KVPY Sample Paper KVPY Stream-SX Model Paper-28

  • question_answer
    The electric field of a plane polarized electromagnetic wave in free space at time\[t=0\]is given by an expression \[\overrightarrow{E}(x,y)=10\hat{j}\,cos[(6x+8z)]\] The magnetic field \[\overrightarrow{B}(x,z,t)\]is given by: (c is the velocity of light)

    A) \[\frac{1}{c}(6\hat{k}+8\hat{i})cos[(6x-8z+10ct)]\]

    B) \[\frac{1}{c}(6\hat{k}-8\hat{i})cos[(6x-8z-10ct)]\]

    C) \[\frac{1}{c}(6\hat{k}+8\hat{i})cos[(6x+8z-10ct)]\]

    D) None of these

    Correct Answer: D

    Solution :

    \[\overrightarrow{E}=10\hat{j}\cos (6x+8z-10ct)\]
    \[{{B}_{o}}=\frac{{{E}_{o}}}{C}=\frac{10}{C}\]
    \[W=10C\]
    \[\because \]       \[\hat{E}\times \hat{B}=\hat{C}\]
    \[\left| \begin{matrix}    {\hat{i}} & {\hat{j}} & {\hat{k}}  \\    0 & 1 & 0  \\    {{B}_{x}} & {{B}_{y}} & {{B}_{z}}  \\ \end{matrix} \right|=\frac{6\hat{i}+8\hat{j}}{10}\]\[\Rightarrow \]\[{{B}_{z}}\hat{i}-0\hat{j}-{{B}_{x}}\hat{k}=\frac{3}{5}\hat{i}+\frac{4}{5}\hat{j}\]
                \[{{B}_{z}}=\frac{3}{5},{{B}_{y}}=0,{{B}_{z}}=\frac{4}{5}\]
    \[\therefore \]\[\overrightarrow{B}=\frac{1}{C}(-8\hat{i}+6\hat{k})cos(6x+8z+10ct)\]


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