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Homework Help: Electrical Charge Question

  1. Oct 17, 2004 #1
    Hey All,
    I'm having some trouble with this problem. Any help would be greatly appreciated.

    Two charged objects A and B have masses of 1.3 X 10-2 kg and 2.6 X 10-2 kg respectively. Their charges are -1.7 X 10-4 C and -3.8 X 10-4 C respectively. They are released from rest when they are 3.6 m apart. What will their speeds be when they are a "large'' distance apart?

    I am really being thrown off by the "large distance" part. I mean, how are we supposed to calculate a number with such a general definition. I understand that as the distance b/w objects gets larger, the repelling force will get smaller. But, if we assume is a conservative system, won't there always be some velocity, no matter how great the distance apart??
    What am I missing/not understanding.
    Thanks for any help!!!
  2. jcsd
  3. Oct 17, 2004 #2


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    "Large distance apart" essentially means infinitely far apart.
  4. Oct 17, 2004 #3
    yes but even at an infinite distance will there not still be some velocity??? how do i solve for this velocity??
  5. Oct 17, 2004 #4


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    This is a potential versus kinetic entergy problem, starting with 2 particles with repelling charge force. At a large distance, both particles will have nearly reached their maximum kinetic energy. The maximum potential energy occurs if the two particles have 0 distance between them. In this case the starting point is 3.6 meters, which is the starting potential energy. So the goal is to caculate the potential energy at 3.6 meters, then assume that all of it is converted into kinetic energy after the particles have traveled a large distance.
  6. Oct 17, 2004 #5


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    A repelling force is invovled the more time and distance between the particles, the faster the speed will be.
  7. Oct 17, 2004 #6
    thanks jeff reid,
    your answer makes sense. One problem i do have is once i find the potential energy of the particles, how do i go about finding kenetic. I understand that energy is conserved so Ep=Ek. So i can use E=KQq/r to find Ep. but once i have this, to find Ek do i add the two masses togeather and use the formula Ek=1/2mv2???
    thanks again
  8. Oct 18, 2004 #7
    Yes, since Potential energy was for the system, you'll find the total kinetic energy of the system ( m1+m2 ) by using energy conservation.

    After that simply take momentum conservation into account. And find the individual velocities from the total kinetic energy.

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