question_answer

A wire carrying current I and other carrying 2I in the same direction produces a magnetic field B at the mid point. What will be the field when 2I wire is switched off    [AFMC 2005]

A)            B / 2                                         

B)            2B

C)            B    

D)            4B

Correct Answer: C

Solution :

           When two parallel conductors carrying current I and 2I in same direction, then magnetic field at the midpoint is \[B=\frac{{{\mu }_{0}}2l}{2\pi r}-\frac{{{\mu }_{0}}I}{2\pi r}=\frac{{{\mu }_{0}}I}{2\pi r}\] When current 2I is switched off then magnetic field due to conductor carrying current I is \[B=\frac{{{\mu }_{0}}I}{2\pi r}\].