Two positive charges of $20$ $coulomb$ and $Q\;coulomb$ are situated at a distance of $60\,cm$. The neutral point between them is at a distance of $20\,cm$ from the $20\,coulomb$ charge. Charge $Q$ is.....$C$

Two charges of equal magnitudes and at a distance $r$ exert a force $F$ on each other. If the charges are halved and distance between them is doubled, then the new force acting on each charge is

Three points charges are placed at the corners of an equilateral triangle of side $L$ as shown in the figure.

Two identical conducting spheres with negligible volume have $2.1\, nC$ and $-0.1\, nC$ charges, respectively. They are brought into contact and then separated by a distance of $0.5 \,m$. The electrostatic force acting between the spheres is $.......... \, \times 10^{-9} \,N$

[Given : $4 \pi \varepsilon_{0}=\frac{1}{9 \times 10^{9}} SI$ unit]

Point charge $q$ moves from point $P$ to point $S$ along the path $PQRS$ (figure shown) in a uniform electric field $E$ pointing coparallel to the positive direction of the $X - $axis. The coordinates of the points $P,\,Q,\,R$ and $S$ are $(a,\,b,\,0),\;(2a,\,0,\,0),\;(a,\, - b,\,0)$ and $(0,\,0,\,0)$ respectively. The work done by the field in the above process is given by the expression

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.....$dynes$