Thermodynamic equilibrium

The equilibrium describes in physics, chemistry, biology, the balance of all potentials and flows in a given system, including any in-or outflows of an open system. The balance mechanism is understood to be the equilibrium of forces and / or the moment equilibrium.

• 2.1 Thermal equilibrium
• 2.2 local thermodynamic equilibrium
• 2.3 Quasi-static processes

Mechanics

Statics

Static is a body in equilibrium when cancel each other out in a body or a coupled system of bodies, all external forces and / or torques.

Formally expressed under the framework of Newtonian mechanics conditions must be met ( equilibrium conditions ):

In the sum of the external forces applied forces are ( generally as forces with specific physical causes) and include constraint forces or moments ( as forces with certain kinematic bonds).

Equilibrium situations occur in many areas of physics and engineering. Consideration must also not necessary to purely mechanical forces restrict itself about gravitation and electrical attraction in a conservative, potential and flux- conserving system can compensate.

The implication of the equilibrium conditions:

For multi- body systems such set of equilibrium conditions is to be set for every body. The principle of virtual work applied, the coercive forces can be eliminated. In static equilibrium, the virtual work of the impressed forces equal to zero.

Equilibrium: stable, unstable, indifferent

According to the measure of their stability are three types of equilibria:

The three types of mechanical equilibrium can be for the body, to which only affects gravity, described by the location of focus and point of application of the torque:

Another illustration is the consideration of a sphere:

• If the ball in a bowl, it is in stable equilibrium, it will roll back upon displacement.
• If the ball lands on one level, it is in neutral equilibrium, it is at the point to which they move, stay here, and after a single force and neglecting the frictional resistance in their state of motion remain ( inertia ).

The hysteresis can transform a stable or unstable equilibrium in an indifferent.

See also: stability, stability

Stable and unstable equilibrium in the pressure system

Two connected balloons - unstable equilibrium

Again, stable equilibrium

Thermodynamics

At the thermodynamic equilibrium principle applies the balance of power from Gibbs free energy:

This means that no energy or potential difference between the respective points is present in the room.

Without internal barriers (eg walls ), and acting force fields (eg gravity) applies the trivial solution. She sits for any two points 1 and 2 of the system requires:

• Thermal equilibrium (see below),
• The mechanical balance (see above) and
• The chemical equilibrium

A system in thermodynamic equilibrium is always stationary, when the fluxes are zero so move no driving potential gradient over the variables in the system.

Reversible processes are possible only along closely spaced points with static equilibrium conditions, otherwise increases the entropy of the system.

Thermal equilibrium

The term thermal balance is used in two different contexts.

• Firstly, in the sense used above as a state of a single thermodynamic system: it is in thermal equilibrium if it can be described by a small number of state variables and this is not time to change. An article in the refrigerator, for example, is to in thermal equilibrium, because its state is uniquely determined by the mass, temperature, pressure and composition, and remains constant over time. Boiling water is, however, not in thermal equilibrium, because for the description of its turbulent flow motion a lot of information are required and it is therefore in the strict sense is not a thermodynamic system.

• Secondly, as a relationship between multiple systems: two bodies are in thermal contact with one another, are each in thermal equilibrium if and only if they have the same temperatures. The ability of systems to be in equilibrium is an equivalence relation. A system is a system with both a B and C with a system in thermal equilibrium, then the systems B and C with each other in thermal equilibrium ( transitivity ). This statement is an important fundamental assumption of thermodynamics and is sometimes called zeroth law of thermodynamics.

Local thermodynamic equilibrium

In thermal equilibrium all processes are in balance, including also the rates of emission and absorption of radiation ( blackbody radiation ).

In many cases, however, the emission and absorption rate is selective: the radiation of gases and liquids over a wide wavelength range optically thin, as only certain energy states are allowed in accordance with the quantum numbers; for the radiation, the energy can not lead to an excitation of the particles, gases or liquids are transparent.

With the local thermodynamic equilibrium (English local thermodynamic equilibrium - abbreviation LTE) describes the ratio of excited to the non-excited molecules, which is dependent on the temperature and the beam intensity. In the isothermal equilibrium of radiation and excitation of the molecule, this ratio is described by the Boltzmann statistics. Deviations from the Boltzmann statistics are lower by multiple shocks; ' hot ' particles, which discontinued while power is supplied to thermalize.

LTE exists, for example in the largest area of ​​the earth's atmosphere. Only at very high altitudes where the collision frequencies are very low due to the low pressure, the deviations from the Boltzmann statistics are essential and there is no more LTE ago.

Quasi-static processes

If a thermodynamic process such that it may be regarded exclusively as a succession of equilibrium states, one calls this process quasi-static or quasi- stationary.

Hydrodynamics

In fluid dynamics, the hydrostatic equilibrium is due to the compensation of a directed force on a body and a pressure gradient in the surrounding fluid dar.

In an expanded sense, however, there may be parts of the fluid itself together into packages and as a body to be treated. Here, a balance between the mechanical and thermodynamic aspects of the system takes place. This one examines about convection and their equilibrium positions, or in meteorology, the stratification stability of the Earth's atmosphere.

Other examples from physics

• The radiative equilibrium of two different hot body in a closed system with emission and absorption of thermal radiation, for further details see Prevostscher sentence.
• The propagation of waves through a dispersive and nonlinear medium. A dynamic interplay of these properties can lead to numerous developmental phenomena, such as a development of so-called solitons.