Two point charges $\mathrm{q}_1$ and $\mathrm{q}_2$ are ' $l$ ' distance apart. If one of the charges is doubled and distance between them is halved. The magnitude of force becomes $n$ times, where $n$ is

' $n$ ' identical small spherical drops of water, each of radius ' $r$ ' and charged to the same potential ' v ' are combined to form a big drop. The potential of a big drop is

Three charges ' $+3 q$ ', ' $Q$ ' and ' $+q$ ' are placed in a straight line of length ' $l$ ' at points at distances $0, \frac{l}{2}$ and $l$ respectively. The value of Q in order to have the net force on +q to be zero, $\mathrm{Q}=\mathrm{xq}$. The value of $x$ is

Two point charges $+10 \mu \mathrm{C}$ and $4 \mu \mathrm{C}$ are placed 10 cm apart in air. The work required to be done to bring them 2 cm closer is

$$ \left(\frac{1}{4 \pi \varepsilon_0}=9 \times 10^9 \text { SI units }\right) $$