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A long straight wire AB… (Class 12 Physics 2023, Q23b)

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PhysicsCBSE Class 12 · Board exam 2023, set 55/5/1 · 2 marks · Very short answer

CBSE Class 12 Physics 2023: question 23(b)

Set in the CBSE Class 12 board exam of 2023, paper 55/5/1, section B, question 23(b), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q23(b) · 2 marks · Very short answerMoving Charges and Magnetism

A long straight wire AB carries a current I. A particle (mass m and charge q) moves with a velocity , parallel to the wire, at a distance d from it as shown in the figure. Obtain the expression for the force experienced by the particle and mention its directions.

Figure in the paper: Vertical long wire AB with current I upward (B at top, A at bottom). A particle at horizontal distance d to the right of the wire, moving with velocity v directed downward (antiparallel to the current). See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 55/5/1.

Hints

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

Hint 1

First find the magnetic field of the wire at the position of the charge, including its direction.

Hint 2

Apply . Remember that antiparallel currents repel.

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. Magnetic field due to the long straight wire at distance : . By the right-hand thumb rule (current upward, point on the right of the wire), this field is directed into the plane of the paper.

    ½ mark

  2. Force on the moving charge: . Here (downward) is perpendicular to (into the page), so .

    ½ mark

  3. Substituting for : .

    ½ mark

  4. Direction: . So for a positive charge the force is to the right, i.e. away from the wire (repulsive). This is expected because the charge moves antiparallel to the current.

    ½ mark

Answer: , directed away from the wire (towards the right)

Where the marks are

  • Field into the page (½ mark)
  • Uses (½ mark)
  • Obtains (½ mark)
  • Direction: away from the wire (½ 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 paper2023 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.