A 10 cm long wire lies along y-axis. It carries a current of 1.0 A in positive y-direction. A magnetic field B = (5 mT)j - (8 mT)k exists in the region. The force on the wire is : (A) (0.8 mN)i (B) -(0.8 mN)i (C) (80 mN)i (D) -(80 mN)i

PhysicsCBSE Class 12 · Board exam 2024, set 55/1/1 · 1 mark · MCQ

CBSE Class 12 Physics 2024: question 5

Set in the CBSE Class 12 board exam of 2024, paper 55/1/1, section A, question 5, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q5 · 1 mark · MCQMoving Charges and Magnetism

A 10 cm long wire lies along y-axis. It carries a current of 1.0 A in positive y-direction. A magnetic field $B = (5 mT) \hat{j} - (8 mT) \hat{k}$ exists in the region. The force on the wire is :

(A) $(0.8 mN) \hat{i}$
(B) $- (0.8 mN) \hat{i}$
(C) $(80 mN) \hat{i}$
(D) $- (80 mN) \hat{i}$

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 55/1/1.

Hints

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

Hint 1

Use $F = I l \times B$ .

Hint 2

Recall $\hat{j} \times \hat{j} = 0$ and $\hat{j} \times \hat{k} = \hat{i}$ .

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.

Use $F = I l \times B$ with $l = 0.1 \hat{j}$ m and $I = 1$ A.
Then $F = (0.1 \hat{j}) \times (5 \hat{j} - 8 \hat{k}) \times 1 0^{- 3} = - 0.8 \times 1 0^{- 3} (\hat{j} \times \hat{k}) = - (0.8 mN) \hat{i}$ , since $\hat{j} \times \hat{j} = 0$ and $\hat{j} \times \hat{k} = \hat{i}$ . Option (B).
1 mark

Answer: $- (0.8 mN) \hat{i}$ , option (B)

Where the marks are

Evaluates the cross product correctly to get −0.8 mN î (1 mark)

Revise the chapter

Chapter: Moving Charges and Magnetism (notes, formulas, where marks are lost and every past question)
Topic: Torque on a Current Loop and Magnetic Dipole

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 paper2024 board paper, set 55/1/1: every question, by section.

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CBSE Class 12 Physics 2024: question 5

Hints

Step-by-step solution

Revise the chapter

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