question_answer

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)  2B                                         

B)  B/4

C)  B/2                                       

D)  B

Correct Answer: B

Solution :

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