Question

a wire of resistance 20 ohm is bent at 180° from middle and both parts are twisted. its new resistance is

Answer 1

resistane is given as

R = ρl/A

 

where 

ρ is the resistivity (stays constant)

l is the length

A is the cross-sectional area

 

now initially R = 20 ohms

 

but if we fold the wire on itself then

A' = 2A

l' = l/2

 

so, new resistance

R' = ρl'/A' = ρl/2/2A

or

R' = R/4

R' = 20/4 

R' = 5 ohms

so, the new resistance will be R' = 5 ohms