✔ 最佳答案
rms speed is not the same as average speed. The rms speed is related to the kinetic energy of gas molecules, but average speed is not.
The pressure of a gas, in microscopic point of view, is the force exerted onto a surface (container walls) when the gas molecules perform elastic impact with the surface. It can be shown from kinetic theory that, the force F (or pressure, which is the force per unit area) is proportional to the square of the rms speed..
When a gas expands so that its volume is doubled, because the pressure remains constant, the gas temperature will be doubled.
[PV = nRT, P is kept constant, n and R are constants, V is thus proportional to T]But molecular kinetic energy is proportional to temperature, doubling the temperature implies doubling the molecular kinetic energy. The mean-square-speed (i.e. square of the rms speed) is also doubled. The rms speed is thus not doubled (it is only square-root [2] of the original rms speed).
SInce average speed increases with rms speed, the rms speed is not doubled indicates that the average speed is also not doubled.
