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Draw the plot of potential… (Class 12 Physics 2026, Q21)

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PhysicsCBSE Class 12 · Board exam 2026, set 55/1/1 · 2 marks · Very short answer

CBSE Class 12 Physics 2026: question 21

Set in the CBSE Class 12 board exam of 2026, paper 55/1/1, section B, question 21, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q21 · 2 marks · Very short answerNuclei

Draw the plot of potential energy of a pair of nucleons as a function of their separation. Write two important conclusions that can be drawn from this plot.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 55/1/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Consider what happens to the energy when two nucleons are pushed very close together, and when they are far apart.

Hint 2

The force is . Look at the sign of the slope on either side of the minimum.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. Graph: Plot potential energy (MeV) on the y-axis against separation (fm) on the x-axis.

    • For small , is large and positive.
    • falls steeply and crosses zero.
    • It reaches a minimum (about MeV) at fm.
    • It then rises and approaches zero asymptotically for beyond about 2–3 fm.
    1 mark

  2. Conclusion 1: The nuclear force is attractive for separations greater than fm, where the slope of is positive. It is strongly repulsive for separations less than , which keeps nucleons from collapsing into one another.

    ½ mark

  3. Conclusion 2: The nuclear force is short-ranged. It becomes negligible beyond a few femtometres, since . It is maximally binding (minimum potential energy) near fm.

    ½ mark

Answer: U–r curve with a minimum at about 0.8 fm. The force is repulsive below 0.8 fm, attractive above it, and short-ranged (vanishes beyond a few fm).

Where the marks are

  • Correct shape of the U versus r plot with its minimum near 0.8 fm (1 mark)
  • Attractive for r > r₀ and repulsive for r < r₀ (½ mark)
  • Short-range nature of the nuclear force (½ mark)

Revise the chapter

  • Chapter: Nuclei (notes, formulas, where marks are lost and every past question)
Practise this chapterBoard-style questions from Nuclei, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 55/1/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.