NEET Sample Paper NEET Sample Test Paper-91

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

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

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

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

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

    Correct Answer: C

    Solution :

    [c] Electric field \[E=-\frac{d\phi }{dr}=-2ar\]      ....(i) By Gauss's theorem \[E=\frac{1}{4\pi {{\varepsilon }_{0}}}\frac{q}{{{r}^{2}}}\]                 ?.(ii) From(i) and (ii), Q\[=-8\,\pi {{\varepsilon }_{0}}a{{r}^{3}}\] \[\Rightarrow \]\[dq=-24\pi {{\varepsilon }_{0}}a{{r}^{2}}dr\] Charge density, \[\rho =\frac{dq}{4\pi {{r}^{2}}dr}=-6{{\varepsilon }_{0}}a\]


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