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adityalakhe

The electric filed at a distance x from the center of a uniformly charged disc of radius R,along the axis passing through the center is given by E = sigma/2e(1-x/rt(x^2+R^2)) where sigma is surface charge density and e is permittivity of free space .Putting x=0 in this eqn gives filed at the center of the disc =sigma/2e ?? How is this possible ??? By symmetry considerations and considering the ring to be made up of concentric rings; for each such ring field at the center will be zero and thus the net field must be zero?? Can anyone explain why it is so ??

  • one year ago
  • one year ago

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  1. adityalakhe
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    help @electrokid

    • one year ago
  2. electrokid
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    Gauss's Law is the answer :) |dw:1366039832463:dw|

    • one year ago
  3. adityalakhe
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    actually i know that but a task is given to me to think abt it without using gauss's law.......

    • one year ago
  4. electrokid
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    then use the fact that since the outside of the disc is positively charged, due to induction the inside would be negatively charged. Then take the intensity at the center due to an infinitesimal element of the disc and then integrate it throughout the resulting vector sum = 0

    • one year ago
  5. electrokid
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    so, you are indirectly asked to proove Gauss's Law

    • one year ago
  6. adityalakhe
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    that is what the question is actually according to symmetry field at centre should be zero but actuallt it is not.... and the charge is uniformly distributed over the disk with surface charge density (say sigma) so the whole disk is positively charged....

    • one year ago
  7. electrokid
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    the only time when the inside of a sphere has non-zero intensity is when it is a non-conductor or an insulator.

    • one year ago
  8. adityalakhe
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    it is a disk...... and since charge is uniformly distributed all over the disk therefore obviously it is an insulator

    • one year ago
  9. adityalakhe
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    please help buddy....

    • one year ago
  10. adityalakhe
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    @.Sam.

    • one year ago
  11. electrokid
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    ok.. look at the denominator of the equation.. rewrite the equation for E using the equation editor

    • one year ago
  12. adityalakhe
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    \[\sigma/2\pi \epsilon(1-\chi/\sqrt{r ^{2}+\chi ^{2}}) \] where sigma is surface charge density, x is dist of from centre on the axis of disk..... now puttin x=0 we are not gettin zero but considerin symmetry it should be zero... why is it so????

    • one year ago
  13. adityalakhe
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    @Vincent-Lyon.Fr

    • one year ago
  14. electrokid
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    |dw:1366042440302:dw|

    • one year ago
  15. electrokid
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    that symmetry argument is invalid!~

    • one year ago
  16. adityalakhe
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    i am sorry but i have told before and even now i am telling you that since charge is uniformly distributed over the whole disk therefore it can not be a conductor and hence it is an insulator |dw:1366077097337:dw| and the charge is positive...

    • one year ago
  17. adityalakhe
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    ??????

    • one year ago
  18. adityalakhe
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    |dw:1366077552708:dw| now the diametrically opposite points will cancel each other's field and the field at centre should be zero.... but keeping x=0 in the equation we don't get electric field zero... why is it so???? hope now u understand my question....

    • one year ago
  19. Vincent-Lyon.Fr
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    The formula given is only valid for x>0, not for x=0. This is because there is a discontinuity at x=0 when you cross the charged sheet. The field vs x looks like this: |dw:1366043565515:dw|

    • one year ago
  20. adityalakhe
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    but how to explain it just by concept of physics... i.e. physically and not mathematically....

    • one year ago
  21. Vincent-Lyon.Fr
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    Finished drawing here: |dw:1366043626646:dw|

    • one year ago
  22. adityalakhe
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    so value of field at centre is not zero... right????

    • one year ago
  23. adityalakhe
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    ???

    • one year ago
  24. adityalakhe
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    ?????? @Vincent-Lyon.Fr

    • one year ago
  25. adityalakhe
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    mathematically its ok but how to explain it physically contradicting the symmetry criteria.????@Vincent-Lyon.Fr

    • one year ago
  26. Vincent-Lyon.Fr
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    Value at centre is 0 because of symmetries.

    • one year ago
  27. adityalakhe
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    are you sure????

    • one year ago
  28. adityalakhe
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    but a question is given to me and it is asked that why field at centre is not zero...???

    • one year ago
  29. Vincent-Lyon.Fr
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    They probably mean: at centre of disc, on the surface (not 'inside'). The field there is \(\sigma/2\epsilon_o\), because the disc is seen as a uniform plane when you get extremely close.

    • one year ago
  30. adityalakhe
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    but we are considering the disk to be a very thin disk and, using this assumption we reach to the expression used above(we are assuming that charge is only at one face of the disk)......

    • one year ago
  31. Vincent-Lyon.Fr
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    Remember a 2D charge distribution is only a model. A real distribution is always 3D, so there is no discontinuity in reality. E-field goes smoothly from \(-\sigma/2\epsilon_o\) on one side to \(+\sigma/2\epsilon_o\) on the other side.

    • one year ago
  32. adityalakhe
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    i know that...... but while deriving the equation we consider the charge to be distributed on the surface... and if 2D arrangement would have been possible we should get field to be zero at the centre... but according to the equation we don't get field zero...(*THE QUESTION IS VALID BECAUSE WE HAVE CONSIDERED THE DISK TO BE A 2D OBJECT WHILE DERIVING THE EXPRESSION OF FIELD DUE TO A DISK....)

    • one year ago
  33. jonatan
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    Hello My Dear First things first, if the disk is uniform, there will be charge at its center too, so there's no way to get the field void at its center. If there is a small hole, the first ring, oh yes we can say that the electric field is zero at its center. Now, this equation was obtained summed up several rings to form a uniform disk, there is no hole in its center so there is no way to get the electric field at x = 0 by it. In fact, you can estiamr when the electric field are near the surface of the disk x << r. Will obtain,\[\sigma/2\pi \epsilon _{0} \] Hope i helped you

    • one year ago
  34. Vincent-Lyon.Fr
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    Actually : \(\sigma/2 \epsilon_o\) without the \(\pi\) in the denominator.

    • one year ago
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