An oil drop of mass \( m \) carrying a charge \( +q \) is held stationary in air by an electric field \( E \). The electric field is correctly given by
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( E = mg/q \) acting upwards
The drop is stationary, so the net force on it is zero: the electric force \( qE \) must balance the weight \( mg \), which acts downwards.
\( \displaystyle qE = mg \;\Rightarrow\; E = \frac{mg}{q} \)
The electric force must act upwards. For a positive charge the force is in the direction of the field, so the field also acts upwards.