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Physics Relativity/Power Question?

  1. Feb 23, 2012 #1
    Physics Relativity/Power Question??

    1. The problem statement, all variables and given/known data
    When a fusion power reactor is possibly created, it is expected to produce about 3 x 10^3 MW of termal power. If a fusion reaction produces 2.808 x 10^-12 Joules, how many fusion reactions must occur per second to generate this power???




    3. The attempt at a solution
    What i did was 3 x 10^9 J/s / 2.808 x 10^-12 J. But my problem is that if i do that i will only get seconds and not reaction per seconds. Please help.
     
  2. jcsd
  3. Feb 23, 2012 #2

    PeterO

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    Re: Physics Relativity/Power Question??

    The key to the question is in red above. So presumably the answer will have units s-1.

    When you do your calculations, looking an the units, it is (Js-1) divide (J) which gives s-1, which seems like a good start.

    By the way, the "reactions" in "reactions per second" is a count, so has no dimensions.
     
  4. Feb 23, 2012 #3
    Re: Physics Relativity/Power Question??

    Ok, can you tell me if this is correct: N = the number of reactions and Eo = 2.808x10^-12J
    P = 3.0 × 10 ⁹ W = 3.0 × 10 ⁹ J / s
    P = E / t --> E = P t
    E = N E₀ --> N = E / E₀
    N = P t / E₀ … --> … N / t = P / E₀ = [ 3.0 × 10 ⁹ J / s ] / [ 2.808 × 10 ^⁻ 12 J ]
    N / t = 1.0683 × 10 ^21 / s
    Is this correct?
     
  5. Feb 23, 2012 #4

    PeterO

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    Re: Physics Relativity/Power Question??

    it looks to be about the right size.

    EDIT: Though you were not asked for the value of N/t, you were asked "how many fusion reactions per second, so a better answer would be

    There would be 1.0683 × 10 ^21 fusion reactions per second.

    If a question is asked in words, it should be answered in words.
    If the question was "what is the number of fusion reactions per second, then you would give just the number.
     
  6. Feb 23, 2012 #5
    Re: Physics Relativity/Power Question??

    Did i do it correctly in terms of the units? So basically isnt it just 3.0 × 10 ⁹ J / s / 2.808 × 10 ^⁻ 12 J?
     
  7. Feb 23, 2012 #6

    PeterO

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    Re: Physics Relativity/Power Question??

    Yes.
     
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