Question

http://openstudy.com/study#/updates/5002de0ee4b0848ddd66d233 electricity question pls help

Answer 1

.If you are calculating the potential an point a, you must also take into account all the charges in the system.
Consider a spherical Gaussian surface with radius just some infinitesimal amount larger than the given sphere.The flux is zero since there is no net charge enclosed inside the sphere.So the field at the surface is zero.Thus the values of the potentials at the surfaces are only due to the point charge that is outside the sphere.So Va=Vb=Vc.
This works if the sphere is sufficiently small in size.If the sphere is too large,then the distribution of the charges change and it will not be consistent with the above answer.Hope this helps.
-kartiksriramk