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Microstate (statistical mechanics)
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Everything about Microstate Statistical Mechanics totally explained

In statistical mechanics, a microstate describes a specific detailed microscopic configuration of a system, that the system visits in the course of its thermal fluctuations. In contrast, the macrostate of a system refers to its macroscopic properties such as its temperature and pressure. In statistical mechanics, a macrostate is characterized by a probability distribution on a certain ensemble of microstates.
   This distribution describes the probability of finding the system in a certain microstate as it's subject to thermal fluctuations.
   Let us now turn to the case of large systems: even if those systems are theoretically able to fluctuate between very different microstates, observing such a fluctuation becomes less and less likely as the size of the system increases. This makes up for the thermodynamic limit. In this limit, the microstates visited by a system during its fluctuations all have the same bulk (or macroscopic) properties.

Microscopic definitions of thermodynamic concepts

The definitions of this section link the thermodynamic properties of a system to its distribution on its ensemble (or set) of microstates. Note that all definitions and expressions of this section are valid even far away from thermodynamic equilibrium.
   In this article we'll consider a system which is distributed on an ensemble of N microstates. p_i is the probability associated to the microstate i, and E_i is its energy. Here microstates form a discrete set, which means we're working in quantum statistical mechanics, and E_i is an energy level of the system.

Internal energy

The internal energy is the mean of the system's energy » U = langle E angle = sum_^N E_i,dp_i So that » ~dU = delta W + delta Q

Examples:
Warning: the two above definitions of heat and work are among the few expressions of statistical mechanics where the sum corresponding to the quantum case can't be converted into an integral in the classical limit of a microstate continuum. The reason is that classical microstates are usually not defined in relation to a precise associated quantum microstate, which means that when work changes the energy associated to the energy levels of the system, the energy of classical microstates doesn't follow this change.

Further Information

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