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Big Bang helium nuclei

  1. Nov 5, 2016 #1
    1. The problem statement, all variables and given/known data
    the relative concentration by mass of helium at 3 minutes after the big bang is 2-5 parts per 100,000. If you had 1 kilogram of the normal matter from the universe at a time 3 minutes after the big bang, how much of it would be normal helium nuclei?

    2. Relevant equations


    3. The attempt at a solution
    I have no idea.can someone help?
     
  2. jcsd
  3. Nov 5, 2016 #2

    mfb

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    If 2% of all apples are bad, and you have 10000 apples, how many apples are bad? The problem is as easy as this.
     
  4. Nov 5, 2016 #3
    your mean is 1kg x 2x10^-5?but the answer is 250g. I only have the answer. I don't know how to get it
     
  5. Nov 5, 2016 #4

    mfb

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    250 g sounds reasonable in terms of cosmology (the actual mass fraction was ~25%), but that doesn't fit to the given "2-5 parts per 100,000". Could that number refer to something else (like helium-3 or deuterium)?
     
  6. Nov 5, 2016 #5
    IMG_1478376939.927361.jpg
    I have table show that
     
  7. Nov 5, 2016 #6

    mfb

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    That's what I expected. 25% 4He, so 250 g in 1 kg. The 3He contribution is negligible.
     
  8. Nov 5, 2016 #7
    how about in 7Li. there isn't have%
     
  9. Nov 5, 2016 #8

    mfb

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    What is unclear about "n parts in x"?
     
  10. Nov 5, 2016 #9
    2-5 parts in 10 billion
     
  11. Nov 5, 2016 #10

    mfb

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    Yes, what is unclear about it?
     
  12. Nov 5, 2016 #11
    So It is 1kg x 2 parts in 10 billion?
     
  13. Nov 5, 2016 #12

    mfb

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    Sure. Well, 2 to 5.
     
  14. Nov 5, 2016 #13
    I have tried it before. 1kgx4x10^(-10)=4x10^(-10)kg. However the answer is 0.4mg=4x10^(-7)kg how to get it?
     
  15. Nov 5, 2016 #14
    there is the Q:If you could take a 1 kg handful of the matter in the universe 3 minutes after the big bang, approximately how much would be lithium nuclei?
     
  16. Nov 5, 2016 #15

    mfb

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    Should be 0.2 to 0.5 µg, not mg.
     
  17. Nov 5, 2016 #16
    my calculation is correct?
     
  18. Nov 5, 2016 #17

    mfb

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    Yes.
     
  19. Nov 5, 2016 #18
    thanks!
     
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