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Homework Help: Energy and Mass Problem

  1. May 6, 2007 #1
    1. The problem statement, all variables and given/known data
    The sun radiates energy at the rate of approximately 4 x 10^26 W. Assume that this energy is produced by a reaction whose net result is the fusion of 4 H nuclei to form 1 He nucleus, with the release of 25 MeV for each He nucleus formed.

    Calculate the sun's loss of mass per day

    2. Relevant equations

    E = mc^2

    3. The attempt at a solution

    I think all the information about the Helium reaction is a red herring, but I'm
    not sure. It's throwing me off. The problem says that the sun radiate energy
    at the rate of 4x10^26 W, but it doesn't say in what time frame.
    What I did at first, was assumed that that rate is per second and I did as follows;

    1 day = 24 hours = 24*60 minutes = 24*60*60 seconds = 86400 s

    Energy output in a day = 4x10^26 W/s * 86400s = 3.456 * 10^31 J

    then I used E = mc^2 -> m = E/c^2 = 3.456 * 10^31 J/(2.0 * 10^8 m/s)^2
    = 3.84 * 10^14 kg to be the amount of mass the sun loses every day.

    Is this right, or do I have to use the given information about the Helium reaction?
  2. jcsd
  3. May 6, 2007 #2
    I believe you are correct, its only asking for a mass equivalent of x amt of energy.
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