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Homework Help: I think i did this electric potential problem correctly but

  1. May 23, 2009 #1
    1. The problem statement, all variables and given/known data

    a helium nucleus (charge =2e, mass = 6.63E-27 kg) traveling at a speed of 6.20E5 m/s enters an electric field, traveling from point A, at a potential of 1.5E3 V, to point B, at 4.0E3 V. what is the speed at point B?

    2. Relevant equations

    charge = 2e = 2 * 1.6E-19 C
    delta V is potential difference.
    i used equation delta V = - (delta KE)/q and derived the relationship:
    delta V = - mass/2q (vf^2 - vi^2)
    3. The attempt at a solution
    since the charge is 2e and e for electron is -1.6E-19 C, so the right hand side is positive. the left side is 4.0E3-1.5E3...finally i solved vf and got 7.91E5 m/s but the answer is 3.78E5 m/s

    i dont get it...where is the mistake?
    Last edited: May 23, 2009
  2. jcsd
  3. May 23, 2009 #2
    I think you are making a sign error. Electric fields point from higher potential to lower potential. The helium nucleus is +2e (not a negative charge). This positive charge is going from A to B, or lower potential to higher potential. This means it is going against the E field. This means that the nucleus will slow down, not speed up. Notice that the answer you want is smaller than the initial velocity.

    Note: I haven't done the math to check your equations. I'm just looking at what the answer, in general, should look like.
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