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Electric field MCQ

  1. Oct 17, 2015 #1
    1. The problem statement, all variables and given/known data

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    2. Relevant equations



    3. The attempt at a solution

    I believe a) and c) are correct options . Regarding option b) if I find charge q as function of radius , then option b) also seems correct . But if I think in terms of electric field lines then I am not sure about option b) .

    Since electric field lines converge at the origin , then suppose if I take a sphere of radius 1 unit centered at say (4,4,4) ,then shouldn't the number of lines entering the sphere be same as that leaving the sphere . Applying Gauss's law the charge enclosed should be zero .

    Is convergence of electric field same as convergence of electric field lines ?

    Please help me in removing the doubt .

    Many Thanks
     

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  3. Oct 17, 2015 #2

    ehild

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    I would say the electric field points to the origin instead of converging towards it. The electric field is a function of the position, it can converge at a value when approaching the origin. This field converges to zero at the origin.
    Can you write the electric field ##E(\vec r)## as function of the position vector ?
    You are right, options a) and c) are correct. That also means c) is wrong.
    b) is easy to answer if you use the differential form of Gauss theorem. ##Div \vec E = \rho(\vec r) /ε_0## where ρ is the charge density at a given position ##\vec r##.
    It is not sure that the net flux is zero in case of a closed surface not centered around the origin.
     
  4. Oct 17, 2015 #3

    rude man

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    Neither (c) nor (d) look right to me.
    4πr2ε(100r) = Q ≠ 3e-9 and also ≠ 3e-13?
     
  5. Oct 17, 2015 #4

    ehild

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    Why? C is correct to 3 digits.
     
  6. Oct 17, 2015 #5

    rude man

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    Right. Quite a coincidence given r = 3 (cm.). But I interpreted the answer 3e-13 as absolute. Shoulkd have done the computation.
     
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