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Abundancy of Uranium 235 when the earth was formed.

  1. Nov 8, 2009 #1
    1. The problem statement, all variables and given/known data
    The Earth is about 4.5 billion years old. If 235U is 0.65% abundant today, how abundant was it when the Earth formed? Note, in this case abundancy is defined as the ratio of Uranium 235 to Uranium 238

    2. Relevant equations
    Half Life = ln(2)/lambda

    3. The attempt at a solution
    I am really unsure how to do this problem. I tried reworking the above equations but I was not able to get the correct answer. I need a little help getting started.
  2. jcsd
  3. Nov 8, 2009 #2
    In your second equation, you can divied by N0 to give N/N0. What would this quantity represent? Think about the total range of values this quantity could take on. Can N be negative? Can N>No?
  4. Nov 8, 2009 #3
    Well, N/N0 would be the probability that a nucleus has decayed in the given period of time which in this case would be -4.5billion years if we take t=0 to be the present. And can't N be larger than N0 if we are going back in time?
  5. Nov 8, 2009 #4
    ok, so i've worked it up to the point where I have the new abundancy = .0065e^(lambda238-lambda235)*4.5billion years but it's wrong. How can i fix this?
  6. Nov 8, 2009 #5
    Got it wrong so now I got an 80 on my HW. Thanks for nothing.
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