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Homework Help: An alpha particle is fired at a uranium atom

  1. Jun 16, 2012 #1
    1. The problem statement, all variables and given/known data
    "If an alpha particle is fired straight at the nucleus of an uranium atom (q = 92e), at a velocity of 5 x 10^5 m/s, how close will it get to the uranium nucleus?

    2. Relevant equations
    Ek = (mv^2)/2
    Ee = (kq1q2)/d

    3. The attempt at a solution
    Since the alpha particle and uranium atom are both positively charged, they will naturally repel each other. Firing the alpha particle at the uranium atom will therefore store electrical energy. At the closest point the alpha particle gets to the nucleus, all of its kinetic energy has been converted into electrical potential energy. Therefore, I equated Ee and Ek, and solved for d, taking the mass of an alpha particle to be quadruple that of a proton. However, the answer I got was 1.02 x 10^-10 m, but the correct answer is 5.1 x 10^-11 as per the provided solution.
  2. jcsd
  3. Jun 16, 2012 #2
    5.5 x 10^-11 is correct, do the math again.
  4. Jun 16, 2012 #3


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    Make sure you use the correct values for the charge and mass of the alpha particle.
  5. Jun 16, 2012 #4
    I accidentally used q = 4e for the alpha particle (since m = 4 x mass of proton)... ^.^'''

    Thanks, guys!
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