Question

a hollow metal sphere A of radius 6 cm is uniformly charged with 10 micro coulomb​​.Another hollow metal sphere B of radius 3 cm is uniformly charged with 6 micro coulomb.A and B are connected externally by a wire. Now from where to where does the charge flow?

Answer 1

A hollow metal sphere with uniform charge, behaves as if the entire charge were concentrated at its centre; and since the electric field is zero inside, the electric potential is the same from surface to centre. 

==> V = k * q / r (V - potential, q - charge, r - radius)
VA =  k * 10 / 6 & VB = k * 6 / 3  ==> VA < VB. 

Now, when the two spheres are connected by a wire, charges flow until their potentials become equal; charges that flow through the wire are free electrons and free electrons will travel from lower potential to higher potential i.e, from sphere A to B.

Note: However, conventional current flows from B to A.