Question

Two small spheres of masses `M_(1)`and `M_(2)` are suspended by weightless insulating threads of lengths `L_(1)` and `L_(2)`. The spheres carry charges `Q_(1)` and `Q_(2)` respectively. The spheres are suspended such that they are in level with one another and the threads are inclined to the vertical at angles `theta_(1)` and `theta_(2)`respectively . Which one of the following conditions is essential for `theta_(1) = theta_(2)` ?
A. If `m_(1)=m_(2)`
B. `|q_(1)|=|q_(2)|`
C. `l_(1)=l_(2)`
D. `(q_(1))/(m_(1))=(q_(2))/(m_(2))`

Answer 1

Correct Answer - a
Since both the small spheres are at same horizontal level, the electrostatic forces on both spheres are in horizontal direction. The `FBD` of left sphere is shown in figure

The sphere is in equilibrium
`T cos theta= mg`....(i)
and `T sin theta= F_(E )`....(ii)
from (i) and (ii) , `tan theta=(F_(E ))/(mg)`
The magnitude of electrostatic force on each sphere is same irrespective of its charge
for `theta_(1)=theta_(2)` the necessary condition is `m_(1)=m_(2)`