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) \[\frac{B}{2}\]         

B) \[y=A\sin (Bx+Ct+D)\]        

C) \[[{{m}^{0}}{{L}^{-1}}{{T}^{0}}]\]      

D) B

Correct Answer: B

Solution :

                For a long straight wire, the magnetic field. \[\frac{1}{3}M{{L}^{2}}\] when,    r = 5cm \[\frac{3}{2}M{{L}^{2}}\]                              ?.... (i)   Similarly, when r = 20 cm \[\frac{3}{4}M{{L}^{2}}\] From Eqs. (i) and (ii), we get \[M{{L}^{2}}\]