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Homework Help: Four Charges

  1. Sep 9, 2005 #1
    Four charges are placed on the corners of a rectangle. What is the resultant force on the positive charge (a = 1.3 m, b = 0.8 m, q = 1.8 × 10-9C)?

    HELP: Use Coulomb's law and superposition.

    HELP: Superposition tells us that we can find the force on the positive charge by looking at the negative charges one at a time. First find the force on the positive charge from just one of the negative charges, then from only the second negative charge, then from only the third negative charge. Then just "add" up these forces...remembering the vector nature of forces. (How do we "add" vectors?)

    HELP: Coulomb's law is F=k(q1)(q2)/r2. Remember, however, that force is a vector so the magnitude of the total force will be the square root of the sum of the forces in the x-direction squared plus the sum of the forces in the y-direction squared. To determine the x- and y-component of the force notice that o ne charge exerts a force in the x-direction only, one charge exerts a force in the y-direction only, and one charge exerts a force that needs to be broken down into x and y components. When breaking the diagonal force down into x and y components, remember cosq = a/r and sinq = b/r.
  2. jcsd
  3. Sep 9, 2005 #2


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    Staff Emeritus
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    This would indicate all charges are + and given by q?


    Well it applies to charge in general. Opposite charges (+ -) attract, like charges (+ +) or (- -) repel, but the magnitude of force would be the same dependent only on magnitudes of charges and distance of separation.

    That is one way, or one simply adds corresponding components of the vectors.
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