Suppose we have a large number of identicall paricles, very small in size. Any two of them at 10cm separation repel with a force of `3xx10^(-10)N`.
(a) If one of them is at 10 cm from a group (of very small size) of n others, how strongly do you expect it to be repelled?
(b) Suppose you measure the repulsion and find it `6 xx 10^(-6)N`. How many particles were there in the group?

Explanation

(a) Since the size of group is small, the group of n small charged particles must behave as single point charge, so that it can have a separation of 10cm from the particle in question. Obviously, Force on this particle due to the group of n particles is n times the force due to a single particle. Hence, force due to group of n particles is
`F=nxx3xx10^(-10)N`.
(b) Here, `F=6xx10^(-6)N`
`n=(F)/(3xx10^(-10))=(6xx10^(-6))/(3xx10^(-10))=2xx10^(4)`.