CBSE Class 12 Physics 2023: question 30(b)

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Step-by-step solution

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1. Graph (described): binding energy per nucleon $E_{bn}$ (MeV) on y-axis vs mass number A on x-axis. It rises steeply for light nuclei (H-2, He-4 peak etc.), reaches a broad maximum about 8.75 MeV near A = 56 (Fe), and then decreases slowly to about 7.6 MeV at A = 238 (U).

   1 mark

2. Feature 1: $E_{bn}$ is nearly constant (about 8 MeV) for nuclei of middle mass numbers 30 < A < 170, showing saturation of nuclear forces.

   ½ mark

3. Feature 2: $E_{bn}$ is lower for both light nuclei (A < 30) and heavy nuclei (A > 170); maximum ≈ 8.75 MeV at A = 56.

   ½ mark

4. Lighter nuclei have low binding energy per nucleon. When two of them fuse, the product nucleus has a higher binding energy per nucleon, so the final system is more tightly bound (more stable) and energy is released. Hence light nuclei tend to undergo fusion.

   1 mark

Answer: Curve peaks ≈ 8.75 MeV at A = 56, nearly flat for 30 < A < 170, lower at both ends; fusion gives higher $E_{bn}$, hence more stable product with energy release.

Revise the chapter

• Chapter: Nuclei (notes, formulas, where marks are lost and every past question)
• Topic: Mass Defect and Binding Energy