Chapter 28, Problem 72P

A beam of electrons whose kinetic energy is K emerges from a thin-foil “window” at the end of an accelerator tube. A metal plate at distance d from this window is perpendicular to the direction of the emerging beam (Fig. 28-53). (a) Show that we can prevent the beam from hitting the plate if we apply a uniform magnetic field such that

$B\le \sqrt{\frac{2mK}{e^2{d}^2}}$

in which m and e are the electron mass and charge, (b) How should $\stackrel{\to }{\text{B}}$ be oriented?

Figure 28-53 Question 72.