# Physics Eduqas question

#1
Hello, I'm revising for my physics exam on Thursday and I came across this question:

Activity=3.3x10^-6 Bq
Number of nuclei=2.88x10^21
mass=1 gram
half life = 1.9x10^19 years

Determine the number of nuclei in 1 gram of Bi (atomic number 83, mass number 209) which will decay in 5 years

In the mark scheme it says 3.3x10^-6x5x365x24x60x60=525

I get this is Activity x time, but I don't understand how this gives the number of nuclei after 5 years?
I initially tried N=N0 e^-(decay constant x time), but when I plugged the values in it was just 0?

Any help is appreciated thank you!
Last edited by stevie67890; 1 month ago
0
1 month ago
#2
Feel like you just misread the question
Activity is the number of decays per second so - (the number of seconds in 5 years) * Activity = n. of nuclei which decay
3.3x10^-6 Bq * 5x365x24x60x60 (the number of seconds in 5 years) = 525 like the mark scheme says?

It's not asking for the number of nuclei remaining, it's asking simply how many will decay. Let's say you do want to know how many remain you would just take the original number of nuclei and subtract the 525 we just calculated, this is pointless though in a sense as it's a pretty negligible amount of nuclei that have decayed and I doubt they're gonna say give your answer to 21 sig figs etc
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