KVPY Sample Paper KVPY Stream-SX Model Paper-20

  • question_answer
    Electric field in a region is \[E=\frac{a\,(x\hat{i}+y\hat{j}+z\hat{k})}{{{x}^{2}}+{{y}^{2}}+{{z}^{2}}}\] where, a is a positive constant. Charge enclosed in a sphere of radius R and centre at origin is

    A) \[4\pi {{\varepsilon }_{0}}aR\]

    B) \[4\pi {{\varepsilon }_{0}}a\log R\]

    C) \[4\pi {{\varepsilon }_{0}}a{{R}^{2}}\]

    D) \[\frac{4\pi {{\varepsilon }_{0}}a}{R}\]

    Correct Answer: A

    Solution :

    \[E=\frac{a\,(x\hat{i}+y\hat{j}+z\hat{k})}{{{x}^{2}}+{{y}^{2}}+{{z}^{2}}}=\frac{ar}{{{r}^{2}}}=\frac{a}{r}\cdot \hat{r}\]
    \[d\phi =Ed\,s=\left( \frac{a}{R}\cdot \hat{r} \right)(ds\cdot \hat{r})\]
    \[\Rightarrow \]\[\phi =\int{d\phi =4\pi aR}\]
    \[\therefore \]\[{{Q}_{\text{enclosed}}}={{\varepsilon }_{0}}\phi =4\pi {{\varepsilon }_{0}}aR\]


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