Q. The decay constant of a radio isotope is λ. If A₁ and A₂ are its activities at times t₁ and t₂ respectively, the number of nuclei which have decayed during the time (t₁ − t₂) is
A) A₁t₁ − A₂t₂
B) A₁ − A₂
C) (A₁ − A₂)/λ
D) λ(A₁ − A₂)
Explanation
[Solution from source] Activity A = λN, so N₁ = A₁/λ and N₂ = A₂/λ. The number decayed between the two instants is N₁ − N₂ = (A₁ − A₂)/λ. The difference A₁ − A₂ alone is a difference of decay rates, not a number of nuclei. Answer (3).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.