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Homework Help: Need help on 2 physics problems

  1. Sep 14, 2005 #1
    ive looked at both of these for about an hour and cant figure them out

    the first one:

    How much work is required to bring three protons initially infinitely apart, to a configuration where each proton is 1.5x10^-13 m from the other two? (This is a typical separation for protons in a nucleus.)

    second one

    A point charge of mass 0.071 kg and charge +6.77x10^-6 C is suspended by a thread between the vertical parallel plates of a parallel-plate capacitor. (a) If the charge deflects to the right of vertical, which of the two plates is at the higher electric potential? (b) If the angle of deflection is 22 degrees, and the separation between the plates is 0.025 m, what is the potential difference between the plates?

    I know that the answer to A is the plate on the left, but i dont know how to find part B.

    Any help would be appreciated, thanks
  2. jcsd
  3. Sep 14, 2005 #2


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    Homework Helper

    For the first one, add the charges one at a time. The first is free, the second must work against the field of the first, etc. For the second problem, add up the torques from gravity and the EM field and find how strong the field must be for there to be no net torque.
  4. Sep 14, 2005 #3
    Im not quite sure what you mean my add the charges one at a time for the first problem? And on the second one, there must be another way to find the answer, i dont think we have gone over anything dealing with torque in our class yet. Got any more ideas?

    Thanks for the help though. Im going to go back to staring at the problem and hoping it magically comes to me.
  5. Sep 14, 2005 #4


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    I'm saying, if you wanted to actually assemble the charges in this arrangement, you'd have to take each one in from very far away (ie, so far away that they have no effect on eachother). The first one takes no work to move into place, because there are no charges nearby. For the second, there is the constant force it feels due to the first that you must work against. And similarly for the third, where this case you must work against both the first and the second charges. The total work is the energy "in" the arrangement, because it is the work you would get out of the arrangement if you dissassembled it (ie, let it explode).

    For the second problem, if you don't know torques, you'll have to include all the forces on the charge: electrical, gravitational, and the tension ofthe string. These are all in a known direction, and must add (as vectors) to zero. Your unknowns are the magnitude of the electric force and the magnitude of the tension, and you'll get two equations from the x and y components of the vectors to solve for these.
  6. Sep 14, 2005 #5
    Okay i think i figured out the first one:

    I took the equation deltaU = k(q)(q1)/r and plugged in the numbers and did that three times and added them togehter.

    And like you said at a great distance they do not affect each other so their original potential is zero

    and from that i got that the deltaU would equal 4.608x10^-31, which if i under stand right is equal to -W.

    Still trying to figure out the second one

    Thanks again for the help
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