The strength of the magnetic field at a point `r` near a long straight current carrying wire is `B`. The field at a distance `r/2` will be

Options

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

Answer

B

Explanation

`B prop 1//r`
`B_(1)//B_(2) = r_(2)//r_(1) = (r//2)/(r )`
`therefore B_(2) = 2B_(1) = 2B`