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Homework Help: Radioactive emission

  1. Jan 25, 2010 #1
    1. The problem statement, all variables and given/known data
    A radioactive source emits electrons. The number of electrons emitted per second
    N(t) decreases with time according to the equation:

    dN/dt = −1.8 × 10−5N(t) ,
    where t is expressed in seconds. If N = 12 electrons/sec at t = 0, after what time t
    has N fallen to the value 4.0 electrons/sec? [7]
    What is the half-life of the radioactive source in seconds? [3]

    2. Relevant equations

    3. The attempt at a solution
    I have the first part done, but am confused about the second part. How can you tell the half life of the material from the information given? N is just the amount of electrons given off per second, and the half life is the amount of time taken for the amount of substance to halve right? I don't see how they can be directly related. Any ideas?
  2. jcsd
  3. Jan 25, 2010 #2
    One way is to just work out how long it takes for the number of electrons given off per second to drop from 12 to 6. That represents the half-life time.
  4. Jan 25, 2010 #3
    OK I was thinking that must be the only way. Thanks :)
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