A) \[4B\]
B) \[B/4\]
C) \[~B/2\]
D) \[2B\]
Correct Answer: A
Solution :
[a] Let \[I\]be current and I be the length of the wire. For 1st case: \[B=\frac{{{\mu }_{0}}In}{2r}=\frac{{{\mu }_{0}}I\times \pi }{l}\]where\[2\pi r=l\] and n = 1 For \[II\,nd\] Case: \[l=2(2\pi r')\Rightarrow r'=\frac{l}{4\pi }\] \[B'=\frac{{{\mu }_{0}}nI}{2r'}=\frac{{{\mu }_{0}}2I}{2\frac{l}{4\pi }}=\frac{4{{\mu }_{0}}I\pi }{l}=4B\]
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