PhysicsCBSE Class 12 · Sample paper, session 2024-25 · 3 marks · Short answer

CBSE Class 12 Physics sample paper 2024-25: question 26

From the official CBSE sample question paper for 2024-25, section C, question 26, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q26 · 3 marks · Short answerCompetency-basedMoving Charges and Magnetism

A boy is holding a smooth, hollow and non-conducting pipe vertically with charged spherical ball of mass 10 g carrying a charge of +10 mC inside it which is free to move along the axis of the pipe. The boy is moving the pipe from East to West direction in the presence of magnetic field of 2T. With what minimum velocity, should the boy move the pipe such that the ball does not move along the axis. Also determine the direction of the magnetic field.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

The ball stays at rest relative to the pipe only if some force balances gravity.

Hint 2

The magnetic force $q v B sin θ$ is largest, so v is smallest, when B ⟂ v.

Hint 3

Use $F = q v \times B$ (or Fleming's left-hand rule) with F up and v towards West.

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.

For the ball not to move along the vertical pipe, the upward magnetic force must balance its weight: $q v B sin θ = m g \Rightarrow v = \frac{m g}{q B sin θ}$
½ mark
v is minimum when $sin θ = 1$ , i.e. B is horizontal and perpendicular to the velocity: $v_{min} = \frac{m g}{q B}$
½ mark
With $m = 10 g = 1 0^{- 2}$ kg and $q = 10 mC = 1 0^{- 2}$ C: $v_{min} = \frac{1 0 ^{- 2} \times 9.8}{1 0 ^{- 2} \times 2}$
½ mark
$v_{min} = 4.9 m s^{- 1}$
½ mark
Direction: $F = q v \times B$ must be vertically upward, with $v$ towards West. Take East = $\hat{i}$ , North = $\hat{j}$ , Up = $\hat{k}$ . Then $(- \hat{i}) \times (- \hat{j}) = + \hat{k}$ , so $B$ is along $- \hat{j}$ . This is consistent with Fleming's left-hand rule: the magnetic field is horizontal, from North to South.
1 mark

Answer: $v_{min} = 4.9$ m/s; magnetic field horizontal, from North to South

Where the marks are

Equates magnetic force to weight, $q v B sin θ = m g$ (½ mark)
Uses $sin θ = 1$ for minimum v (½ mark)
Correct substitution in SI units (½ mark)
Result 4.9 m/s (½ mark)
Direction of B: North to South, with reasoning (1 mark)

Revise the chapter

Chapter: Moving Charges and Magnetism (notes, formulas, where marks are lost and every past question)

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 paperSample paper 2024-25: every question, by section.

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CBSE Class 12 Physics sample paper 2024-25: question 26

Hints

Step-by-step solution

Revise the chapter

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