JEE Main & Advanced Physics Electrostatics & Capacitance JEE PYQ-Electrostatics and Capacitance

  • question_answer
    The electrostatic potential inside a charged spherical ball is given by \[\phi =an{{r}^{2}}+b\] where r is the distance from the centre; a, b are constants. Then the charge density inside the ball is                                             [AIEEE 2011]

    A)             \[-6\,\,a{{\varepsilon }_{0}}\]

    B) \[-24\,\pi \,a{{\varepsilon }_{0}}\gamma \]          

    C)             \[-6\,a{{\varepsilon }_{0}}\gamma \]

    D) \[-24\,\,\pi \,a{{\varepsilon }_{0}}\gamma \]        

    Correct Answer: A

    Solution :

    [a] \[\phi =a{{r}^{2}}+b\]
                \[\frac{d\phi }{dr}=-2ar\Rightarrow E=-2ar\]
    \[E\times 4\pi {{r}^{2}}=\frac{q}{{{\varepsilon }_{0}}}\]
    \[\Rightarrow \,\,q=-8\pi {{\varepsilon }_{0}}a{{r}^{3}}\]
    \[\Rightarrow \,\,\rho =\frac{dq}{dV}\]
                \[=\frac{-24\pi {{\varepsilon }_{0}}{{r}^{2}}dr}{4\pi {{r}^{2}}dr}\]
    \[=-6{{\varepsilon }_{0}}a\]


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