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Why can't two magnetic field… (Class 10 Science 2025, Q36b)

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ScienceCBSE Class 10 · Board exam 2025, set 31/6/1 · 5 marks · Long answer

CBSE Class 10 Science 2025: question 36(b)

Set in the CBSE Class 10 board exam of 2025, paper 31/6/1, section D, question 36(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q36(b) · 5 marks · Long answerMagnetic Effects of Electric Current

Why can't two magnetic field lines cross each other ? Draw magnetic field lines showing the direction of the magnetic field due to a current carrying long straight solenoid. State the conclusion which can be drawn from the pattern of magnetic field lines inside the solenoid. Name any two factors on which the magnitude of the magnetic field due to this solenoid depends.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/6/1.

Hints

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

Hint 1

What would a compass needle do at a point where two lines cross?

Hint 2

The field pattern outside a solenoid looks like that of a bar magnet.

Hint 3

Look at the spacing and shape of the lines inside the solenoid. What does that tell you about the field strength there?

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. Two field lines cannot cross. At the point of intersection, the magnetic field would have two directions, and a compass needle placed there would point two ways at once. This is not possible.

    1 mark

  2. Figure (described in words): Draw a long coil (solenoid) with the current shown by arrows entering at one end and leaving at the other.

    Inside the solenoid, draw straight, parallel, equally spaced lines running along the axis. Outside, draw closed curves from one end to the other, like those of a bar magnet. Mark the end where the lines emerge as N and the other end as S.

    Put arrows on the lines: inside the solenoid from S to N, outside from N to S.

    1 mark

  3. Conclusion: the field lines inside the solenoid are parallel straight lines, so the magnetic field is uniform, i.e. the same at all points inside the solenoid.

    1 mark

  4. Factor 1: the field is stronger when the number of turns in the solenoid is larger.

    1 mark

  5. Factor 2: the field is stronger when the current through the solenoid is larger. (Another factor is the nature of the core material, e.g. a soft iron core increases the field.)

    1 mark

Answer: Crossing would give two directions of field at one point, which is impossible. Inside a solenoid the lines are straight, parallel and equally spaced, so the field is uniform. The field depends on the number of turns and the current (also the core material).

Where the marks are

  • Reason: two directions of field at one point is impossible (1 mark)
  • Correct diagram of solenoid field lines with arrows (1 mark)
  • Conclusion: uniform field inside the solenoid (1 mark)
  • Any two factors: number of turns, current, core material (1 + 1 marks)

Revise the chapter

Practise this chapterBoard-style questions from Magnetic Effects of Electric Current, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 31/6/1: 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.