Question

The resistance of a wire is 20 Ohm. It is stretched so that its radius becomes half. The new resistance is :​

Answer 1

180Ω

Resistance, R=$\frac{ρl}{A}$

When length l increases three times, the cross section A will decease three times of previous. So length becomes 3l and cross section becomes A/3.

Now resistance becomes ,R′= $\frac{ρ3l}{ \pi{A/3}}$

=9R=9×20=180Ω