CGPET 2015 Solved Paper

Resistance of a wire is given by
R=

ρL

A

where, ρ = resistivity
L= length of the wire
A = area of cross - section of the wire
Equating initial and final volumes of the wire, we can write
AL=A′L′=A′×(2L)
⇒A′=

A

2

∴

R′

R

=

ρL′∕A′

ρL∕A

=(

L′

L

)(

A′

A

)
= (

2L

L

)(

A

A∕2

)=4
⇒R′=4R=4(5)=20Ω