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
A)2 B
B)B/4
C)B/2
D)B
Correct Answer:
B
Solution :
For a long straight wire, the magnetic field \[B=\frac{{{\mu }_{0}}2I}{4\pi r}\] When \[r=5\,cm\] \[\therefore \] \[B=\frac{{{\mu }_{0}}2I}{4\pi (5)}\] ?(i) Similarly, when r = 20 cm \[B'=\frac{{{\mu }_{0}}2I}{4\pi (20)}\] ?(ii) From Eqs. (i) and (ii), we get \[B'=\frac{B}{4}\]