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Find thekinetic energy of the sphere from line of charge

  1. Mar 1, 2017 #1
    1. The problem statement, all variables and given/known data
    A small sphere with mass 5.00×10−7 kg and charge +6.00 μC is released from rest a distance of 0.400 m above a large horizontal insulating sheet of charge that has uniform surface charge density σ = +8.00 pC/m2.Using energy methods, calculate the speed of the sphere when it is
    0.100 m above the sheet.

    2. Relevant equations
    V=integrate(E dl) from a to b
    E=σ/2ε
    3. The attempt at a solution
    σql/2ε=0.5mv^2
    (8*10^-12)(6*10^-6)(0.4-0.1)/2ε=0.5(5*10^-7)v^2
    v=1.8m/s
    But mastering physics keeps saying it is wrong
    I don't know what did I did wrong
     
  2. jcsd
  3. Mar 1, 2017 #2

    BvU

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    Hello Wormwood, :welcome:

    What if gravity were 19.62 m/s2 instead of 9.81 m/s2 ? Would it go just as fast then ?
     
  4. Mar 1, 2017 #3
    What about gravity?
     
  5. Mar 1, 2017 #4

    haruspex

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    Both charges are positive, so why did it fall?
     
  6. Mar 1, 2017 #5
    Ah, I totally ignored gravity >.<
    With gravity the formula is:
    mgh=σql/2ε+0.5mv^2
    (5*10^-7)(9.81)(0.4-0.1)=(8*10^-12)(6*10^-6)(0.4-0.1)/2ε+0.5(5*10^-7)v^2
    v=1.62m/s
    The answer is finally correct.
    Thanks for all the help >.<
     
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