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

Four charges are arranged at the corners of a square $ABCD$, as shown. The force on a $+ve$ charge kept at the centre of the square is

Why Coulomb’s law is associated with Newton’s $3^{rd}$ law ?

There are two metallic spheres of same radii but one is solid and the other is hollow, then

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 electrons each are fixed at a distance $'2d'$. A third charge proton placed at the midpoint is displaced slightly by a distance $x ( x << d )$ perpendicular to the line joining the two fixed charges. Proton will execute simple harmonic motion having angular frequency : $( m =$ mass of charged particle)