Question No. 8 and 9 are based on the following paragraph. Consider a block of conducting material of resistivity '$\rho$ ' shown in the figure. Current '$I$' enters at 'A' and leaves from 'D'. We apply superposition principle to find voltage ' $∆V$' 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 $E=\rho j$ , 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), (iii) for current '$I$' leaving 'D' and superpose results for 'A' and 'D'.