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Quiz 7 - quick check

  1. Oct 11, 2008 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 11, 2008 #2


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    All looks okay except for 34 and 37.

    34. There are more flux lines pointing inwards than outwards. What is the direction of the flux lines for a positive charge, as opposed to a negative one?

    37. Not true, it's only required that the volume is equipotential for there to be zero E-field, but not necessarily zero potential. Consider a uniformly charged ring. What are the electric potential and the electric field in the centre of the ring, respectively?
  4. Oct 11, 2008 #3
    For 34, I thought all flux lines enter then leave, so I it seemed that there would be a net positive charge since the balanced lines would be leaving the box. Flux lines for a positive charge point outward, and negative charge point inwards.

    For 37, you are correct. The difference is zero, but there is still balanced electric potential.
  5. Oct 11, 2008 #4


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    No, why would the flux lines entering the box leave? Imagine if you have a box and a negative charge. Put the negative charge inside the box. What does Gauss law tell you about the amount of charge enclosed in the box if all the flux lines which entered the box somehow left it?
  6. Oct 11, 2008 #5
    If there's no charge in the box the flux lines entering would also leave, right?

    If they all left, the charge enclosed would be zero.
  7. Oct 12, 2008 #6


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    Yes that's right, but in the quiz question above you can clearly see that there are net E-flux lines entering the box.
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