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

  • question_answer
    Two identical charged spheres suspended from a common point by two massless strings of lengths \[l,\] are initially at a distance \[d\,(d<<l)\] apart because of their mutual repulsion. The charges begin to leak from both the spheres at a constant rate. As a result, the spheres approach each other with a velocity v. Then v varies as a function of the distance x between the spheres, as: [NEET - 2016]

    A) \[v\propto {{x}^{\frac{1}{2}}}\]

    B) \[v\propto x\]

    C) \[v\propto {{x}^{-\frac{1}{2}}}\]

    D) \[v\propto {{x}^{-1}}\]

    Correct Answer: C

    Solution :

    [c] \[\tan \theta =\frac{{{F}_{e}}}{mg}\simeq \theta \]
                            \[\frac{K{{q}^{2}}}{{{x}^{2}}mg}=\frac{x}{2\ell }\]
                or         \[\]                                …..(1)
                or         \[{{x}^{3/2}}\propto q\]                                   …..(2)
    differentiate eq.(i) w. r .t. time \[3{{x}^{2}}\frac{dx}{dt}\propto 2q\frac{dq}{dt}\] but \[\frac{dq}{dt}\] is constant so \[{{x}^{2}}(v)\propto q\]replace q from eq. (2)
    \[{{x}^{2}}(v)\propto {{x}^{3/2}}\]
    or         \[\]


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