuum in order to disprove
Aristotle's long-held supposition that 'nature
abhors a vacuum'. Shortly after Guericke, the Irish physicist and
chemist
Robert Boyle had learned of
Guericke's designs and, in 1656, in coordination with English
scientist
Robert Hooke, built an air
pump. Using this pump, Boyle and Hooke noticed a correlation
between
pressure,
temperature, and
volume.
In time,
Boyle's Law was formulated,
which states that pressure and volume are
inversely proportional. Then, in 1679,
based on these concepts, an associate of Boyle's named
Denis Papin built a
bone digester, which was a closed vessel with
a tightly fitting lid that confined steam until a high pressure was
generated.
Later designs implemented a steam release valve that kept the
machine from exploding. By watching the valve rhythmically move up
and down, Papin conceived of the idea of a piston and a cylinder
engine. He did not, however, follow through with his design.
Nevertheless, in 1697, based on Papin's designs, engineer
Thomas Savery built the first engine. Although
these early engines were crude and inefficient, they attracted the
attention of the leading scientists of the time. Their work led 127
years later to
Sadi
Carnot, the "father of thermodynamics", who, in 1824, published
Reflections
on the Motive Power of Fire, a discourse on heat, power,
and engine efficiency. The paper outlined the basic energetic
relations between the
Carnot engine,
the
Carnot cycle, and
Motive power. It marked the start of
thermodynamics as a modern science..The term
thermodynamics was coined by
James
Joule in 1849 to designate the science of relations between
heat and
power.
By 1858, "thermo-dynamics", as a functional term, was used in
William Thomson's
paper
An Account of Carnot's Theory of the Motive Power of
Heat. The first thermodynamic textbook was written
in 1859 by William
Rankine, originally trained as a physicist and a civil and
mechanical engineering professor at the University of
Glasgow
.
Classical thermodynamics is
the original early 1800s variation of thermodynamics concerned with
thermodynamic states, and properties as energy, work, and heat, and
with the laws of thermodynamics, all lacking an atomic
interpretation. In precursory form, classical thermodynamics
derives from
chemist Robert Boyle’s 1662 postulate that the pressure
P of a given quantity of gas varies inversely as its
volume
V at constant temperature; i.e. in equation form:
PV = k, a constant. From here, a semblance of a
thermo-science began to develop with the construction of the first
successful atmospheric steam engines in England by
Thomas Savery in 1697 and
Thomas Newcomen in 1712. The first and
second laws of thermodynamics emerged simultaneously in the 1850s,
primarily out of the works of
William Rankine,
Rudolf Clausius, and
William Thomson (Lord
Kelvin).
With the development of atomic and molecular theories in the late
1800s and early 1900s, thermodynamics was given a molecular
interpretation. This field, called
statistical mechanics or
statistical thermodynamics, 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. The statistical approach is in contrast to
classical thermodynamics, which is a more phenomenological approach
that does not include microscopic details. The foundations of
statistical thermodynamics were set out by physicists such as
James Clerk Maxwell,
Ludwig Boltzmann,
Max
Planck,
Rudolf Clausius and
J. Willard Gibbs.
Chemical thermodynamics is
the study of the interrelation of
energy with
chemical reactions or with a
physical change of
state within
the confines of the
laws of
thermodynamics. During the years 1873-76 the American
mathematical physicist
Josiah
Willard Gibbs published a series of three papers, the most
famous being
On the
Equilibrium of Heterogeneous Substances, in which he
showed how
thermodynamic
processes could be graphically analyzed, by studying the
energy,
entropy,
volume,
temperature and
pressure
of the
thermodynamic system, in
such a manner to determine if a process would occur spontaneously.
During the early 20th century, chemists such as
Gilbert N. Lewis,
Merle
Randall, and
E. A. Guggenheim began to apply the mathematical
methods of Gibbs to the analysis of chemical processes.
The Four Laws
The present article is focused on classical thermodynamics, which
is focused on systems in
thermodynamic equilibrium. It is
wise to distinguish classical thermodynamics from
non-equilibrium
thermodynamics, which is concerned with systems that are not in
thermodynamic
equilibrium.
