I had been reading Feynmann lectures , and in it he has shown an argument which proves that E field inside an empty cavity of a conductor is zerK. Now he says a similar argument can be used to show that if there is some charge in a cavity of a conductor than the field outside must be zero. Electrostatic shielding works both ways. Doubt: But then if we consider a gaussian surface containing the conductor , then the net charge is not zero => integral(E.da) is non zero, but E is zero. HOW?