The electric field between the two spheres of a charged spherical condenser

Two charges $\mathrm{q}$ and $-3\mathrm{q}$ are placed fixed on $x-$ axis separated by distance $\mathrm{'d'}$. Where should a third charge $2\mathrm{q}$ be placed such that it will not experience any force ?

If two charges of $1$ coulomb each are placed $1 \,km$ apart, then the force between them will be ........... $N$

A paisa coin is made up of $\mathrm{Al - Mg}$ alloy and weighs $0.75\, g$ It is electrically neutral and contains equal amounts of positive and negative charge of magnitude $34.8$ $\mathrm{kC}$. Suppose that these equal charges were concentrated in two point charges separated by :

$(i)$ $1$ $\mathrm{cm}$ $(\sim \frac{1}{2} \times $ diagonal of the one paisa coin $)$

$(ii)$ $100\,\mathrm{m}$ $(\sim $ length of a long building $)$

$(iii)$ $10^6$ $\mathrm{m}$ (radius of the earth).

Find the force on each such point charge in each of the three cases. What do you conclude from these results ?

The distance between charges $5 \times {10^{ - 11}}\,C$ and $ - 2.7 \times {10^{ - 11}}\,C$ is $0.2\, m$. The distance at which a third charge should be placed in order that it will not experience any force along the line joining the two charges is......$m$

Two charges are at a distance $‘d’$ apart. If a copper plate (conducting medium) of thickness $\frac{d}{2}$ is placed between them, the effective force will be