BCECE Engineering BCECE Engineering Solved Paper-2008

  • question_answer
    If the electric flux entering and leaving an enclosed surface respectively are \[{{\phi }_{1}}\] and \[{{\phi }_{2}}\] the electric charge inside the surface will be

    A)  \[\frac{{{\phi }_{2}}-{{\phi }_{1}}}{{{\varepsilon }_{0}}}\]             

    B)         \[\frac{{{\phi }_{1}}+{{\phi }_{2}}}{{{\varepsilon }_{0}}}\]

    C)  \[\frac{{{\phi }_{1}}-{{\phi }_{2}}}{{{\varepsilon }_{0}}}\]             

    D)         \[{{\varepsilon }_{0}}\,({{\phi }_{1}}+{{\phi }_{2}})\]

    Correct Answer: D

    Solution :

    According to this law, the net electric flux through any dosed surface is equal to the net charge inside the surface divided by \[{{\varepsilon }_{0}}.\] Therefore,         \[\phi \text{=}\frac{q}{{{\varepsilon }_{0}}}\] Let \[-{{q}_{1}}\]be the charge, due to which flux \[{{\phi }_{1}}\] is entering the surface \[\therefore \]  \[\phi \text{=}\frac{-{{q}_{1}}}{{{\varepsilon }_{0}}}\] or            \[-{{q}_{1}}={{\varepsilon }_{0}}{{\phi }_{1}}\] Let \[+\,{{q}_{2}}\]be the charge, due to which flux \[{{\phi }_{2}}\] is leaving, the surface \[\therefore \]  \[{{\phi }_{2}}=\frac{{{q}_{2}}}{{{\varepsilon }_{0}}}\] or            \[{{q}_{2}}={{\varepsilon }_{0}}{{\phi }_{2}}\] So, electric charge inside the surface \[={{q}_{2}}-{{q}_{1}}\] \[={{\varepsilon }_{0}}{{\phi }_{2}}+{{\varepsilon }_{0}}{{\phi }_{1}}\] \[={{\varepsilon }_{0}}({{\phi }_{2}}+{{\phi }_{1}})\]


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