Question

the wire of length L and resistance R is stretched so that its radius of cross section is halved what is it's new resistance?​

Answer 1

Answer:

The wire of length L and resistance R is stretched so that it's radius of cross section is halved. what is it new resistance ...

Explanation:

Resistance should be doubled since resistance is directly proportional to area of cross section

Answer 2

Answer:

R = ρ*l/A

where R is resistance in ohm, ρ is resistivity in ohm-meter, l is length in meter & A is cross sectional area in m^2

Since the current becomes ‘2l’ & the ‘A’ becomes ‘A/2’, so the new resistance will be,

R’ = ρ*2l/(A/2)

or, R’ = ρ*2*2*l/A

or, R’ = 4*ρ*l/A

or, R’ = 4R

So the new resistance will be 4 times than previous.

Coming to your second question, Resistivity cannot be changed for a same material. It is constant term. Every material has their own fixed resistivity.