Write the limitations of Coulomb's law.

Two identical particles of mass $m$ and charge $q$ are shot at each other from a very great distance with an initial speed $v$. The distance of closest approach of these charges is

The force between two point charges $q_1$ and $q_2$ placed in a vacuum at a distance $r \ cm$ apart is $F$. The force between them when placed in a medium having a dielectric constant $K=5$ at a distance $r/5 \ cm$ apart will be:

Four charges are arranged at the corners of a square $ABCD$,as shown in the adjoining figure. The force on the charge kept at the centre $O$ is

The ratio of Coulomb's electrostatic force to the gravitational force between an electron and a proton separated by some distance is $2.4 \times 10^{39}$. The ratio of the proportionality constant $K = \frac{1}{4 \pi \varepsilon_0}$ to the gravitational constant $G$ is nearly (Given that the charge of the proton and electron each $= 1.6 \times 10^{-19} \; C$,the mass of the electron $= 9.11 \times 10^{-31} \; kg$,the mass of the proton $= 1.67 \times 10^{-27} \; kg$):

Two small spheres,each having an equal positive charge $Q$ (Coulomb),are suspended by two insulating strings of equal length $L$ (metre) from a rigid hook. The whole setup is taken into a satellite where there is no gravity. The two balls are now held by electrostatic forces in a horizontal position. The tension in each string is then: