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    kristynejh's Avatar
    kristynejh Posts: 1, Reputation: 1
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    #1

    Feb 5, 2010, 12:34 PM
    How long will it take for 20% of the {\rm U}-238 atoms in a sample of {\rm U}-238 to
    How long will it take for 20% of the {\rm U}-238 atoms in a sample of {\rm U}-238 to decay?
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #2

    Feb 5, 2010, 12:50 PM

    That will depend on the half life of Uranium-238. Google half-life formula, and you'll surely find the formula used to calculate it. If you don't know how to use it, post back.
    ebaines's Avatar
    ebaines Posts: 12,131, Reputation: 1307
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    #3

    Feb 5, 2010, 12:59 PM

    The percentage of atoms that remain as U238 (not yet decayed) at time t is:

    where t = time and H = half life. The half-life of U238 is about 4.47 Billion years. So you want to solve for t:



    Take the log of both sides and rearrange to find t.
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