Walker, Halliday & Resnick — Magnetic Fields: Problem 36

36 An electron is accelerated from rest through potential difference V and then enters a region of uniform magnetic field, where it undergoes uniform circular motion. Figure 28-37 gives the radius r of that motion versus \(V^{1/2}\). The vertical axis scale is set by \(r_s = 9.0\text{ mm}\), and the horizontal axis scale is set by \(V_s^{1/2} = 40.0\text{ V}^{1/2}\). What is the magnitude of the magnetic field?