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Homework Help: More electrical charges

  1. Aug 29, 2005 #1

    My question:

    Two point charges, Q1=- microcolumbs and Q2= microcolumbs are seperated by a distance of 12cm. The electric field at the point P is zero. How far from Q1 is P?

    I know that E=F/q, but I'm not sure if I equal this to zero. To figure out this problem, do I first devise a portion to help me find the answer??? Not sure

    Thank You :smile:
  2. jcsd
  3. Aug 29, 2005 #2


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    Science Advisor

    Aren't there supposed to be numbers in here?

  4. Aug 29, 2005 #3
    Yep sorry

    Two point charges, Q1=-25 microcolumbs and Q2= 50 microcolumbs are seperated by a distance of 12cm. The electric field at the point P is zero. How far from Q1 is P?

    Thank You
  5. Aug 29, 2005 #4

    Doc Al

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    Staff: Mentor

    The first thing for you to figure out is where is point P. Is it between the two charges? Or on the far side of one of them? (Which one?) This will help you visualize what's going on.

    To solve for the position of point P, try this: Call the distance between point P and Q1 by the variable "x". Now write the equations for the field from each charge at point P. The fields must be equal and opposite to cancel, so set the magnitudes of the fields from each charge equal to each other. Solve for x.
  6. Aug 29, 2005 #5


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    Homework Helper

    jena, they're asking questions about the E-field because
    they want you to get used to using it in its own right,
    not just a "math trick" computed from Force per charge.

    an E-field surrounds any source charge Q, as E=kQ/r^2 ,
    even when there's no other charge *at* the distant place.
    (like gravity field "g" exists even where it's not pulling on a rock)

    Once you find the E-field at some place, caused by source charges Q,
    then you get the Force on another charge there by F = qE . (like mg)
  7. Sep 1, 2005 #6

    I think I figured it out I have to make a equation like

    Kq/r^2 + kq/(r+12)^2= E

    This will allow me to solve the problem.

    Thank You
  8. Sep 1, 2005 #7

    Doc Al

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    Staff: Mentor

    Good! Be careful with units.
    [itex]E_{total} = k q_1/ r^2 + k q_2/(r + 0.12)^2[/itex]
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