MHT CET · Physics · Nuclear Physics

Two different radioactive elements with half lives ' \(\mathrm{T}_1\) ' and ' \(\mathrm{T}_2\) ' have undecayed atoms ' \(\mathrm{N}_1\) ' and ' \(\mathrm{N}_2\) ' respectively present at a given instant. The ratio of their activities at that instant is

A \(\frac{\mathrm{N}_1 \mathrm{~T}_1}{\mathrm{~N}_2 \mathrm{~T}_2}\)
B \(\frac{\mathrm{N}_2 \mathrm{~T}_2}{\mathrm{~N}_1 \mathrm{~T}_1}\)
C \(\frac{\mathrm{N}_1 \mathrm{~T}_2}{\mathrm{~N}_2 \mathrm{~T}_1}\)
D \(\frac{\mathrm{N}_1 \mathrm{~N}_2}{\mathrm{~T}_1 \mathrm{~T}_2}\)

Verified Solution

Answer & Solution

Correct Answer

(C) \(\frac{\mathrm{N}_1 \mathrm{~T}_2}{\mathrm{~N}_2 \mathrm{~T}_1}\)

Step-by-step Solution

Detailed explanation

\(\therefore\) Activity is given as:
\(
\begin{aligned}
\therefore \mathrm{A} & =\lambda \mathrm{N} \\
\lambda & =\frac{\ln 2}{\mathrm{~T}}
\end{aligned}
\)
\(\therefore\) Ratio of two different radioactive elements will be:
\(
\begin{aligned}
& \frac{\mathrm{A}_1}{\mathrm{~A}_2}=\frac{\lambda_1 \mathrm{~N}_1}{\lambda_2 \mathrm{~N}_2} \\
& \frac{\mathrm{A}_1}{\mathrm{~A}_2}=\frac{\frac{\ln 2}{\mathrm{~T}_1} \mathrm{~N}_1}{\frac{\ln 2}{\mathrm{~T}_2} \mathrm{~N}_2} \\
& \frac{\mathrm{A}_1}{\mathrm{~A}_2}=\frac{\mathrm{N}_1 \mathrm{~T}_2}{\mathrm{~N}_2 \mathrm{~T}_1}
\end{aligned}
\)

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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