The total energy of a → closed system
is constant. This means that energy can be changed from one form to another,
or transferred from one system to another, but it cannot be created or destroyed.
A mathematical formulation of the first law is:
δQ = δU + δW,
where δQ is the heat transferred to the system, δU
the change in internal energy (resulting in a rise or fall of temperature), and
δW is the work done by the system.

The assumption that all distribution functions characterizing the
material and its interaction with the radiation field at a point in
the star are given by → thermodynamic equilibrium relations at local
values of the temperature and density.

1) Heat cannot be transferred from a colder to a hotter body without some other effect, i.e.
without → work being done. Expressed in terms of
→ entropy: the entropy of an
→ isolated system tends toward a maximum and its
available energy tends toward a minimum.
2) In language of → statistical physics,
an isolated physical system will tend toward an
equilibrium → macrostate
with as large a total → entropy
as possible, because then the number of → microstates
is the largest. See also
→ Kelvin's postulate,
→ Clausius's postulate.

The condition of a → thermodynamic system
in which the available → energy is distributed uniformly
among all the possible forms of energy. Furthermore,
all → thermodynamic process es must be exactly balanced by
their reverse processes. For example, inside a star
there will be as many → ionizations of helium per second
as there are → recombinations of
free electrons and helium ions. Se also
→ local thermodynamic equilibrium (LTE).

A quantity of substance or a working machine which in a well-defined way is set apart from
its → environment. The boundary between the system and its
surroundings can be real or an imaginary mathematical envelope. A thermodynamic system
is not necessarily bound to a predefined geometry. Thermodynamic systems can be divided into
three types:
→ open systems,
→ closed systems, and
→ isomated systems.

A temperature scale, measured in → kelvin (K),
that is related to the energy possessed by matter; it was formerly known as
→ absolute temperature.
The zero point on the scale (0 K) is absolute
zero. Thermodynamic temperature can be converted to temperature on
the → Celsius scale
by subtracting 273.15.

A branch of physics concerned with the relations between heat and
other forms of energy and how these affect temperature, pressure, volume,
mechanical action, and work.

→ thermo-; → dynamics, coined by
the Scottish physicist William Thomson (Lord Kelvin, 1824-1907), in 1849.

third law of thermodynamics

قانون ِ سوم ِ گرماتوانیک

qânun-e sevom-e garmâtavânik

Fr.: troisième loi de la thermodynamique

The → entropy of an idealized state of maximum order is
zero at the temperature of → absolute zero.
Another version of this law: As a system approaches absolute zero, all processes
cease and the entropy of the system approaches a minimum value.