1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook