Two point charges of $+2\,\mu C$ and $+6\,\mu C$ repel each other with a force of $12\,N$. If each is given an additional charge of $-4\,\mu C$,then the force will become:

In a medium,the force of attraction between two point charges,distance $d$ apart,is $F$. What distance apart should these point charges be kept in the same medium,so that the force between them becomes $16F$?

$(a)$ In a quark model of elementary particles,a neutron is made of one up quark [ charge $\frac{2}{3}e$ ] and two down quarks [ charges $-\frac{1}{3}e$ ]. Assume that they have a triangle configuration with side length of the order of ${10^{ - 15}} \ m$. Calculate the electrostatic potential energy of the neutron and compare it with its mass $939 \ MeV$. $(b)$ Repeat the above exercise for a proton which is made of two up and one down quark.

How did Coulomb determine the law of electric force between two point charges?

Assertion : The Coulomb force is the dominating force in the universe.
Reason : The Coulomb force is weaker than the gravitational force.

Two identical conducting spheres having unequal positive charges $q_1$ and $q_2$ are separated by a distance $r$. If they are made to touch each other and then separated again to the same distance $r$,the electrostatic force between the spheres in this case will be (neglect induction of charges):