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In order to obtain magnetic… (Class 10 Science 2025, Q39)

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ScienceCBSE Class 10 · Board exam 2025, set 31/5/1 · 4 marks · Case-based

CBSE Class 10 Science 2025: question 39

Set in the CBSE Class 10 board exam of 2025, paper 31/5/1, section E, question 39, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q39 · 4 marks · Case-basedCompetency-basedMagnetic Effects of Electric Current

In order to obtain magnetic field lines around a bar magnet, a student performed an experiment using a magnetic compass and a bar magnet. The magnet was placed on a sheet of white paper fixed on a drawing board. Using magnetic needle he obtained on the paper a pattern of magnetic field lines (as shown below) around the bar magnet.

  1. (a)

    By convention, the field lines emerge from north pole and merge at south pole. Why? Give reason.

    1 mark
  2. (b)

    State the relationship between strength of the magnetic field and the degree of closeness of the field lines.

    1 mark
  3. (c)

    (A) (i) No two field lines can ever intersect each other. Give reason. (ii) The magnetic field in a given region is uniform. Draw a diagram to represent it.

    2 marks

  4. OR

    (c)

    (B) Draw the pattern of the magnetic field lines through and around a current carrying solenoid. What does the pattern of field lines inside the solenoid represent?

    2 marks

Figure in the paper: Magnetic field lines around a bar magnet lying horizontally, with S pole on the left and N pole on the right. Field lines emerge from the N end, loop around the outside and enter the S end; loops are closer together near the poles. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/5/1.

Hints

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

Hint 1

Think about which way the north pole of a small compass points when it is placed near a magnet.

Hint 2

Field lines show direction at each point. What would two directions at one point mean for a compass?

Hint 3

A uniform field has the same strength and direction everywhere. How would you draw lines to show that?

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. (a) The direction of a field line is taken as the direction in which the north pole of a compass needle moves when placed in the field. The needle's north pole is repelled by the magnet's N pole and attracted to its S pole. So outside the magnet the lines emerge from N and merge at S.

    1 mark

  2. (b) The closer together the field lines, the stronger the magnetic field in that region. The field is strongest near the poles, where the lines are most crowded.

    1 mark

  3. (c)(A)(i) If two field lines crossed, there would be two tangents (two field directions) at the point of intersection. A compass needle there would have to point in two directions at once, which is impossible. So field lines never intersect.

    1 mark

  4. (c)(A)(ii) Diagram: Draw several straight, parallel, equally spaced lines, all with arrows pointing in the same direction. This represents a uniform magnetic field (as found inside a solenoid).

    1 mark

  5. (c)(B) Diagram: Draw a solenoid (coil of many turns) with the current direction marked. Inside it, draw straight, parallel lines along the axis. Outside, draw closed loops from one end (N) to the other (S), like the field of a bar magnet. Mark arrows from N to S outside and from S to N inside.

    1 mark

  6. The parallel, straight field lines inside the solenoid show that the magnetic field there is uniform: it has the same strength and direction at all points.

    1 mark

Answer: (a) By convention, a field line's direction is the direction a compass north pole moves, which is away from N and towards S; (b) closer lines mean a stronger field; (c)(A)(i) intersection would give two field directions at one point, which is impossible; (ii) equally spaced parallel straight lines with arrows in one direction; (c)(B) bar-magnet-like pattern with parallel lines inside, showing a uniform field inside the solenoid.

Where the marks are

  • Relates field-line direction to the movement of a compass north pole (1 mark)
  • Closer field lines indicate a stronger field (1 mark)
  • (c)(A)(i) Intersection would mean two directions of the field/compass at one point, which is impossible (1 mark)
  • (c)(A)(ii) Diagram of equidistant parallel lines with arrows in one direction (1 mark)
  • (c)(B) Correct field pattern of a current-carrying solenoid (1 mark)
  • (c)(B) The field inside the solenoid is uniform (1 mark)

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 31/5/1: 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.