Question

An electron gun with its collector at a potential of 100 V fires out electrons in a spherical bulb containing hydrogen gas at low pressure (~10^-2 mm of Hg). A magnetic field of 2.83 x 10^-4 T curves the path of the electrons in a circular orbit of radius 12.0 cm. (The path can be viewed because the gas ions in the path focus the beam by attracting electrons, and emitting light by electron capture; this method is known as the ‘fine beam tube method). Determine e/m from the data.

Answer 1

V = 100 V
B = 2.83 × 10^-4 T
r = 12 cm = 12 x 10^-2 m
Due to the applied potential difference between the emitter and the collector, an emitted electron gains kinetic energy which is given by,

When a magnetic field is applied normal to the path of electron, the force due to the filed makes the electron to move along the circular path. The radius of the circular path is given by,