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Homework Help: How to find final kinetic energy for a charged particle

  1. Feb 9, 2015 #1
    1. The problem statement, all variables and given/known data
    An alpha particle (helium nucleus, charge +2e) starts from rest and travels a distance of 2.5 cm under the influence of a uniform electric field of magnitude 12.5 kV/m. What is the final kinetic energy of the alpha particle?

    2. Relevant equations
    U = 1/2 QV = 1/2 C(V^2) = (Q^2)/2C
    Q = CV
    C = (Eo A)/d

    where U = potential energy, Q = charge, C= capacitance, A= area, d = distance, and V = potential difference
    3. The attempt at a solution
    I tried using U = 1/2 QV by calculating that the charge is 2 times the constant charge of an electron (1.602x10^-19) and used 12.5 kv/m as V (but changed it back to just volts.
    U = 0.5 (3.204x10^-19) (12500 v/m)
    U = 2.0025x10^-15
    But this is wrong. Also, in a practice version of the problem it said that given magnitude = 18.5 kv/m and distance = 1.6 cm the answer would be 9.48x10^-17.
    Any help with this problem would be greatly appreciated.
  2. jcsd
  3. Feb 9, 2015 #2


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    Homework Helper

    How can you get from the E-field (V/m) to an Energy (Joule = CV)?
    This is not a Capacitor ... the general Potential Energy formula is U = Q_total V_average ...
    (this gives a ½ for Energy STORED in a Capacitor, but at maximum charge Q and maximum voltage V.)
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