Electric Charges and Fields - Result Question 15
15. An electron falls from rest through a vertical distance $h$ in a uniform and vertically upward directed electric field $E$. The direction of electric 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
[2018]
(a) smaller
(b) 5 times greater
(c) equal
(d) 10 times greater
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Answer:
Correct Answer: 15. (a)
Solution:
- (a) As we know, $F=qE=ma$
$\Rightarrow a=\frac{q E}{m}$
$h=\frac{1 q E}{2 m} t^{2}$
$\therefore \quad t=\sqrt{\frac{2 h m}{q E}}$
i.e., time $t \propto \sqrt{m}$ as ’ $q$ ’ is same for electron and proton.
Since, electron has smaller mass so it will take smaller time.
