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Physics 30 electricity problem

  1. Oct 13, 2005 #1
    Ok so im on the last 2 questions of my electricity and electric fields assignment and im totally stumped.
    heres the first one
    12. Object A with a mass of 1.4 X 10-3 kg has a charge of 3.8 X 10-10 C. This object is at rest a distance of 48 cm from a fixed object B of charge 4.2 X 10-5 C. The two objects will repel each other. With what speed will object A be moving at when it is 96 cm from object B?
    (4 marks)
    Ok so far this i figured that first i ahve to find the total energy at rest at 48cm and ET=EP because Ek=0 cuz its at rest. however i cant think of a formula for EP because for EP=mgh i dont have any hieght and for EP=1/2kx^2 i dont have k or x. then i know i find the EP at 96 cm and solve for v, but i cant figure out how to get there.

    THe next question is sort of the same:
    13. 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? (6 marks)
    For this one im totally confused because i dont know how to start. I mean i wrote down the variables but im not sure how to approach this question.
    Please help me out... my grade depends on it.
     
  2. jcsd
  3. Oct 15, 2005 #2

    andrevdh

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    Homework Helper

    Try the work done by the electrostatic force as a mass is moved form
    [tex]48\ cm\rightarrow\ 96\ cm[/tex]
    is equal to the change in it's kinetic energy.
     
  4. Oct 15, 2005 #3

    andrevdh

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    Homework Helper

    In the second case all their potential energy gets converted into their kinetic energy.
     
  5. Apr 10, 2011 #4
    try using the potential at the two points then finding out the work done and hence the kinetic energy
    then find the velocity
     
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