## thermodynamicsA division of physics concerned with the interconversion of heat, work, and other forms of energy, and with the states of physical systems. Being concerned only with bulk matter and energy, classical thermodynamics is independent
of theories of their microscopic nature; its axioms are sturdily empirical,
and from them theorems are derived with mathematical rigor. Classical thermodynamics
is basic to engineering, parts of geology, metallurgy, and physical chemistry.
Building on earlier studies of the thermodynamic functions temperature and heat, Sadi Carnot pioneered the science by his investigations of the cyclic heat engine in 1824, and in 1850 Clausius stated the first two laws. Thermodynamics was first developed by Joshua Gibbs, Hermann von Helmholtz, William Thomson (Lord Kelvin), and James Clerk Maxwell. In thermodynamics, a system is any defined collection of
matter: a closed system is one that cannot exchange matter
with its surroundings; an isolated system can exchange
neither matter nor energy. The state of a system is specified by
determining all its properties such as pressure, volume, etc. A system in
stable equilibrium is said to be in
an equilibrium state, and has an equation of state (e.g., the general gas
law) relating its properties. (See also phase
equilibria.) A process is a change from one state A
to another B, the path being specified by all the intermediate states. A
state function is a property or function
of properties which depends only on the state and not on the path by which
the state was reached; a differential
dX of a function X (not necessarily a state function)
is termed a perfect differential if it can be integrated
between two states to give a value X_{AB} (= integral from
A to B of dX) which is independent of the path from A to B. If
this holds for all A and B, X must be a state function. ## Laws of thermodynamicsThere are four basic laws of thermodynamics, all having many different formulations that can be shown to be equivalent.The zeroth law states that, if two systems are each in
thermal equilibrium with a third system, then they are in thermal equilibrium
with each other. This underlies the concept of temperature. The first law states that for any process the difference
of the heat Q supplied to the system and the work W done
by the system equals the change in the internal
energy U: ΔU = Q - W. U
is a state function, though neither Q nor W separately
is. Corollaries of the first law include the law of conservation
of energy, Hess' law (see thermochemistry),
and the impossibility of perpetual motion machines of the first kind. For
more, see first law of thermodynamics.
The second law (in Clausius' formulation) states that heat
cannot be transferred from a colder to a hotter body without some other
effect, i.e., without work being done. Corollaries include the impossibility
of converting heat entirely into work without some other effect, and the
impossibility of perpetual motion machines of the second kind. It can be
shown that there is a state function entropy,
S, defined by ΔS = ∫dQ/T,
where T is the absolute temperature. The entropy change ΔS
in an isolated system is zero for a reversible process and positive for
all irreversible processes. Thus entropy tends to a maximum. It also follows
that a heat engine is most efficient when it works in a reversible Carnot
cycle between two temperatures T_{1} (the heat source)
and T_{2} (the heat sink), the efficiency
being (T_{1} - T_{2})/T_{2}.
For more, see second law
of thermodynamics. The third law states that the entropy of any finite system
in an equilibrium state tends to a finite value (defined to be zero) as
the temperature of the system tends to absolute
zero. The equivalent Nernst heat theorem states that the energy
change for any reversible isothermal
process tends to zero as the temperature tends to zero. Hence absolute entropies
can be calculated from specific heat
data. Other thermodynamic functions, useful for calculating equilibrium
conditions under various constraints, are: enthalpy
(or heat content) H = U + pV; the Helmholtz free
energy A = U - TS; and the Gibbs
free energy G = H - TS. The free energy represents
the capacity of the system to perform useful work. For more, see third
law of thermodynamics. ## Quantum statistical thermodynamicsQuantum statistical thermodynamics, based on quantum mechanics, arose in the 20th century. It treats a system as an assembly of particles in quantum states. The entropy is given byS = k
log P, where k is the Boltzmann
constant and P the statistical probability of the state of
the system. Thus entropy is a measure of the disorder of the system. ## Related category• HEAT AND THERMODYNAMICS | |||||

Home • About • Copyright © The Worlds of David Darling • Encyclopedia of Alternative Energy • Contact |