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)muOmega`
B. `(1875)/(64)muOmega`
C. `(1875)/(49)muOmega`
D. `(2775)/(132)muOmega`

Answer

B

Explanation

`R_(Fe)=(rho_(Fe)xx50xx10^(-3))/((2xx10^(-3))^(2))=1250muOmega`
`R_(Al)=(rho_(Al)xx50xx10^(-3))/((49-4)xx10^(-6))=30muOmega`
`R_(eq)=(1250xx30)/(1280)=(1875)/(64)muOmega`