7. A metal sphere with a hollow center has a charge of -2q, initially all on the outer surface. An additional charge of +q is placed in the hollow center of the sphere. The charge on the outer surface of the sphere becomes: (Draw electric lines of field clearly.) Solution If a charge Q hollow is given in cavity of conductor than it will induce - Q hollow on inner wall of cavity and Q hollow on outer surface of cavity So +q will be added to outer surface of conductor HEnce net charge onouter surface of metal sphere Q= -2q+q = -q Electric field will be radially outward in cavity But radially inward outside the sphere . in the vicinity of sphere No field will be present .