Math

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After 150 thousand years, only 1/8 of the original amount of a particular radioactive waste will remain. The half-life of this radioactive waste is how many thousand years?

  • Math -

    N = (No)e^(-kt)
    where
    No is the original amount
    k is constant
    t is time
    substitute,
    (1/8)*No = No * e^(-k*150000)
    the No cancels:
    1/8 = e^(-150000k)
    ln (1/8) = -150000k
    k = -150000/ ln(1/8)
    after solving for k, substitute to this equation and get the value of t (half-life):
    1/2 * No = No * e^(-kt)

  • Math -

    What does the math symbols such as ^ and * mean? I still don't understand exactly what your trying to solve for. Am I trying to figure out the half-life of radioactive waste? Am I trying to figure out how many thousand years it takes in a half-life of radioactive waste? What does the term in math mean when saying "half-life"? Is the 1/8 suppose to stay as a fraction or be turned into a decimal or percent?

  • Math -

    the symbol ^ means "raise to". the symbol * means "multiplied by" as in for example
    3^2 = 3*3 = 9

    Yes, we're looking for the half-life. Half-life is the time it takes for a substance to achieve 1/2 of its original amount when it undergoes radioactive decay. the formula used for radioactive decay is
    N = (No)*e^(-k*t)
    where
    N = remaining amount after time, t
    No = the original amount
    k = some constant
    t = time
    since we don't know the k value (nothing is given), as well as the original amount (the No), express N in terms of No so we can cancel them on both sides. that's why the equation becomes
    (1/8)*No = No * e^(-k*150000)
    It is stated in the problem that after 150000 years (this is the variable t), the remaining amount will only be 1/8 of the original (the original is No, right?).
    since the equation above contains No term, we can cancel them:
    (1/8) = e^(-k*150000)
    then get the natural log of both sides. That will cancel the e on the right side, leaving only the exponent:
    ln (1/8) = ln (e^(-k*150000))
    ln (1/8) = -150000k
    k = -150000/ ln(1/8)
    Get a calculator and solve the above equation to get k.

    Now, after getting k, you need to substitute it to the original equation:
    N = (No)*e^(-k*t)
    The N here will be equal to (1/2)*No since half-life (remember the definition?). Again, No will be cancelled on both sides. The equation becomes:
    1/2 * No = No * e^(-kt)
    1/2 = e^(-k*t)
    ln (1/2) = ln(e^(-k*t))
    ln (1/2) = -k*t
    k = (-ln(1/2)) / t
    now you know the value of k, solve for t.

  • Math -

    i mean,
    t = (-ln(1/2)) / k
    we're solving for t. the units are in years.

  • Math -

    Wow - that's a lot of work.

    each half-life diminishes the amount by half.
    1/8 = 1/2 * 1/2 * 1/2

    So, 150,000 years is 3 half-lives
    The half-life is thus 50,000 years.

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