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Homework Help: Find the potential difference in a rectangle

  1. Oct 15, 2014 #1
    1. The problem statement, all variables and given/known data

    Figure 20-3, referred to below, is 0.800m wide and 0.400m tall with "A" in the top left corner, "+4 microC" charge in the top right corner, "+2 microC" charge in the bottom left corner, and "B" in the bottom right corner.

    Two point charges of magnitude +4.00 μC and +2.00 μC are placed at the opposite corners of a rectangle as shown in Figure 20-3.
    (a) What is the potential at point A due to these charges?
    (b) What is the potential at point B due to these charges?
    (c) What is the potential difference between points A and B?

    2. Relevant equations

    U=(kQq)/r where k=8.99E9 Nm^2/C^2



    3. The attempt at a solution

    I honestly do not know where to begin this problem, other wise I wouldn't have posted it. I started by trying to plug values into "U=(kQq)/r" but quickly realized that would only help me if I knew the charge on points A and B.

    This is what I had done before I realized I had no idea what I was doing:
    U=(8.99E9*4*2)/0.89 (0.89 is the hypotenuse of the rectangle)

    I was then going to plug that number into U=qV and solve for V, but I have no q for points A or B so I'm stuck.
  2. jcsd
  3. Oct 15, 2014 #2
    Here is the Figure

    Attached Files:

  4. Oct 15, 2014 #3
    Use the formula for potential. This is the quantity you have to calculate.
    Calculate potential at A produced by each charge, separately. Then add the two potentials.
  5. Oct 16, 2014 #4


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    Homework Helper

    This is the potential energy of a charge q in the field of a a point charge Q. You need the potential, V, the potential energy of a unit positive charge. You wrote correctly that
    You need the potential V=kQ/r. that q in the formula for the potential energy is 1 C.

    You do not need U. You need V. Recall, that the potential at distance r from a point charge Q is kQ/r.
    The potential at A is the sum of the potentials from both the ##2 \mu C## charge and the
    ##4 \mu C## charge. Substitute the appropriate distances in the formula kQ/r. How far is A from both charges?

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