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Answer the following. (Science 2026 31/3/1 Q39b)

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ScienceCBSE Class 10 · Board exam 2026, set 31/3/1 · 5 marks · Long answer

CBSE Class 10 Science 2026: question 39(b)

Set in the CBSE Class 10 board exam of 2026, paper 31/3/1, section C, question 39(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q39(b) · 5 marks · Long answerMagnetic Effects of Electric Current

Answer the following.

  1. (i)

    With the help of a diagram, show that a current-carrying conductor placed in an external magnetic field, experiences a force. Indicate the direction of the field, current and force clearly.

    0 marks
  2. (ii)

    The diagram given below shows an electron moving in a uniform magnetic field.
    (I) What will be the direction of the force acting on the electron ?
    (II) What will happen to the direction of the force acting on the electron, if the direction of the magnetic field as well as the direction of motion of the electron, both are reversed ?

    0 marks

Figure in the paper: Four parallel horizontal field lines with arrows pointing left, labelled Magnetic field. A vertical line crosses them with an upward arrow at its top labelled Electron, i.e. the electron moves upward, perpendicular to the leftward field. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 31/3/1.

Hints

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

Hint 1

Recall the NCERT activity with an aluminium rod hung between the poles of a horseshoe magnet.

Hint 2

An electron's motion is opposite to the direction of conventional current. Find the current direction first.

Hint 3

Apply Fleming's left-hand rule: forefinger for the field, middle finger for the current, thumb for the force. Then think what happens when two quantities are reversed together.

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. (i) Activity: suspend a small aluminium rod horizontally by two connecting wires between the poles of a horseshoe magnet, and connect it in series with a battery, a key and a rheostat. When the key is closed and current passes, the rod is displaced. This shows that a current-carrying conductor in a magnetic field experiences a force. Reversing the current reverses the displacement.

    1 mark

  2. (i) Diagram (described): draw a horseshoe magnet with its N pole above and S pole below, and the rod AB lying between the poles, joined to the circuit.

    ½ mark

  3. Mark the current from A to B along the rod (for example, into the page).

    ½ mark

  4. Mark the magnetic field vertically downward, from N to S.

    ½ mark

  5. Mark the force on the rod, perpendicular to both the field and the current (here, horizontally, as given by Fleming's left-hand rule).

    ½ mark

  6. (ii)(I) The electron moves upward, so the conventional current is downward. The field points to the left. By Fleming's left-hand rule (forefinger to the left for the field, middle finger downward for the current), the thumb points into the plane of the paper. So the force on the electron is into the plane of the paper.

    1 mark

  7. (ii)(II) Reversing the field alone would reverse the force, and reversing the motion alone would also reverse it. Reversing both cancels out, so the direction of the force remains the same (into the plane of the paper).

    1 mark

Answer: (i) The rod is displaced when current flows, so a current-carrying conductor in a field experiences a force perpendicular to both field and current. (ii)(I) Into the plane of the paper; (II) the direction of force remains the same.

Where the marks are

  • Explanation that the conductor experiences a force and is displaced when current flows (1 mark)
  • Diagram of rod between magnet poles (½ mark)
  • Direction of current shown (½ mark)
  • Direction of magnetic field shown (½ mark)
  • Direction of force shown, consistent with Fleming's left-hand rule (½ mark)
  • Force on electron into the plane of the paper (1 mark)
  • Force direction remains the same when both field and motion are reversed (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 paper2026 board paper, set 31/3/1: 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.