What is Maxwell's Principle (Thermodynamics)?

[FaNgS]

Homework Statement

I'm trying to calculate the saturation pressure and the densities of a liquid and vapor using an equation of state. I figured out how to do it and know how to proceed except that I don't understand Maxwell's Principle which is stated in number 2 below.

Homework Equations

∫P.dV - Psat (Vg - Vl) = 0
integration limits of the ∫P.dV are from Vl to Vg

Where P is the pressure,
Psat is the saturation pressure,
Vg is the gas volume,
Vl is the liquid volume

The Attempt at a Solution

I don't understand how principle is derived. The only equation which comes to mind that relates Pressure and Saturation Pressure is Raoult's Law which is y*P=x*Psat (y is the gas mole fraction and x is the liquid mole fraction).

I would also appreciate it if I can be guided to some references which explains this prinicple, the book lacks this principle.

 

[Greg Bernhardt]

Maxwell's principle is a physical principle that states that any change in the electric field is accompanied by a change in the magnetic field, and vice versa.