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: Proving the half-life of Potassium-40

  1. Mar 18, 2013 #1
    1. The problem statement, all variables and given/known data

    "K-40... decays by two radioactive processes. It can decay by electron capture or β- emission.

    It is found that a sample containing 4.0x10^18 nuclei of K-40 emits a total of 68 β- particles and photons each second. This shows the half life is 1.3x10^9 years."

    Use the data in the passage to show that the half life is 1.3x10^9 years.

    2. Relevant equations

    I'm assuming...

    T1/2 = ln2/λ; A=λN; A=A0e^(-λ)(t)

    3. The attempt at a solution

    Using A=λN --> A/N=λ --> (68)/(4*10^18) = 1.7^10-17
    then ln2/(1.7^10-17) = 4.1*10^16 ->> waayy too big.

    I then tried various other combinations of the above, but to no success.

    Please help!! I know I'm missing something VERY obvious.
  2. jcsd
  3. Mar 18, 2013 #2


    User Avatar
    Science Advisor

    You calculated (correctly) the half-life in seconds. Now convert to years.
  4. Mar 18, 2013 #3

    You legend, thank you :D
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted