Question

electric charge is conserved in isolated system.why??​

Answer 1

Electric charges are conserved in a isolated system because of the structure the universe has with some symmetries. This basically states that the charge in a isolated system is equal at all points. This law is know as the Law of Conservation of Change. This law says that if a property that's particularly measured in an isolated system doesn't change if the system changes/evolves with time.

Answer 2

Charge is conserved for an isolated system means that the net charge of isolated system remains constant. Both balls will have now -3Q charge. (i.e. net -6Q). No matter what operation you would do in this isloated system of two balls net charge will always remain -6Q.