17. 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 A.5 R B.5 R/4 C.4 R/5 D.R/5
Option D Solution: 1st copper wire: R = pl/A = pl/ πr2 2ndcopper wire: R’ = p’l’/A’ = pl/(4 πr2) = R/4 If joined in parallel, (1/Req) = (1/R) + (1/R’) = 1/R + 4/R = 5/R Req = R/5
