A) \[7.16\times {{10}^{-7}}kg-{{m}^{2}}\]
B) \[14.32\times {{10}^{-7}}kg-{{m}^{2}}\]
C) \[5.58\times {{10}^{-7}}kg-{{m}^{2}}\]
D) None of these
Correct Answer: A
Solution :
Magnetic moment of bar magnet \[M=5\times {{10}^{-5}}\Omega -m\] \[B=8\pi \times {{10}^{-4}}T\] \[T=2\pi \sqrt{\frac{l}{MB}}\] \[{{T}^{2}}=4{{\pi }^{2}}\frac{l}{MB}\] \[l=\frac{{{T}^{2}}}{4{{\pi }^{2}}}MB\] \[=\frac{{{(15)}^{2}}}{4\times {{(3.14)}^{2}}}\times 5\times {{10}^{-5}}\times 8\pi \times {{10}^{-4}}\] \[=7.16\times {{10}^{-7}}kg-{{m}^{2}}\]
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