And here, in an isothermal expansion, The bath is putting back the energy that the gas is expanding or using to expand, and so the energy is flowing back into the gas through the environment in the isothermal expansion.

而在等温膨胀中，恒温环境将气体膨胀对外做功,而损失的能量又返还回来，在等温膨胀中,能量从环境,流向系统。

热力学与动力学课程节选 : 麻省理工公开课