Question

An electron falls from rest through a vertical distance $h$ in a uniform and vertically upward directed electric field $E$. The direction of electrio field is now reversed, keeping its magnitude the same. A proton is allowed to fall from rest in it through the same vertical distance $h$. The time of fall of the electron, in comparison to the time of fall of the proton is

• A. equal
• B. smaller
• C. 10 times greater
• D. 5 times greator

Answer 1

Answer: (B)

$h=\frac{1}{2}\frac{eE}{m}{t}^2$
$\therefore t=\sqrt{\frac{2hm}{eE}}$
$\therefore t\propto \sqrt{m}$ as ‘ $e$’ is same for electron and proton.
$\therefore$ Electron has smaller mass so it will take smaller time.