Saturday, August 7, 2010

Measuring the Molecular Velocity Distribution

Just a quick question (or few).
It’s in regard to the experiment described in 3.2.2 where the distribution of molecular velocities is measured using a box of gas with a pinhole, a series of rotating discs and a detector.

It says that only molecules with the selected speed can pass through to the detector. What happens to the molecules that escape through the pinhole but aren’t at the selected speed? Do they bounce straight back into the box?
I would assume that in order for this to happen the pinhole would have to be small enough so that the molecules passed through at an orientation perpendicular to the discs. But wouldn’t that restrict the ‘randomness’ of the experiment? Or do they hold the experiment over such a large time to accommodate this?
Alternatively are the molecules just lost into the vacuum if they are not at the correct velocity? And if this does happen wouldn’t that also skew the obtained results?
I must be missing something.


  1. Today I was reading a paper that discusses spectroscopy experiments and it got me thinking that I tended to view instruments as a black box without physical reality and associated errors. The issues you raise would add to the error or uncertainty of the measurements - such as molecules lost to vacuum and molecules trapped between the two rotating wheels. If you can quantify and account for the error there is no problem with this. If the gas is kept at a low enough concentration(pressure?) this should help eliminate some of the error as it would minimise unwanted interactions.

  2. To even add more complexity... how thick are the box walls? How many times does a particle bounce in the hole? Are the walls of the hole bore elastic?

    Do these details matter if the distribution of velocities along orthogonal directions is independent?

    How would you change the experiment to test the independence of distributions in the x,y & z directions?