Balance thermodynamic
In Thermodynamic, a thermodynamic system is in thermodynamic balance when it is at the same time in thermal balance, mechanical and chemical. The local state of a system in thermodynamic balance is determined by the values of its intensive parameters, like the Pression or the Température.
More specifically, thermodynamic balance is characterized by the minimum of a thermodynamic potential, like
- the free energy of Helmholtz for the systems at constant temperature and volume:
- the free enthalpy of Gibbs for the systems with pressure and temperature constants:
The process leading to thermodynamic balance is called Thermalization. An example is a system of particles in interaction isolated from any external influence. While interacting, the particles will exchange energy and moment between them, and will reach a state where the total statistics of the system will remain invariant in time.
Definitions
- Two systems are in thermal balance when them Température is identical.
- Two systems are in mechanical balance when them Pression is identical.
- Two systems are in balance diffusive (or chemical) when chemical Potentiel is identical for them.
Equilibrium conditions
- For a completely isolated system, ΔS = 0 with balance.
- For a system at constant temperature and volume, ΔF = 0 with balance.
- For a system at temperature and pressure constants, ΔG = 0 with balance.
These relations can be given by considering the differential forms of the thermodynamic Potentiels.
Balance thermal
Thermal balance is reached when the observable macroscopic ones of a system ceased varying with time. For example, a Perfect gas whose function of distribution was stabilized according to some Statistique of Maxwell-Boltzmann is in thermal balance. This makes it possible to allot a temperature and a single pressure to the whole of the system.
Balance thermodynamic room
It is useful to differentiate total and local thermodynamic balance. In thermodynamics, the exchanges inside a system and with outside are controlled by intensive parameters. For example, the Temperature control heat transfers. The total thermodynamic balance (ETG) means that these intensive parameters are homogeneous in all the system, while the local thermodynamic balance (LTE) means that these parameters can vary in space and time, but that this variation is so slow that for any point, one supposes that there exists a vicinity balances some around this point.If the description of the system supposes very important variations of these intensive parameters, the assumptions made to define these intensive parameters are not valid any more and the system will be neither in total balance, nor in local balance. For example, a particle needs to carry out of a certain number of collisions in order to arrive at balance with its environment. If the average distance that she will have traversed moves it out of the vicinity which she tried to enter in balance, she will never arrive at balance, and there will be no LTE. The temperature is, by definition, proportional to energy interns average of a vicinity in balance. Considering there is no more vicinity in balance, the concept even of temperature does not have here any more a direction.
The concept of local thermodynamic balance applies only to massive particles. In a radiant gas, the emitted and absorptive photons do not need to be in thermodynamic balance between them or with their environment so that there is a LTE.
For example, a LTE exists in water glass containing an ice floe melting. The temperature in glass can be defined in any point, but is lower close to the ice floe. The energy of the particles close to the ice floe will be distributed according to a distribution of Maxwell-Boltzmann for a certain temperature. The energy of particles located in another point will follow a distribution of Maxwell-Boltzmann for a different temperature.
Local thermodynamic balance is not a steady balance, except if it is maintained by exchanges between the system and outside. For example, it could be maintained in our water glass while adding to it of the ice progressively to compensate for fusion. The Phénomènes of transport are processes which lead of one of local balance to a total balance. To return to our example, the diffusion of heat will lead our water glass to the total balance, for which the temperature will be identical in any point.
Related articles
Simple: Thermodynamic equilibrium
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