why does glass rod develop +ve charge when rubbed with silk cloth and plastic rod develop -ve charge?both should have developed either -ve or +charge.

conservation of charge

initial charge on system (rod+cloth) zero

for final to be zero one is +ve and other -ve

in the case of +ve charge electrons will be loosing..and in the case of negative charge electrons will be added. so that according to the conservation of charges total remains zero