If a copper wire is stretched to make it 0.2% longer. Find the percentage change in its resistance.

Explanation

Resistance `R = (rhoL)/A`
`L xx A` = volume = V = const.
`therefore (DeltaL)/L + (DeltaA)/A =0, (DeltaL)/L = (DeltaA)/A` (numerically)
`R = (rhoL)/A`
`(DeltaR)/R = (DeltaL)/L + (DeltaA)/A = (2DeltaL)/L (2 xx 0.2) = 0.4%` increase.