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Re[2]: magnetic lines like these?



From Ludwik Kowalski

have a related question (about plasma "pushed by a pressure difference").
uppose a mixture of two gases, such as He and Ar in thermal equilibrium,
s inside a long cylindrical tube. A thin alluminum window, at one end of
he tube, breakes and the mixture is ejected into the surrounding vacuum.
ach atom gains a non-random velocity component along the axis. Is the
verage non-random velocity component for the He atoms the same as for
?he Ar atoms?
Ludwik Kowalski


I think the thermal velocities of the atoms before the window breaks will
determine their velocity components along the axis of the tube after the
window breaks.

To simplify this analysis, I will assume ideal gases. Thus, I assume
that thermal equilibrium was established only by collisions between the
atoms and the walls of the tube. The gases are in thermal equilibrium
(at the same temperature) before the window breaks; thus, helium, being
the less massive atom, has the higher rms velocity. Suppose, at the
instant the window breaks, an atom is approaching the window. This atom
will leave the tube with the same velocity as before the breakage,
because no force is exerted on the atom. The window is gone and can
exert no force on the atom. In the absence of interatomic collisions,
other atoms will exert no force on the outgoing atom. By extension, the
velocities of all of the atoms leaving the tube will have the same
distribution as before they left the tube. But these velocities are
determined by the gas temperature at equilibrium. Therefore, helium,
having the higher rms velocity at equilibrium, will move out of the tube
at the higher average speed.