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Homework Help: Half life questions

  1. Apr 11, 2010 #1
    i am taking upgrading, and one of the main reasons i failed physics 30 to start with was due to half life. the book cant explain it good enough to me and either could my teacher. so i have a few questions that i would appreciate a starting point for.

    How much time elapses before 90% of the radioactivity of a sample of 72/33As dissapears as measured by its activity? the half life is 26h.


    A sample of radioactive material contains 10^15 atoms and has an activity of 6.00 x 10^11 Bq. What is the half life?

    these are just a couple of the many questions i am struggling with.
     
  2. jcsd
  3. Apr 11, 2010 #2
    [tex]N(t)=N_0 e^{-t/\tau}[/tex]

    Is all you need to know, along with the definition that the half life is the time it takes for half a given sample to decay.

    After a time [tex]t=t_{1/2}[/tex] it holds that: [tex]N(t)=\tfrac{1}{2}N_0[/tex]

    Looking at the general form, we find that:
    [tex]\tfrac{1}{2}=e^{-t_{1/2}/\tau}[/tex]

    Some algebra provides: [tex]\tau = \frac{t_{1/2}}{\ln{2}}[/tex]

    Now try and solve the problems and show us where you have difficulty.
     
  4. Apr 11, 2010 #3
    Don't feel bad if you don't agree that logarithmic manipulations aren't algebraic, I don't either. Doesn't mean they don't work.
     
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