CEE Kerala Engineering CEE Kerala Engineering Solved Paper-2009

  • question_answer
    Two long parallel wires carry currents\[{{i}_{1}}\]and\[{{i}_{2}}\]such that\[{{i}_{1}}>{{i}_{2}}\]. When the currents are in the same direction, the magnetic field at a point midway between the wires is\[6\times {{10}^{-6}}T\]. If the direction of\[{{i}_{2}}\]is reversed, the field becomes\[3\times {{10}^{-5}}T.\]The ratio\[\frac{{{i}_{1}}}{{{i}_{2}}}\]is

    A)  \[\frac{1}{2}\]                  

    B)         2

    C)  \[\frac{2}{3}\]                  

    D)         \[\frac{3}{2}\]

    E)  \[\frac{1}{5}\]

    Correct Answer: D

    Solution :

    When the currents are in the same direction then magnetic field \[B=\frac{{{\mu }_{0}}}{4\pi }\times \frac{1}{d}[{{i}_{}}_{1}-{{i}_{2}}]\] \[6\times {{10}^{-6}}=\frac{{{\mu }_{0}}}{4\pi }\times \frac{2}{d}[{{i}_{1}}-{{i}_{2}}]\]           ...(i) When the currents are in the reversed direction then magnetic field                 \[{{B}_{2}}=\frac{{{\mu }_{0}}}{4\pi }\times \frac{2}{d}[{{i}_{1}}-(-{{i}_{2}})]\] Or           \[{{B}_{2}}=\frac{{{\mu }_{0}}}{4\pi }\times \frac{2}{d}[{{i}_{1}}+{{i}_{2}}]\] or            \[3\times {{10}^{-5}}=\frac{{{\mu }_{0}}}{4\pi }\times \frac{2}{d}[{{i}_{1}}+{{i}_{2}}]\]       ...(ii) Dividing Eq. (i) by Eq. (ii)                 \[\frac{{{i}_{1}}-{{i}_{2}}}{{{i}_{1}}+{{i}_{2}}}=\frac{6\times {{10}^{-6}}}{3\times {{10}^{-5}}}\] Or           \[\frac{{{i}_{1}}-{{i}_{2}}}{{{i}_{1}}+{{i}_{2}}}=\frac{2}{10}\]          Or           \[5{{i}_{1}}-5{{i}_{2}}={{i}_{1}}+{{i}_{2}}\] Or           \[4{{i}_{1}}=6{{i}_{2}}\] Or           \[\frac{{{i}_{1}}}{{{i}_{2}}}=\frac{6}{4}\] Or           \[\frac{{{i}_{1}}}{{{i}_{2}}}=\frac{3}{2}\]


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