An early method testing Maxwell’s theoretical prediction for the distribution of molecular speeds is shown in Figure 8-34. In 1925 Otto Stern used a beam of Bi₂ molecules emitted from an oven at 850 K. The beam defined by slit S₁ was admitted into the interior of a rotating drum via slit S₂ in the drum wall. The identical bunches of molecules thus formed struck and adhered to a curved glass plate fixed to the interior drum wall, the fastest molecules striking near A, which was opposite S₂, the slowest near B, and the others in between depending on their speeds. The density of the molecular deposits along the glass plate was measured with a densitometer. The density (proportional to the number of molecules) plotted against distance along the glass plate (dependent on v) made possible determination of the speed distribution. If the drum is 10 cm in diameter and is rotating at 6250 rpm,

(a) Find the distance from A where molecules traveling at v_m, (v), and v_rms will strike. 

(b) The plot in (a) must be corrected slightly in order to be compared with Maxwell’s distribution equation. Why? 

(c) Would N₂ molecules work as well as Bi₂ molecules in this experiment? Why or why not?

Transcribed Image Text:

Drum A Glass plate S2 B W Oven