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A school has installed… (Class 10 Science SQP 2026-27, Q38)

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ScienceCBSE Class 10 · Sample paper, session 2026-27 · 4 marks · Long answer

CBSE Class 10 Science sample paper 2026-27: question 38

From the official CBSE sample question paper for 2026-27, section C, question 38, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q38 · 4 marks · Long answerCompetency-basedMagnetic Effects of Electric Current

A school has installed an automatic electromagnetic door-locking system at the main entrance. The locking mechanism uses a solenoid controlled by a switch at the security desk. When electricity flows through the solenoid, it creates a magnetic field strong enough to pull an iron bolt, unlocking the door. As soon as the switch is turned off, the magnetic field disappears, and a spring pushes the bolt back, locking the door.

  1. A

    Why is an iron core kept inside the solenoid in this locking mechanism?

    0 marks
  2. B

    How can we determine the magnetic poles in this solenoid?

    0 marks
  3. C

    Attempt either subpart (C) or (D). If the current in the solenoid is increased from 1 A to 2 A, explain how the magnetic effect changes and its effect on unlocking the door.

    0 marks

  4. OR

    D

    If the source of the current is fixed, how can we increase the magnetic field in the solenoid?

    0 marks

Figure in the paper: Solenoid assembly: a coil of wire wound around a soft iron core, with a plunger rising vertically above it. Labelled: Plunger (arrow pointing up), Switch, Electric current energizes a coil, Soft iron core, and a second Switch terminal connected to the circuit. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

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

Hint 1

What happens to a piece of soft iron when it is placed in a magnetic field?

Hint 2

A solenoid behaves like a bar magnet. How do you test which end of a magnet is North?

Hint 3

The field of a solenoid depends on the current and on the number of turns per unit length.

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. Soft iron is a magnetic material. Placed inside the solenoid, it becomes strongly magnetised and concentrates the field lines, so the magnetic field becomes much stronger. This gives a stronger pull on the iron bolt. Soft iron also loses its magnetism as soon as the current is switched off, so the spring can push the bolt back and lock the door.

    1 mark

  2. B. Use the right-hand rule for a solenoid. Curl the fingers of the right hand in the direction of the current in the turns, and the thumb points towards the North pole. Seen from an end, clockwise current marks a South pole and anticlockwise current a North pole. To check, bring the known North pole of a bar magnet near one end of the solenoid. Repulsion shows that end is a North pole.

    1 mark

  3. C. The magnetic field inside a solenoid is directly proportional to the current through it (). Increasing the current from 1 A to 2 A therefore doubles the strength of the magnetic field, other factors being unchanged.

    1 mark

  4. C (contd.) A stronger field gives a stronger magnetic pull on the iron bolt. The bolt is pulled in more firmly and quickly against the spring, so the door unlocks more effectively and reliably.

    1 mark

  5. D (alternative to C). With the current fixed, the field can be increased by: (i) increasing the number of turns of the coil, (ii) winding the same turns over a shorter length so there are more turns per unit length, or (iii) inserting a soft iron core inside the coil. Any two of these.

    2 marks

Answer: A. Iron core concentrates and strengthens the field (and demagnetises when switched off); B. Right-hand rule for a solenoid / repulsion with a known N pole; C. Field doubles (B ∝ I), stronger pull, door unlocks more effectively; D. More turns, shorter coil (more turns per length), soft iron core (any two)

Where the marks are

  • Iron core strengthens/concentrates the magnetic field, giving a stronger pull (1 mark)
  • Method to find poles: right-hand rule or repulsion test with a known North pole (1 mark)
  • (C) Field directly proportional to current, so it doubles (1 mark)
  • (C) Stronger pull, so the bolt is unlocked more effectively (1 mark)
  • (D) Any two correct ways: more turns, more turns per unit length, soft iron core (2 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 paperSample paper 2026-27: 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.