AFMC AFMC Solved Paper-2001

  • question_answer
    A long vertical current carrying wire produces a neutral point with the earth.  Magnetic field at a distance of 5cm from the wire. If the horizontal component of the earth's magnetic induction is 0.18 gauss, then the current in the wire is :

    A) 0.036 A                                

    B) 0.9 A

    C) 0.45 A                                   

    D) 4.5 A

    Correct Answer: D

    Solution :

    Let due to be a straight conductor carrying a current of i ampere, magnetic field at point is \[B=\frac{{{\mu }_{0}}}{4\pi }\frac{i}{R}(sino{{|}_{1}}+sino{{|}_{2}})\]                 For a conductor of infinite length, we have \[\text{o}{{\text{ }\!\!|\!\!\text{ }}_{1}}=\text{o}{{\text{ }\!\!|\!\!\text{ }}_{2}}={{90}^{o}}\] \[\therefore \]              \[B=\frac{{{\mu }_{0}}}{4\pi }\frac{2i}{R}\] Given, \[R=5\times {{10}^{-2}}m\] \[\therefore \]               \[B={{10}^{-7}}\left( \frac{2i}{5\times {{10}^{-2}}} \right)\] \[=(4i)\times {{10}^{-2}}\]gauss At neutral point, \[4i\times {{10}^{-2}}=0.18\] \[\Rightarrow \]   \[i=4.5\,A\]


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