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In [[thermodynamics]], a '''phase transition''' is the transformation of a thermodynamic system from one [[phase]] to another.  

In [[thermodynamics]], a '''phase transition''' is the transformation of a thermodynamic system from one [[phase]] to another. At phase-transition point, physical properties may undergo abrupt change- for instance, volume of the two phases may be vastly different. As an example imagine transition of liquid water into vapour at boiling point.   

 

At phase-transition point, physical properties may undergo abrupt change- for instance, volume of the two phases may be vastly different.  

As an example imagine transition of liquid water into vapour at boiling point.   

   

   

In the English vernacular, the term is most commonly used to describe transitions between [[solid]], [[liquid]] and [[gas]]eous [[states of matter]], in rare cases including [[Plasma (physics)|plasma]].

In the English vernacular, the term is most commonly used to describe transitions between [[solid]], [[liquid]] and [[gas]]eous [[states of matter]], in rare cases including [[Plasma (physics)|plasma]]. Phase transitions happen when the [[Thermodynamic free energy|free energy]] of a system is [[analytic function|non-analytic]] for some choice of thermodynamic variables - see [[phases]]. This non-analyticity generally stems from the interactions of an extremely large number of particles in a system, and does not appear in systems that are too small.

 

Phase transitions happen when the [[Thermodynamic free energy|free energy]] of a system is [[analytic function|non-analytic]] for some choice of thermodynamic variables - see [[phases]]. This non-analyticity generally stems from the interactions of an extremely large number of particles in a system, and does not appear in systems that are too small.

To put it simply, at phase-transition point (for instance, [[boiling point]] for water) the two phases of water - [[liquid]] and [[vapour]] have identical free energies and therefore are equally likely to exist. Below the boiling point, liquid-water is more stable state of the two. At [[boiling point]] [[liquid]] and [[vapour]] are equally stable and above boiling point [[vapour]] is more stable than liquid state of [[water]].

To put it simply, at phase-transition point (for instance, [[boiling point]] for water) the two phases of water - [[liquid]] and [[vapour]] have identical free energies and therefore are equally likely to exist. Below the boiling point, liquid-water is more stable state of the two. At [[boiling point]] [[liquid]] and [[vapour]] are equally stable and above boiling point [[vapour]] is more stable than liquid state of [[water]].

== Magnetic phases ==

== Magnetic phases ==

Often also ''magnetic'' phases are used as the basis of a theory, and for introductory motivation. However, usually these are similar to the  well-known liquid (ferromagnetic) or gaseous  paramagnetic) phases, as can be seen by the two equivalent interpretations, the ''magnetic'' one ("up" or "down" spins) or the ''lattice-gas'' interpretation ("occupied" or "unoccupied" sites) of a prominent binary model, the [[Ising model]].  

Often also ''magnetic'' phases are used as the basis of a theory, and for introductory motivation. However, usually these are similar to the  well-known liquid (ferromagnetic) or gaseous  paramagnetic) phases, as can be seen by the two equivalent interpretations, the ''magnetic'' one ("up" or "down" spins) or the ''lattice-gas'' interpretation ("occupied" or "unoccupied" sites) of a prominent binary model, the [[Ising model]].