You have prepared a nice sample of N0 radioactive nuclei of decay constant λ, and want to show it off to your friends. When they arrive units of time later, you open the lid and find there are only ___nuclei left intact.
A
N0eλt
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B
N0e−λt
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C
N0
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D
N0λt
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Solution
The correct option is BN0e−λt The decay constant given is λ . Also, at t = 0 , there were N0 nuclei in the sample.
If you have N nuclei in the sample at a time , the number of nuclei which will decay in the next time interval dt will be - dN=−λNdt⇒dNN=−λdt⇒∫NN0dNN=−λ∫t0dt⇒InNN0=−λt⇒NN0=e−λt⇒N=N0e−λt
So if you had started with N0 nuclei in the sample, when your friends arrive units of time later, there will be only N=N0e−λt
P.S.: If you ever get your hands on a nice sample of radioactive nuclei in a jar, do not 'open the lid'. Even better, never get your hands on a nice sample of radioactive nuclei.