Question 21: NCERT Solutions for 12th Class Physics: Chapter 11-Dual Nature of Radiation and Matter
(a) A monoenergetic electron beam with electron speed of 5.20 × 106 m s-1 is subjected to a magnetic field of 1.30 × 10-4 T, normal to the beam velocity. What is the radius of the circle traced by the beam, given e/m for electron equals 1.76 × 1011 C
kg-1.
(b) Is the formula you employ in (a) valid for calculating radius of the path of a 20 MeV electron beam? If not, in what way is it modified?
Solution:

The normal magnetic field provides necessary centripetal force to the electron beam so that it can follow a circular path. Thus Force on an electron = Centripetal force due to magnetic field on an electron
The formula for radius of circular path is not valid at very high energies because such high energy
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NCERT Solutions for 12th Class Physics: Chapter 11-Dual Nature of Radiation and Matter