Thermodynamics Understanding

[bluesky]

In the development of first law of thermodynamics,Sears and Salinger proceeds in the following way:

 

They have referred to a PV diagram where a gas has been taken from one eqlbm point a to another point b.They have considered an adiabatic free expansion from a to c,followed by an adiabatic reversible expansion from c to b.

Again they have taken another path where the gas undergoes an adiabatic reversible expansion from a to d,followed by an adiabatic free expansion from d to b.

 

Ultimately, they have said that the work in all adiabatic paths are equal between two states.

 

In this formulation they have neglected any dissipative work in the adiabatic free expansion process.(That is the process from a to c,from d to .They have mentioned this,but did not justify.

 

Why should we neglect the dissipative work in that path?Not referring to 1st law of thermodynamics please clarify what would be the problem if we do not neglect the work.All my friends are saying that since first law is correct,hence...

I do not want this type of answer.I need to understand in physical terms why in the adiabatic free expansion process dissipative work is not changing the configuration.