For current entering at A, the electric field at a distance r from A. is Consider a block of conducting material of resistivity r shown in the figure. Current I enters at A and leaves from D.We apply superposition principle to find voltage ΔV developed between B and C. The calculation is done in the following steps:
(i) Take current I entering from A and assume it to spread over a hemispherical surface in the block.
(ii) Calculate field E(r) at distance r from A by using Ohm’s law E = ρj, where j is the current per unit area at r.
(iii) From the r dependence of E(r), obtain the potential V(r)at r.
(iv) Repeat (i), (ii) and (iii) for current I leaving D and superpose results for A and D.
