In a Millikan's oil drop experiment the charge on an oil drop is calculated to be $6.35 \times {10^{ - 19}}C$. The number of excess electrons on the drop is
$3.9$
$4$
$4.2$
$6$
In an electron gun the control grid is given a negative potential relative to cathode in order to
The specific charge of an electron is
The ratio of momenta of an electron and an $\alpha - $particle which are accelerated from rest by a potential difference of $100 V$ is
When electron beam passes through an electric field, they gain kinetic energy. If the same beam passes through magnetic field, then