A copper wire of radius r and length l has a resistance of R A second copper wire with radius 2r and...........

General Science (CDS) Physics

A copper wire of radius r and length l has a resistance of R. A second copper wire with radius 2r and length l is taken and the two wires are joined in a parallel combination. The resultant resistance of the parallel combination of the two wires will be

5 R

5/4 R

4/5 R

R/5

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Explanation:

1 st copper wire: R = pl/A = pl/ πr2 2 nd copper wire: R’ = p’l’/A’ = pl/(4 πr2 ) = R/4 If joined in parallel, (1/Req) = (1/R) + (1/R’) = 1/R + 4/R 1/Req = 5/R Req = R/5