In thermodynamics, there are four laws that do not depend on the
details of the systems under study or how they interact. Hence
these laws are very generally valid, can be applied to systems
about which one knows nothing other than the balance of energy and
matter transfer. Examples of such systems include
Einstein's prediction, around the turn of the 20th
century, of
spontaneous
emission, and ongoing research into the thermodynamics of
black holes.
These four laws are:
- :If two thermodynamic
systems are separately in thermal equilibrium with a third,
they are also in thermal equilibrium with each other.
- If we grant that all systems are (trivially) in thermal
equilibrium with themselves, the Zeroth law implies that thermal
equilibrium is an equivalence
relation on the set of thermodynamic systems. This law is
tacitly assumed in every measurement of temperature. Thus, if we
want to know if two bodies are at the same temperature, it is not necessary to bring them
into contact and to watch whether their observable properties
change with time.
- :The change in the internal
energy of a closed thermodynamic system is equal to the
sum of the amount of heat energy supplied to or
removed from the system and the work done on or by the system or we can
say " In an isolated system the heat is constant".
- :The total entropy of any isolated thermodynamic system always
increases over time, approaching a maximum value or we can say " in
an isolated system, the entropy never decreases".
- :As a system asymptotically
approaches absolute zero of temperature all processes virtually
cease and the entropy of the system asymptotically approaches a
minimum value; also stated as: "the entropy of all systems and of
all states of a system is zero at absolute zero" or equivalently
"it is impossible to reach the absolute zero of temperature by any
finite number of processes".
- :See also: Bose–Einstein condensate
and negative temperature.
Potentials
As can be derived from the energy balance equation (or Burks'
equation) on a thermodynamic system there exist energetic
quantities called
thermodynamic
potentials, being the quantitative measure of the stored energy
in the system. The five most well known potentials are:
Other thermodynamic potentials can be obtained through
Legendre transformation. Potentials
are used to measure energy changes in systems as they evolve from
an initial state to a final state. The potential used depends on
the constraints of the system, such as constant temperature or
pressure. Internal energy is the internal energy of the system,
enthalpy is the internal energy of the system plus the energy
related to pressure-volume work, and Helmholtz and Gibbs energy are
the energies available in a system to do useful work when the
temperature and volume or the pressure and temperature are fixed,
respectively.
System models
An important concept in thermodynamics is the “system”. Everything
in the universe except the system is known as surroundings. A
system is the region of the universe under study. A system is
separated from the remainder of the universe by a
boundary which may be imaginary or
not, but which by convention delimits a finite volume. The possible
exchanges of
work,
heat, or
matter between the
system and the surroundings take place across this boundary.
Boundaries are of four types: fixed, moveable, real, and
imaginary.
Basically, the “boundary” is simply an imaginary dotted line drawn
around a volume of
something when there is going to be a
change in the
internal energy of
that
something. Anything that passes across the boundary
that effects a change in the internal energy of the
something needs to be accounted for in the energy balance
equation. That
something can be the volumetric region
surrounding a single atom resonating energy, such as
Max Planck defined in 1900; it can be a body of
steam or air in a
steam engine, such as
Sadi Carnot defined
in 1824; it can be the body of a
tropical cyclone, such as
Kerry Emanuel theorized in 1986 in the field
of
atmospheric
thermodynamics; it could also be just one
nuclide (i.e. a system of
quarks) as some are theorizing presently in
quantum thermodynamics.
For an engine, a fixed boundary means the piston is locked at its
position; as such, a constant volume process occurs. In that same
engine, a moveable boundary allows the piston to move in and out.
For closed systems, boundaries are real while for open system
boundaries are often imaginary. There are five dominant classes of
systems:
- Isolated Systems – matter and energy may not cross the
boundary
- Adiabatic Systems – heat must not cross the
boundary
- Diathermic Systems - heat may cross boundary
- Closed Systems – matter may not cross the
boundary
- Open Systems – heat, work, and matter may cross the
boundary (often called a control
volume in this case)
As time passes in an isolated system, internal differences in the
system tend to even out and pressures and temperatures tend to
equalize, as do density differences. A system in which all
equalizing processes have gone practically to completion, is
considered to be in a
state of
thermodynamic
equilibrium.
