Question

A wire of 12ohm resistance is stretched to double it's length.find out the new resistance of the wire..

Answer 1

As we know that ;

Resistance = Resistivity*length/area

Here we note that the resistance is directly proportional to length and inversely proportional to area of cross section .... 

Now a/c to question,;

Length of wire is double then area is reduced to half...

Now 

New resistance = resistivity*2length/area/2

New resistance =(resistivity*length/area)*4

New resistance = 4resistance