question_answer

A gas does work during adiabatic expansion. What is the source of mechanical energy so produced?

Answer:

                By first law of thermodynamics,                 \[\Delta Q=\Delta U+\Delta W.\] But for an adiabatic process, \[\Delta Q=0,\]so\[\Delta W=-\Delta U\]. Thus the source of energy required for doing mechanical work during adiabatic expansion is the internal energy of the gas itself.