In an aluminium (Al) bar of square cross section, a square hole is drilled and is filled with iron (Fe) as shown in the figure. The electrical resistivities of Al and Fe are `2.7 xx (10^-8) Omega m and 1.0 xx (10^-7) Omegam,` respectively. The electrical resistance between the two faces P and Q of the composite bar is

Options

A. `2475/64 mu Omega`
B. `1875/64 mu Omega`
C. `1875/49 mu Omega`
D. `2432/175 mu Omega`

Explanation

`R_(Al)=(pl)/A=(2.7 times 10^-8 times 50 times 10^-3)/((7^2-2^2) times 10^-6)=30 mu Omega, R_(Fe)=(1 times 10^-7 times 50 times 10^-3)/(4 times 10^-6)=`
Both the bars are in parallel , `R_(eq)=(R_(A1) times R_(Fe))/(R_(Al)+R_(Fe))=1875/64 mu Omega`