JCECE Engineering JCECE Engineering Solved Paper-2015

  • question_answer
    Find the electric field vector at \[P(b,\,\,b,\,\,b)\] due to three infinitely long lines of charges along \[x,\,\,y\] and \[z-\]axes, respectively. The charge density, \[i.e.\] charge per unit length of each wire is\[\sigma \].

    A) \[\frac{\sigma }{\pi {{\varepsilon }_{0}}b}(\widehat{\mathbf{i}}+\widehat{\mathbf{j}}+\widehat{\mathbf{k}})\]               

    B) \[\frac{\sigma }{2\pi {{\varepsilon }_{0}}b}(\widehat{\mathbf{i}}+\widehat{\mathbf{j}}+\widehat{\mathbf{k}})\]

    C) \[\frac{\sigma }{\pi {{\varepsilon }_{0}}b}(\widehat{\mathbf{i}}+\widehat{\mathbf{j}}-\widehat{\mathbf{k}})\]               

    D)  \[\frac{\sigma }{2\pi {{\varepsilon }_{0}}b}(\widehat{\mathbf{i}}+\widehat{\mathbf{j}}-\widehat{\mathbf{k}})\]

    Correct Answer: B

    Solution :

    Electric field at\[P\], due to wire 3 only is \[{{\mathbf{E}}_{\mathbf{3}}}=\frac{\sigma }{2\pi {{\varepsilon }_{0}}{{({{b}^{2}}+{{b}^{2}})}^{1/2}}}(\widehat{\mathbf{i}}\cos {{45}^{o}}+\widehat{\mathbf{k}}\cos {{45}^{o}})\] Similarly, electric field due to wires 1 and 2                 \[{{\mathbf{E}}_{\mathbf{1}}}=\frac{\sigma }{2\sqrt{2}\pi {{\varepsilon }_{0}}b}\times \frac{1}{\sqrt{2}}(\widehat{\mathbf{j}}+\widehat{\mathbf{k}})\] and        \[{{\mathbf{E}}_{\mathbf{2}}}=\frac{\sigma }{2\sqrt{2}\pi {{\varepsilon }_{0}}b}\times \frac{1}{\sqrt{2}}(\widehat{\mathbf{i}}+\widehat{\mathbf{j}})\] \[{{\mathbf{E}}_{net}}={{\mathbf{E}}_{\mathbf{1}}}+{{\mathbf{E}}_{\mathbf{2}}}+{{\mathbf{E}}_{\mathbf{3}}}=\frac{\sigma }{2\pi {{\varepsilon }_{0}}b}(\widehat{\mathbf{i}}+\widehat{\mathbf{j}}+\widehat{\mathbf{k}})\]


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