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

The magnetic field due to the straight wire at a distance r is given by
`B = (mu_(0)i)/(2pi r)`
or `B prop (1)/(r) or (B_(2))/(B_(1)) = (r_(1))/(r_(2))`
`therefore" "(B_(1))/(B) = (5)/(20) = (1)/(4) or B_(2) = (B)/(4)`