JEE Main & Advanced Physics Electrostatics & Capacitance Question Bank Critical Thinking

  • question_answer
    An infinite number of electric charges each equal to 5 nano-coulomb (magnitude) are placed along X-axis at \[x=1\]cm, \[x=2\]cm, \[x=4\]cm \[x=8\]cm ???. and so on. In the setup if the consecutive charges have opposite sign, then the electric field in Newton/Coulomb at \[x=0\] is  \[\left( \frac{1}{4\pi {{\varepsilon }_{0}}}=9\times {{10}^{9}}N-{{m}^{2}}/{{c}^{2}} \right)\]                                  [EAMCET 2003]

    A)            \[12\times {{10}^{4}}\]

    B)                                      \[24\times {{10}^{4}}\]

    C)            \[36\times {{10}^{4}}\]    

    D)            \[48\times {{10}^{4}}\]

    Correct Answer: C

    Solution :

               \[E=\frac{1}{4\pi {{\varepsilon }_{0}}}.\left[ \frac{5\times {{10}^{-9}}}{{{(1\times {{10}^{-2}})}^{2}}}-\frac{5\times {{10}^{-9}}}{{{(2\times {{10}^{-2}})}^{2}}}+\frac{5\times {{10}^{-9}}}{{{(4\times {{10}^{-2}})}^{2}}} \right.\]\[\left. -\frac{(5\times {{10}^{-9}})}{{{(8\times {{10}^{-2}})}^{2}}}+..... \right]\] \[\Rightarrow E=\frac{9\times {{10}^{9}}\times 5\times {{10}^{-9}}}{{{10}^{-4}}}\left[ 1-\frac{1}{{{(2)}^{2}}}+\frac{1}{{{(4)}^{2}}}-\frac{1}{{{(8)}^{2}}}+... \right]\] \[\Rightarrow E=45\times {{10}^{4}}\left[ 1+\frac{1}{{{(4)}^{2}}}+\frac{1}{{{(16)}^{2}}}+... \right]\]\[-45\times {{10}^{4}}\left[ \frac{1}{{{(2)}^{2}}}+\frac{1}{{{(8)}^{2}}}+\frac{1}{{{(32)}^{2}}}+... \right]\] \[\Rightarrow E=45\times {{10}^{4}}\left[ \frac{1}{1-\frac{1}{16}} \right]-\frac{45\times 10{{\,}^{4}}}{(2){{\,}^{2}}}\left[ 1+\frac{1}{{{4}^{2}}}+\frac{1}{{{(16)}^{2}}}+.. \right]\] \[E=\text{ 48}\times \text{1}{{0}^{\text{4}}}\text{ 12}\times \text{1}{{0}^{\text{4}}}=\text{ 36}\times \text{1}{{0}^{\text{4}}}N/C\]


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