An electric current passes through a long straight wire. At a distance 5 cm from the wire, the magnetic field is B. The field at 20 cm from the wire would be

Options

A. 2B
B. `B/4`
C. `B/2`
D. B

Answer

B

Explanation

Magnetic field due to straight wire, `B=(mu_(0)i)/(2pir)`
`(B_(1))/(B_(2))=(r_(2))/(r_(1))`
`implies (B)/(B_(2))=20/5`
`thereforeB_(2)=B/4`