An electric current passes through a long straight copper wire. At a distance of 5 cm from the straight wire, the magnetic field is B. What is the magnetic field at a distance of 20 cm from the wire ?

Options

A. B/6
B. B/4
C. B/3
D. B/2

Answer

B

Explanation

Magnetic field at a distance r from the straight wire.
`B = (mu_(o)i)/(2pi r) " "B prop (1)/(r)`
`(B)/(B.) = (20)/(5) " "B. = (B)/(4)`