A point charge of $40$ stat coulomb is placed $2$ $cm$ in front of an earthed metallic plane plate of large size. Then the force of attraction on the point charge is…..$dynes$

When the distance between the charged particles is halved, the force between them becomes

$ABC$ is a right angled triangle in which $AB = 3\,cm$ and $BC = 4\,cm$. And  $\angle ABC = \pi /2$. The three charges $ + 15,\; + 12$ and $ – 20\,e.s.u.$ are placed respectively on $A$, $B$ and $C$. The force acting on $B$ is…….$dynes$

In hydrogen like system the ratio of coulombian force and gravitational force between an electron and a proton is in the order of:

Two small spheres each of mass $10 \,mg$ are suspended from a point by threads $0.5 \,m$ long. They are equally charged and repel each other to a distance of $0.20 \,m$. The charge on each of the sphere is $\frac{ a }{21} \times 10^{-8} \, C$. The value of $a$ will be …… .

$\left[\right.$ Given $\left.g=10 \,ms ^{-2}\right]$