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Problem 18 - Entrance Test

A wire of length L and area A has a resistance R. If the length is doubled and the area is halved, what is the new resistance?

Correct: D

Resistance R = ρL/A, where ρ is the resistivity. If L' = 2L and A' = A/2, then R' = ρ(2L)/(A/2) = 4ρL/A = 4R.