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Homework Help: Fusion Reaction Problem

  1. Mar 9, 2008 #1
    1. The problem statement, all variables and given/known data
    In a fusion reaction, the nuclei of two atoms join to form a single atom of a different element. In such a reaction, a fraction of the rest energy of the original atoms is converted to kinetic energy of the reaction products. A fusion reaction that occurs in the Sun converts hydrogen to helium. Since electrons are not involved in the reaction, we focus on the nuclei.
    Hydrogen and deuterium (heavy hydrogen) can react to form helium plus a high-energy photon called a gamma ray:

    Particle # of protons # of neutrons Charge Rest Mass (atomic mass units)
    1H (proton) 1 0 +e 1.0073
    2H (deuterium) 1 1 +e 2.0136
    3He (helium) 2 1 +2e 3.0155
    gamma ray 0 0 0 0

    A proton (1H nucleus) and a deuteron (2H nucleus) start out far apart. An experimental apparatus shoots them toward each other (with equal and opposite momenta). If they get close enough to make actual contact with each other, they can react to form a helium-3 nucleus and a gamma ray (a high energy photon, which has kinetic energy but zero rest energy). Consider the system containing all particles.

    A)The deuterium nucleus starts out with a kinetic energy of 6.2e-14 joules, and the proton starts out with a kinetic energy of 1.23e-13 joules. The radius of a proton is 0.9e-15 m; assume that if the particles touch, the distance between their centers will be twice that. What will be the total kinetic energy of both particles an instant before they touch?

    B) Now that the proton and the deuterium nucleus are touching, the reaction can occur. 1.What is the kinetic energy of the reaction products (helium nucleus plus photon)?

    2.What was the gain of kinetic energy in this reaction? (The products have more kinetic energy than the original particles did when they were far apart. How much more?)

    3. If a mole of hydrogen and a mole of deuterium underwent this fusion reaction, how much kinetic energy would be generated?

    2. Relevant equations
    c (speed of light)= 2.9979e8 m/s
    e (charge of a proton) =1.6022e-19 coulomb
    atomic mass unit =1.6605e-27 kg
    coulomb's constant=8.9875e9 N·m2 /C2
    Kf + Uf = Ki+Ui

    3. The attempt at a solution
    The only one I have attempted thus far is A because I believe I need that answer to solve the three questions in part B. What I did for the first part was take:

    Kf + Uf = Ki+Ui
    Ui=0, Ki=6.2e-14j+1.23e-13j
    Could the answer to the first part be 0 for the final kinetic energy, since all the energy has been transferred to potential energy?
  2. jcsd
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