Electrons in a beam are accelerated from rest through a potential difference 0V. The beam enters an experimental chamber through a small hole. As shown in Figure P29.59, the electron velocity vectors lie within a narrow cone of half angle ǿ oriented along the beam axis. We wish to use a uniform magnetic field directed parallel to the axis to focus the beam, so that all of the electrons can pass through a small exit port on the opposite side of the chamber after they travel the length d of the chamber. What is the required magnitude of the magnetic field? Hint: Because every electron passes through the same potential difference and the angle ǿ is small, they all require the same time interval to travel the axial distance d.

Exi port Entrance port