Question

Two ideal gas systems undergo reversible expansion starting from the same P and V. At the end of the expansion, the two systems have the same volume. The pressure in the system that has undergone adiabatic expansion is lower than that in the system that has undergone isothermal expansion. Explain this observation without using equations.

Answer 1

The work done in a system undergoing Adiabatic expansion generally comes through the lowering of Internal Energy of the system (U) and the temperature. In the system undergoing adiabatic expansion, all the work done must come through the lowering of ΔU, and therefore of the temperature. But, in case of isothermal expansion work done is carried out by the flow of heat across the boundary between the system and the surroundings.

That's the reason, pressure in the system is lowered in an adiabatic expansion rather than an isothermal expansion.