PMT Predicted Paper 2 - Query on Q27.
11
Hey, I've just completed the PMT Predicted 2 paper for A Level Physics, I got Question 27, but unsure why it's incorrect.
A radioactive nuclide with a half-life of 6 hours has an initial count rate of 420 counts per minute. If the background count rate is 20 counts per minute, what would the average count rate of the source be after 24 hours?
A - 25
B - 26
C - 45
D - 100
I put C yet, the mark scheme says it's A.
A radioactive nuclide with a half-life of 6 hours has an initial count rate of 420 counts per minute. If the background count rate is 20 counts per minute, what would the average count rate of the source be after 24 hours?
A - 25
B - 26
C - 45
D - 100
I put C yet, the mark scheme says it's A.
Reply 1
JHaria
OP
11
Wrong forum lol, if it's possible can it be moved...
Reply 2
Original post
by JHariaHey, I've just completed the PMT Predicted 2 paper for A Level Physics, I got Question 27, but unsure why it's incorrect.
A radioactive nuclide with a half-life of 6 hours has an initial count rate of 420 counts per minute. If the background count rate is 20 counts per minute, what would the average count rate of the source be after 24 hours?
A - 25
B - 26
C - 45
D - 100
I put C yet, the mark scheme says it's A.
A radioactive nuclide with a half-life of 6 hours has an initial count rate of 420 counts per minute. If the background count rate is 20 counts per minute, what would the average count rate of the source be after 24 hours?
A - 25
B - 26
C - 45
D - 100
I put C yet, the mark scheme says it's A.
So initially there is a count rate of 420, however because the background count is 20, this means the actually activity is 400 counts (400 - 20). Over 24 hours, it halves 4 times (24 hours/6 = 4) Therefore 400 halved 4 times, 400,200,100,50,25, so the final activity is 25. Hope that helps! (I assume your error was adding back on the 20 background count at the end of the process (25 +20 = 45) but it asks for the count rate of the source, so it's not included. It's a bit unclear in the question, but it's an unofficial paper so I assume it would be more clear in an exam, as the question is a bit vague.)
(edited 9 months ago)
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