Physics Help
Thermodynamics
Thermodynamics
Thermodynamics is the study of
energy, its conversions between various forms such as
heat,
and the ability of energy to do
work. It is closely related to
statistical mechanics from which many themodynamic relationships can be
derived.
It can be argued that thermodynamics was misnamed as it does
not actually relate to rates of change as such and therefore would probably have
been better called thermostatics as a field. Thermodynamics relates to whether
certain chemical reactions are possible but not how quickly they occur.
The field covers a wide range of topics including, but not
limited to: efficiency of
heat engines and
turbines,
phase equilibria,
PVT relationships.
gas laws (both
ideal and
non ideal),
energy balances,
heats of reactions, and
combustion reactions. It is governed by 4 basic
laws (in brief):
Alternative statements are given for each law. These
statements are, for the most part, mathematically equivalent.
-
Zeroth law: A fundamental concept within thermodynamics, however, it was
not termed a law until after the first three laws were already widely in use,
hence the zero numbering. There is some discussion about its
status. Stated as:
-
1st Law: Is stated as follows:
-
2nd Law: A far reaching and powerful law, it can be stated many ways, the
most popular of which is:
-
It is impossible to obtain a process such that the unique
effect is the subtraction of a positive heat from a reservoir and the
production of a positive work.
-
A system operating in contact with a thermal reservoir
cannot produce positive work in its surroundings (Kelvin)
-
A system operating in a cycle cannot produce a positive
heat flow from a colder body to a hotter body (Clausius)
-
The
entropy of a closed system never decreases (see
Maxwell's demon)
-
3rd Law: This law explains why it is so hard to cool something to
absolute zero:
-
All processes cease as temperature approaches zero.
-
As temperature goes to 0, the entropy of a system
approaches a constant
The three original laws have been humorously summarised as:
(1) you can't win; (2) you can't break even; (3) you can't get out of the game.
This is a brief summary and collection of the major concepts
in thermodynamics. To learn more about each, just click on the corresponding
links:
U stands for the
internal energy, T stands for
temperature, S stands for
entropy, P stands for
pressure, V stands for
volume, ρ stands for
density, F stands for
Helmholtz free energy, H stands for
enthalpy, G stands for
Gibbs free energy, μ stands for
chemical potential and N stands for particle number.
The rest of this discussion is about systems in equilibrium
only. For
nonequilibrium thermodynamics, see ...
Blackbody radiation is an example. The reason why this is the
case is because photon number isn't conserved. The state is completely described
by its temperature except at phase transitions and perhaps spontaneous symmetry
breaking in the ordered phase. given the internal energy as a function of
temperature, we can define F=U-TS.
Most "pure" nonmagnetic substances fall into this category.
This state is completely described by its temperature and pressure, except at
phase transitions and perhaps spontaneous symmetry breaking in the ordered
phase. Given U and V (or the density ρ) as a function of T and P, we can define
the Helmholtz energy as before and the Gibbs energy as G=U-TS+PV and the
enthalpy as H=U+PV.
If there are more than one kind of atom/molecule, a substance
would fall into this category. This state is completely described by its
temperature, pressure and chemical potentials, except at phase transitions and
perhaps spontaneous symmetry breaking in the ordered phase.
If a substance is a ferromagnet or a superconductor, for
example, it would fall into this category. It is completely described by its
temperature and magnetic field, except at phase transitions and perhaps
spontaneous symmetry breaking in the ordered phase.
A thermodynamic system is that part of the universe that is
under consideration. A real or imaginary boundary separates the system from the
rest of the universe, which is referred to as the surroundings. Often
thermodynamic systems are characterized by the nature of this boundary as
follows:
-
Isolated systems are completely isolated from their
surroundings. Neither heat nor matter can be exchanged between the system and
the surroundings. An example of an isolated system would be an insulated
container, such as an insulated gas cylinder. (In reality, a system can never
be absolutely isolated from its environment, because there is always at least
some slight coupling, even if only via minimal gravitational attraction).
-
Closed systems are separated from the surroundings by an
impermeable barrier. Heat can be exchanged between the system and the
surroundings, but matter cannot. A greenhouse is an example of a closed
system.
-
Open systems can exchange both heat and matter with their
surroundings. Portions of the boundary between the open system and its
surroundings may be impermeable and/or
adiabatic, however at least part of this boundary is subject to heat and
mass exchange with the surroundings. The ocean would be an example of an open
system.
A key concept in thermodynamics is the state of a system.
When a system is at equilibrium under a given set of conditions, it is said to
be in a definite state. For a given thermodynamic state, many of the
system's properties have a specific value corresponding to that state. The
values of these properties are a function of the state of the system and are
independent of the path by which the system arrived at that state. The number of
properties that must be specified to describe the state of a given system is
given by
Gibbs phase rule. Since the state can be described by specifying a small
number of properties, while the values of many properties are determined by the
state of the system, it is possible to develop relationships between the various
state properties. One of the main goals of Thermodynamics is to understand these
relationships between the various state properties of a system.
Equations of State are examples of some of these relationships.
Thermodynamics also touches upon the fields of:
Home | Up | Classical Mechanics | Thermodynamics | Statistical Mechanics | Electromagnetism | Special relativity
General Relativity | Quantum Mechanics | Quantum Field Theory | Standard Model | Fluid Mechanics
Physics Help, made by MultiMedia | Free content and software
This guide is licensed under the GNU
Free Documentation License. It uses material from the Wikipedia.
| 
