Question

A radioactive substance has a half-life of 4.33 days. If a sample is presently undergoing 4000decays/sec, how long will it take for its activity to decrease to 125 decays/sec.?

          A radioactive substance has a half-life of 4.33 days. If a sample is presently undergoing 4000decays/sec, how long will it take for its activity to decrease to 125 decays/sec.?
        

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University Physics with Modern Physics
University Physics with Modern Physics
Hugh D. Young 14th Edition
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A radioactive substance has a half-life of 4.33 days. If a sample is presently undergoing 4000decays/sec, how long will it take for its activity to decrease to 125 decays/sec.?
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00:01 In this problem the half -life of radioactive substance is 140 days and we have to find out the time after which 15 gram decays from a 16 gram sample of it.
00:17 So the decay constant is natural log 2 upon half -life.
00:26 Half -life is 140 days.
00:30 Now the amount at time t will be equal to initial amount times e to the power minus lambda t.
00:35 So amount at time t is 1 gram because 15 gram has decayed from 16 gram and initial amount is 16 gram.
00:52 From this we get e to the power minus lambda t equal to 1 upon 16...
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