Thermodynamics, also as caloric theory designation, is a subsection of classical physics. It originated in in the process 19. Century on the basis of the work of James Prescott joule, Nicolas Sadi Carnot, Julius Robert von Mayer and Hermann von Helmholtz. It is the theory of the energy, its manifestation and ability to perform work. It proves as versatile applicable in chemistry, biology and technology. With its assistance one can explain for example, why spontaneously to run off and other one did not determine chemical reactions. Thermodynamics is a purely macroscopic theory, which assumes the physical characteristics of a system can be described sufficiently well with macroscopic variables of state.

Intensive variables of state, for example temperature T, are differentiated pressure p and chemical Potenzial , from extensive variables of state, for example internal energy U, entropy S and volume V. The work W and the warmth Q are not variables of state, since they do not characterize the system in clear way at a fixed time. The particle number of N (or the mass m) of a system is likewise no variable of state, since by its default a system is only defined.

The equations, which supply concrete connections between the variables of state for special physical systems (e.g. ideal gas), are called equations of state.

Thermodynamics develops on (at present) four axioms, the four main clauses of thermodynamics. These axioms are in their original formulation - according to their emergence being based on empirical observations - pure experience sets. The elegant mathematical structure received thermodynamics by the work from Josiah wanting pool of broadcasting corporations Gibbs, which as the first recognized the meaning of the fundamental equation and formulated its characteristics.

By the statistic mechanics after James Clerk Maxwell and Ludwig Boltzmann can be confirmed many aspects of thermodynamics on the basis microscopic theories. In its entire representation it keeps however further the excellent status of its own physical theory. Their applicability must be limited however to suitable systems, i.e. such, which of sufficient many individual systems, thus usually particles, to consist.

Zeroth main clause (sometimes also 4. Main clause mentioned)

If a system A with a system B as well as B are with a system C in the thermal equilibrium, then also A with C is in the thermal equilibrium.

Differently formulated, the equilibrium is transitiv. This permits it, a new variable of state to introduce the empirical temperature so that two systems have exactly the same temperature if they are in the thermal equilibrium. This law was only formulated after the three other main clauses. Since it forms an important basis, it was designated later than zeroth main clause. It explains why a thermometer, which is located in contact with the object which can be measured, which can measure temperature.

Instead of the temperature if the entropy is introduced not only for all thermodynamic systems, but as primary term in the phenomenological sense, then the zeroth main clause is unnecessary.

First main clause

The first main clause of thermodynamics is the set of the energy conservation: Each system possesses an internal energy U (=extensive variable of state). This can change only by the transport of energy in the form of work W and warmth Q over the border of the system, i.e.:

\ qquad \ mathrm dU= \ delta Q + \ delta W

The energy of an final system remains unchanged. Different forms of energy can be converted therefore into one another, but energy cannot be produced nor destroyed neither from that anything.

In the reality always energy in the form of friction and heat transfer are lost to the environment during a transformation (see efficiency). Therefore a Perpetuum mobile of first kind is impossible (no system verichtet without heat transfer or change of the internal energy work).

A restriction of the commutability of warmth in work results only from the second main clause of thermodynamics.

See also: Energy equation

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