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A radioactive source has a half-life of 80 s - OCR Gateway - GCSE Physics - Question 15 - 2021 - Paper 4
Question 15
A radioactive source has a half-life of 80 s. How long will it take for ¾ of the source to decay? A 10 s B 70 s C 240 s D 640 s Your answer __
Worked Solution & Example Answer:A radioactive source has a half-life of 80 s - OCR Gateway - GCSE Physics - Question 15 - 2021 - Paper 4
Step 1
How long will it take for ¾ of the source to decay?
Answer
To determine how long it takes for ¾ of a radioactive source to decay, we start by recognizing that after each half-life, half of the remaining substance decays.
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Understanding Half-Life: The half-life of this source is 80 seconds. After each 80 seconds, half of the original amount remains.
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Calculating the Decay Process:
- After 1 half-life (80 s): 50% remains
- After 2 half-lives (160 s): 25% remains
- Therefore, after 2 half-lives, 75% or ¾ of the source has decayed.
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Conclusion: Adding up the time, it takes 160 seconds to allow ¾ of the source to decay. However, the timeline according to the half-life directly shows that to reach ¾ decay, we are calculating the remaining percentage—which directly correlates to 2 half-lives.
Thus, the time it takes for ¾ of the source to decay is 240 seconds.