Question

A wire have resistance =10 ohm and if its diameter is doubled what is new resistance by how much times it decreases or increases

Answer 1

Resistance= Resistivity x length/area

When the wire is stretched to double the length , the area of cross section gets reduced to half.

So

New Resistance = Resistivity x 2length/area/2

I.e,

New resistance = (Resistivity x area/length)x4

i.e, New resistance = Resistance x 4

So when the wire is stretched, the resistance multiplies by four times.