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.

A numerical method for modeling → compressible
hydrodynamic flows, which
uses particles to simulate a continuous fluid flow. Because the system of hydrodynamical
basic equations can be analytically solved only for few exceptional cases, the SPH
method provides a numerical algorithm to solve systems of coupled
→ partial differential equations
for continuous field quantities. The main advantage of the method is
that it does not require a computational grid to calculate spatial
→ derivatives and that it is a Lagrangian method, which automatically
focuses attention on fluid elements. The equations of motion and
continuity are expressed in terms of ordinary differential equations
where the body forces become classical forces between particles. This
method was first independently developed by Lucy (1977, AJ 82, 1013)
and Gingold & Monaghan (1977, MNRAS 181, 375).

Smoothed Particle Hydrodynamics, first used by Gingold & Monaghan (1977);
→ smooth; → particle;
→ hydrodynamics.

The field of astrophysics that describes systems of many
→ point mass particles whose mutual
gravitational interactions determine their orbits. Theses systems include
→ star clusters, → globular clusters,
and galaxies (→ galaxy) consisting of about
10^{2}-10^{3}, 10^{4}-10^{6}, and up to about
10^{12} members respectively. Stellar dynamics deals with systems in
which each member contributes importantly to the overall
gravitational field and is usually concerned with the statistical properties of many
orbits. It can be compared to the
→ kinetic theory of gases developed in the
late 19th century. In contrast, → celestial mechanics
deals with systems where the gravitational force of a massive planet or star
determines the orbits of its satellites.

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.