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Four long straight thin wires… (Class 12 Physics 2026, Q3)

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PhysicsCBSE Class 12 · Board exam 2026, set 55/5/1 · 1 mark · MCQ

CBSE Class 12 Physics 2026: question 3

Set in the CBSE Class 12 board exam of 2026, paper 55/5/1, 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

Four long straight thin wires are held vertically at the corners A, B, C and D of a square of side 'a', kept on a table and carry equal current 'I'. The wire at A carries current in upward direction whereas the current in the remaining wires flows in downward direction. The net magnetic field at the centre of the square will have the magnitude :

  1. (A) and directed along OC
  2. (B) and directed along OD
  3. (C) and directed along OB
  4. (D) and directed along OA

Figure in the paper: Square ABCD seen from above with A top-left, B top-right, C bottom-right, D bottom-left, centre O; diagonals dotted. Wire at A is marked with a dot (current out of page/upward); wires at B, C, D are marked with crosses (downward). See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 55/5/1.

Hints

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

Hint 1

Pair the wires at opposite corners.

Hint 2

Same-direction currents at opposite corners cancel at O; opposite currents add.

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. Each wire is at distance from O, so each field has magnitude .

  2. B and D are opposite corners with currents in the same direction, so their fields cancel. A and C are opposite corners with opposite currents, so their fields add. By the right-hand rule both point along OB.

  3. The net field is , directed along OB.

    1 mark

Answer: along OB, option (C)

Where the marks are

  • Correct magnitude and direction (1 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 paper2026 board paper, set 55/5/1: 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.