Question

An electron travelling west to east enters a chamber having a uniform magnetic field in a north to south direction. Specify the direction in which a uniform magnetic field should be set up to prevent the electron deflecting from its straight line path.

Answer 1

Under the action of electrostatic field, the electron will be deflected towards north (towards positive plate). It will remain undeflected, if force due to magnetic field is towards south. For \(\vec V\)  velocity from west to east, the expression for magnetic Lorentz force

\(\vec F_m = -e\ (\vec v \times \vec B)\)

or Fleming left hand rule tells that \(\vec B\)  should be applied along vertical and in downward direction.