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A long straight wire of… (Class 12 Physics SQP 2024-25, Q3)

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PhysicsCBSE Class 12 · Sample paper, session 2024-25 · 1 mark · MCQ

CBSE Class 12 Physics sample paper 2024-25: question 3

From the official CBSE sample question paper for 2024-25, section A, question 3, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q3 · 1 mark · MCQMoving Charges and Magnetism

A long straight wire of circular cross section of radius 'a' carries a steady current I. The current is uniformly distributed across its cross section. The ratio of magnitudes of the magnetic field at a point a/2 above the surface of wire to that of a point a/2 below its surface is

  1. (A)4:1
  2. (B)1:1
  3. (C)4:3
  4. (D)3:4

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

The point above the surface is at from the axis.

Hint 2

Inside a uniformly carrying wire, .

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. Outside, at : .

    ½ mark

  2. Inside, at : . The ratio is , so the answer is (C).

    ½ mark

Answer: 4:3, option (C)

Where the marks are

  • Field outside the wire at (0.5 mark)
  • Field inside at and ratio 4:3 (0.5 mark)

Revise the chapter

Practise this chapterBoard-style questions from Moving Charges and Magnetism, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2024-25: 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.