question_answer

An electric current passes through a long straight copper wire. At a distance 5cm from the straight wire, the magnetic field is B, The magnetic field at 20cm from the straight wire would be:

A) \[\frac{B}{6}\]                          

B) \[\frac{B}{4}\]     

C) \[\frac{B}{3}\]                          

D) \[\frac{B}{2}\]

Correct Answer: B

Solution :

Magnetic field due to straight wire at point A \[{{\Beta }_{\Alpha }}=\Beta =\frac{{{\mu }_{\operatorname{o}}}\operatorname{I}}{2\pi \left( 5\times {{10}^{-2}} \right)}\]             ?.(1) \[{{\Beta }_{\operatorname{B}}}=\frac{{{\mu }_{\operatorname{o}}}\operatorname{I}}{2\pi \times \left( 20\times {{10}^{-2}} \right)}\]                      ?.(2) by (1) & (2) \[{{\Beta }_{\operatorname{B}}}=\frac{B}{4}\]