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Homework Help: Electric field on a point charge

  1. Jan 29, 2009 #1
    1. The problem statement, all variables and given/known data

    The distance between the oxygen nucleus and each of the oxygen nuclei in an H2O molecule is 9.58*10^-11m; the angle between the H atoms is 105 degrees. Find the electric field produced by the nuclear charges (positive charges) at the point P at a distance of 1.20*10^-10m to the right of the oxygen nucleus.

    2. Relevant equations
    The law of cosines.

    3. The attempt at a solution
    I used the law of cosines to find the distance from the oxygen atom to the point P, but I don't know where to go from there.
  2. jcsd
  3. Jan 29, 2009 #2


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    You have a mistake in your statement of the problem on the first line. I corrected it above for you. Here's the deal: you need to draw a picture of the situation first. Don't worry about equations until you have sketched your three nuclei and placed your point. Then at the point draw in a little vector arrow representing the electric field due to each charge. Hint: the vector for a given nucleus will point away from that nucleus. Now you are going to have to figure out the magnitude of that vector at the point. There's this nice equation that's got a q in the numerator and an r-squared in the denominator (also a constant out front) that gives you the magnitude of the electric field. Your job then is to figure out the components of each of the electric field vectors in the x and y directions and add them up (x components with x components and y components with y components). So you see the law of cosines is only part of it. I haven't see your drawing so I'm not sure where P is, but I'll bet that if you draw your picture with the x and y axes in the right place you can save yourself some work.

    Hope this gets you started!
    Last edited: Jan 29, 2009
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