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 froe the wire would be

A)  25            

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     \[r=5\text{ }cm\] \[\therefore \] \[B=\frac{{{\mu }_{0}}2I}{4\pi (5)}\] ?.(i) Similarly, when \[r=20\text{ }cm\] \[B=\frac{{{\mu }_{0}}2I}{4\pi (20)}\] ?.(ii) From Eqs. (i) and (ii), we get \[B=\frac{B}{4}\]