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Homework Help: Work required to disassemble a helium atom

  1. Apr 28, 2012 #1
    1. The problem statement, all variables and given/known data

    A helium atom has a rest mass of He = 4.002603u. When disassembled into its constituent particles (2 protons, 2 neutrons, 2 electrons), the well-separated individual particles have the following masses: p = 1.007276u, n= 1.008665u, e = 0.000549u.

    How much work is required to completely disassemble a helium atom? (Note: 1 u of mass has a rest energy of 931.49 MeV.)

    2. Relevant equations


    3. The attempt at a solution

    2(1.007276u + 1.008665u + 0.000549u) = 4.03298u

    4.03298u - 4.002603u = .030377u

    E = (0.030377u)*(931.49 MeV/u)*(3.0*10^8)^2

    E = 2.5466*10^18 MeV

    *I tried using c = 2.9979*10^8, however it is still wrong.
  2. jcsd
  3. Apr 28, 2012 #2


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    Staff Emeritus
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    Did you notice that the units of (0.030377u)*(931.49 MeV/u)*(3.0*10^8)^2 are m4/s4?

    The conversion 931.49 MeV/u is not a conversion between one mass and another mass, so multiplying by this and then using E=mc2 afterwards is pretty suspect
  4. Apr 28, 2012 #3
    I had considered this. I'm honestly not sure where else to use the conversion. Can you give a hint? Am I correct to solve for the change in mass?
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