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Metal sphere

  1. Jul 4, 2008 #1
    A metal sphere of radius R = 10 cm carries a total charge Q = 0.4 μC.

    A. What is the magnitude of the electric field just outside the sphere, and in which direction is it pointing?

    B. What is the magnitude and direction of the electric field just inside the sphere?

    I have already found the magnitude of the electric field for each, but I am not sure about the direction.
     
  2. jcsd
  3. Jul 4, 2008 #2

    Hootenanny

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    Let's start with case A. How did you determine the magnitude? (Your method should also help you determine the direction).

    For case B, the fact that you are attempting to determine the direction of the electric field tells me you have calculated it's magnitude incorrectly.
     
  4. Jul 4, 2008 #3
    I used the equation E=q/(4pi*e*r^2)
     
  5. Jul 4, 2008 #4

    Hootenanny

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    That is the electric field for a point charge q, so in what direction does the electric field of a point charge point?
     
  6. Jul 4, 2008 #5
    Outwards, since the charge is positive?
     
  7. Jul 4, 2008 #6
    Btw do you mean that the equation I used is wrong?
     
  8. Jul 4, 2008 #7

    Hootenanny

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    Nope your equation is quite correct, I was simply giving you a hint.
    Radially outwards from the centre of the sphere, correct :approve:.
     
  9. Jul 4, 2008 #8
    Okay thanks! For B then I know that the field is 0. Does this mean there is no direction?
     
  10. Jul 4, 2008 #9

    Hootenanny

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    Correct again.
     
  11. Jul 4, 2008 #10
    Thanks for your help! =)
     
  12. Jul 4, 2008 #11

    Hootenanny

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    A pleasure :smile:
     
  13. Jul 4, 2008 #12
    im doing a question similar to this, except i need to find the charge on the ball. What does the "e" stand for in your equation?
     
  14. Jul 4, 2008 #13
    that's the electric constant.
     
  15. Jul 4, 2008 #14
    ohh ok. so, the question is:

    The field just outside a 3.61 cm radius metal ball is 2.86×102N/C and points toward the ball. What charge resides on the ball?

    my equation should look like:

    286(4pi)(0.0361^2)(8.85E-12) = q

    right?
     
  16. Jul 4, 2008 #15
    yah that should be right. check the sign of ur charge though.
     
  17. Jul 4, 2008 #16
    yup. the charge was neg since the feild pointed toward the center of the ball. Thanks!
     
  18. Jul 4, 2008 #17
    No prob =)
     
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