Where is w=-nRT ln V2/V1 come from?

W= Fs
P = F/A   so   F = PA
so W= PAs
Because s= distance and A = Area so As= V and
because Pressure from surrounding is constant so

dW= -P dV (In physics maybe dW = P dV ; up to consider from surrounding or system.)           next step –>

P = n R T / V —> W = – ∫ (n R T / V) dV = – n R T ln (Vf / Vi).
integral of 1/v * dv = ln V
dln(x)/dx = 1/x

Since T is constant, Pi Vi = Pf Vf
(Vf / Vi) = (pi / pf).

W = n R T ln (Vf / Vi) = n R T ln (pi / pf)

Credit:
http://en.wikiversity.org/wiki/Thermodynamics/The_First_Law_Of_Thermodynamics
http://hubpages.com/hub/MECHANICAL-ENGINEERING-Thermodynamics—PV-Work-compression-expansion-work
http://www.physicsforums.com/showthread.php?t=424175

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