AFMC AFMC Solved Paper-2001

  • question_answer
    The electric field required to keep a water drop of mass m just to remain suspended, when charged with one electron is :

    A) \[\frac{em}{g}\]                                              

    B) \[\frac{mg}{e}\]

    C) emg                                      

    D) mg

    Correct Answer: B

    Solution :

    Key Idea: A charge placed in an electric field experiences an electrostatic force. The intensity of electric field at a point in an electric field is the ratio of the force acting on  the test charge placed at the point to the magnitude of test charge. \[\vec{E}=\frac{{\vec{F}}}{{{q}_{0}}}\] Also, the force acting on a charge 'q' placed at that point is \[F={{q}_{0}}E\] Also, F = mg = weight, where g is gravity. \[\therefore \]      \[mg=eE\](given\[{{q}_{0}}=e\]) \[\therefore \]         \[E=\frac{mg}{e}\]


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