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We know from previous lectures :

Stability condition: Is this(4) |

- 1.
- If
*T*is large enough, the first term in (4) dominates--system is always stable - 2.
- If
*T*is small, first term wins only for and --system is unstable at intermediate*V*! - 3.
- Borderline case: just in
*one*point (critical point*K*)

Phases:

- 1.
- At
*T*>*T*_{c}one phase (fluid)--no phase transitions - 2.
- At
*T*<*T*_{c}two phases (liquid & gas)--first order phase transition - 3.
- At
*T*=*T*_{c}--second order phase transition

Equation for *T*_{c}:

Let us measure everything in critical units:

Van der Waals law:

This is same for all van der Waals gases! AllLet us determine binodal for van der Waals gas. In coexisting phases or

If , we have , or It means that dashed areas below are equal!© 1997

Tue Oct 7 22:16:36 EDT 1997