In thermodynamic equilibrium, a system's properties are, by
definition, unchanging in time. Systems in equilibrium are much
simpler and easier to understand than systems which are not in
equilibrium. Often, when analysing a thermodynamic process, it can
be assumed that each intermediate state in the process is at
equilibrium. This will also considerably simplify the situation.
Thermodynamic processes which develop so slowly as to allow each
intermediate step to be an equilibrium state are said to be
reversible
processes.
Conjugate variables
The central concept of thermodynamics is that of
energy, the ability to do
work.
By the
First Law, the
total energy of a system and its surroundings is conserved. Energy
may be transferred into a system by heating, compression, or
addition of matter, and extracted from a system by cooling,
expansion, or extraction of matter. In
mechanics, for example, energy transfer equals the
product of the force applied to a body and the resulting
displacement.
Conjugate
variables are pairs of thermodynamic concepts, with the first
being akin to a "force" applied to some
thermodynamic system, the second being
akin to the resulting "displacement," and the product of the two
equalling the amount of energy transferred. The common conjugate
variables are:
Instrumentation
There are two types of thermodynamic instruments, the
meter and the
reservoir. A
thermodynamic meter is any device which measures any parameter of a
thermodynamic system. In some
cases, the thermodynamic parameter is actually defined in terms of
an idealized measuring instrument. For example, the
zeroth law states that if two
bodies are in thermal equilibrium with a third body, they are also
in thermal equilibrium with each other. This principle, as noted by
James Maxwell in 1872, asserts
that it is possible to measure temperature. An idealized
thermometer is a sample of an ideal gas at
constant pressure. From the
ideal gas
law PV=nRT, the volume of such a sample can be used as
an indicator of temperature; in this manner it defines temperature.
Although pressure is defined mechanically, a pressure-measuring
device, called a
barometer may also be
constructed from a sample of an ideal gas held at a constant
temperature. A
calorimeter is a device
which is used to measure and define the internal energy of a
system.
A thermodynamic reservoir is a system which is so large that it
does not appreciably alter its state parameters when brought into
contact with the test system. It is used to impose a particular
value of a state parameter upon the system. For example, a pressure
reservoir is a system at a particular pressure, which imposes that
pressure upon any test system that it is mechanically connected to.
The Earth's atmosphere is often used as a pressure reservoir.
It is important that these two types of instruments are distinct. A
meter does not perform its task accurately if it behaves like a
reservoir of the state variable it is trying to measure. If, for
example, a thermometer were to act as a temperature reservoir it
would alter the temperature of the system being measured, and the
reading would be incorrect. Ideal meters have no effect on the
state variables of the system they are measuring.
States & processes
When a system is at equilibrium under a given set of conditions, it
is said to be in a definite
state. The
thermodynamic state of the system can be described
by a number of
intensive
variables and
extensive
variables. The properties of the system can be described by an
equation of state which specifies
the relationship between these variables. State may be thought of
as the instantaneous quantitative description of a system with a
set number of variables held constant.
A
thermodynamic process may be defined as the
energetic evolution of a thermodynamic system proceeding from an
initial state to a final state. Typically, each thermodynamic
process is distinguished from other processes, in energetic
character, according to what parameters, as temperature, pressure,
or volume, etc., are held fixed. Furthermore, it is useful to group
these processes into pairs, in which each variable held constant is
one member of a
conjugate pair. The
seven most common thermodynamic processes are shown below:
- An isobaric process occurs at
constant pressure.
- An isochoric process, or
isometric/isovolumetric process, occurs at constant
volume.
- An isothermal process occurs
at a constant temperature.
- An adiabatic process occurs
without loss or gain of energy by heat.
- An isentropic process
(reversible adiabatic process) occurs at a constant entropy.
- An isenthalpic process
occurs at a constant enthalpy.
- A steady state process occurs
without a change in the internal
energy of a system.
See also
Applied fields
Other
Lists and timelines:
Thermodynamic:
Variable:
Theorem:
Other Related Topics
Heat:
Thermal:
Physical
chemistry:
Statistical
Mechanics:
Thermoelectricity:
Sundry:
Wikibooks
References
Further reading
- A nontechnical introduction, good on historical and
interpretive matters.
The following titles are more technical:
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