# measured between B and CDirections : Consider a block of conducting material of resistivity   shown in the figure. Current I enters at A    and leaves from  D  . We apply superposition principle to find voltage 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   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 B Option 1) Option 2) Option 3) Option 4)

As we learnt in

If current density is uniform -

- wherein

Current density

Normal cross-section

Current is spread over an area       The current    is a surface current.

Current density, '

Option 1)

This option is incorrect.

Option 2)

This option is incorrect.

Option 3)

This option is incorrect.

Option 4)

This option is correct.

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