IB Physics SL E.3 Radioactive decay questions

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Question 1

Multiple choiceEasy1 mark

For each nucleus in a sample of 500 radioactive nuclei, the probability of decay during a specified interval is 0.20. What is the expected number of nuclei that decay during the interval?

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Answer: C. The expected number of decays is the probability of decay per nucleus multiplied by the number of nuclei: 0.20×500=1000.20 \times 500 = 100 nuclei. Because decay is random, this is an expected number, not a guarantee of the exact number that will decay.

A: converts the probability 0.20 to 20% and reports 20 as the number of nuclei, without applying the probability to the sample of 500.
B: uses half the stated probability, 0.10, giving 50 nuclei.
D: calculates the expected number that do not decay, 0.80×500=4000.80 \times 500 = 400 nuclei.
Remember: probability predicts an expected fraction of a sample, not which individual nuclei decay.

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