Figure (a)shows two concentric conducting shells .Some charge Q1 is given to the outer shell.No charge is developed on the inner shell.I want to know why?When charge is on inner sphere ,charges also develop on outer shell then why not charge develops on inner shell when there are charges on outer shell?But when we see (b)we get to know there is a case when inner shell does develop charges due to charges present on outer shell. I want to know when charges develop on inner shell due to charges on outer shell and when they don't? I hope it's not confusing! In the figure (b),the inner shell is earthed and hence some charges Q2 is developed on it so that it's potential becomes zero. I did not understand the underlined sentence. My understanding(may be wrong):I know earthed conductor is always at zero potential due to flow of charge either from the earth to conductor or from conductor to earth.So that means charge has to be there on the earthed shell in order to be at zero potential so that charges (if negative)can flow to the earth or charges(if positive)get neutralized by flow of negative charges from earth to conductor.But then ,both of these conditions result in zero charges but that was the case initially there was no charge on the inner shell (see a)then how for being at zero potential Q2 has to be there on the inner shell?in other words how presence of Q2 on inner shell results in zero potential of it?