I had a query in th following question:

A given wire of resistece is stretched to double its length. What will be its new resistance?

How come the answer is 4 ?

ρ= RA/l

R = ρl/A

When length is doubled l =2l and area of cross section is reduced to half

ρ is constant for wire.

 

Thus,

ρ = R'(A/2)/(2l)

ρ= R'A/(4l)

Thus,

R' = 4(ρl/A)

R'= 4R 

 

Thus, when length is doubled then resistance becomes 4 times its original value.