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Homework Help: Coulomb's Law

  1. Oct 5, 2008 #1
    1. The problem statement, all variables and given/known data
    There is a washer (inner radius is a and outer radius is b) on the x-y axis with a constant surface charge density σ . What is the force felt on charge q on z-axis distance D from center of washer.

    2. Relevant equations

    3. The attempt at a solution
    I don't even know where to begin...
    I was thinking about drawing a triangle to a random spot on the washer from the origin to the charge q...but I don't know if that will work since it's a washer.
    I don't think I can pick a gaussian surface...can I?
    Please give some hints!
  2. jcsd
  3. Oct 5, 2008 #2


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    Gauss's law is only useful under cylindrically symmetric cases when the cylinder extends to infinity, or it is very long and you are far from the ends; neither of these conditions are met by this problem, so that method is out.

    Your idea of drawing a triangle to a random spot on the washer (x,y,0) is good. What is the separation vector [itex]\vec{r}[/itex] from the point (x,y,0) to the point (0,0,D)?How much charge [itex]dq[/itex] is contained in the infinitesimal area [itex]dxdy[/itex]? What does that make [itex]\vec{E}[/itex]? How about [itex]\vec{F}[/itex]?
  4. Oct 6, 2008 #3
    Thank you very much for the help!

    But I'm still a little puzzled about the problem...If I pick a random spot on the washer, then I won't know the exact distance from the origin, so I can't have a specific value for the separation vector. Right?

    So, can I pick a spot on the outer radius b?
  5. Oct 6, 2008 #4


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    Well, give your random spot the coordinates (x,y,0), what is the separation vector in terms of x and y? Remember, you are going to end up integrating over all possible random spots, so you can't restrict yourself to only those spots that are along the outer radius.
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