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\sect{Effusion of electrons from a box in magnetic field}

Electrons are in a given box, in the presence of a magnetic field
${B}$. We drill a hole in the box and measure the spin of the
emitting electrons, with velocity ${v<v'<v+dv}$.
Find ${\alpha\equiv \frac{\triangle N_{t}}{\triangle N_{v}}}$ using
${m,r,B,\beta\mu}$. Assume
${H_{1}=\frac{p^{2}}{2m}-rB\sigma_{z}}$.



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