The Resistance of a Wire Varies Directly as Its Length $50$

Essay

The resistance of a wire varies directly as its length and inversely as the square of its diameter. A wire $50$ m long and $0.01$ m in diameter has a resistance of $25$ $\Omega$ . Write an equation to describe its resistance, and find the resistance of a wire made of the same material that is $20$ m long and has diameter $0.02$ m.