天同師兄,有個小問題

You have said in your quesion that the gas expands isothermally. That is, the temperature remains constant throughout the expansion. In the actual fact, the equation describing an isthermal change of a gas is Boyle's Law. This is done by setting the right hand side of the ideal gas equation to be a constant (as T remains constant throughout an isothermal process). The increases in volume leads to a reduction in pressure.

Notice that the use of the ideal gas equation is for quasi-static process that links the initial and final states of a gas. In reality, no process is quasi-static, the cooling of a gas under its own expansion and the inflow of heat to the gas occur at an instant, and for which the application of gas equation is not appropriate.