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Homework Help: Energy Problem-what did I do wrong?

  1. Oct 11, 2007 #1
    Energy Problem--what did I do wrong???

    Fusion (or "thermonuclear") reactions take place inside a star such as our Sun. One fusion reaction results from a collision between a proton (1H) and a deuteron (2H, the nucleus of "heavy" hydrogen, consisting of a proton and a neutron). When these two nuclei touch, they undergo a nuclear reaction, forming a helium-3 (3He) nucleus (containing two protons and one neutron) and a high energy photon, called a gamma ray.

    1H + 2H 3He +

    One "unified atomic mass unit", denoted by the symbol "u", is equal to 1.66 10-27 kg. The rest mass of the proton is 1.0073 u, the rest mass of the deuteron is 2.0136 u, the rest mass of the helium-3 nucleus is 3.0155 u, and the gamma ray is massless.


    rest masses
    proton = 1.0073 u
    deuteron = 2.0136 u
    helium-3 nucleus = 3.0155 u

    What is the kinetic energy of the helium nucleus plus the energy of the photon in the final state? Because you will be calculating a small difference of large quantities, you will need to use 5 or more significant figures in this calculation.
    KHe + E = joules

    *I've aready found out that the equation is:

    mprotonc^2 + mdeuteronc^2 +Ui = mheliumc^2 +Khelium + Egamma

    *Eand earlier in the problem I got that the Ui between the proton and the deuteron is 1.152e-13 J

    so to find out mc^2 for the proton, deuteron, and helium,

    (for the proton)

    = (1.0073u) *(1.66e-27 kg) *(3e8)^2 = 1.5049062 J

    (for deteron)
    = 3.0083184 J

    (for helium)

    = 4.505158 J

    so you would sove for Khelium + Egamma and get

    Khelium + Egamma = mprotonC^2 +mdeutronC^2 +Ui - mheliumc^2

    and i get 9.5876e-13....which is wrong?

    Where have I gone wrong? Any help is greatly appreciated!!! thanks!
  2. jcsd
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