In an electron gun,the control grid is kept at a negative potential with respect to the cathode because...

Cathode rays are

The ratio of thermionic currents $(I/I_0)$ for a metal when the temperature is slowly increased from $T_0$ to $T$ is shown in the figure. ($I$ and $I_0$ are currents at $T$ and $T_0$ respectively). Which curve correctly represents this relationship?

The number of secondary electrons emitted per number of primary electrons depends on:

Which of the following has the highest specific charge?

$(a)$ Estimate the speed with which electrons emitted from a heated emitter of an evacuated tube impinge on the collector maintained at a potential difference of $500\;V$ with respect to the emitter. Ignore the small initial speeds of the electrons. The specific charge of the electron,$i.e.$,the $e/m$ is given to be $1.76 \times 10^{11}\; C\; kg^{-1}$.
$(b)$ Use the same formula you employ in $(a)$ to obtain electron speed for a collector potential of $10\;MV$. Do you see what is wrong? In what way is the formula to be modified?