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How does the impact… (Class 12 Physics SQP 2025-26, Q21I)

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PhysicsCBSE Class 12 · Sample paper, session 2025-26 · 2 marks · Very short answer

CBSE Class 12 Physics sample paper 2025-26: question 21(I)

From the official CBSE sample question paper for 2025-26, section B, question 21(I), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q21(I) · 2 marks · Very short answerAtoms

How does the impact parameter affect the trajectory of a α – particles scattered by a heavy nucleus? What is the value of impact parameter for head on collision of α – particles with the nucleus?

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

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

Hint 1

Impact parameter is the perpendicular distance of the initial velocity from the centre of the nucleus.

Hint 2

Closer approach means stronger Coulomb repulsion.

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. The smaller the impact parameter , the closer the α-particle passes to the nucleus, the stronger the repulsion and the larger the scattering angle; as large, the α-particle is hardly deflected and nearly keeps its original straight path.

    1 mark

  2. For a head-on collision, the impact parameter is zero () and the α-particle rebounds back (scattering angle ≈ 180°).

    1 mark

Answer: Smaller b → larger deflection; larger b → small deflection; b = 0 for head-on collision

Where the marks are

  • Relation between impact parameter and scattering angle (1 mark)
  • Impact parameter zero for head-on collision (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Atoms, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2025-26: 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.