Class JEE Physics ALL Q #1182
COMPETENCY BASED
APPLY
4 Marks 2026 JEE Main 2026 (Online) 21st January Morning Shift MCQ SINGLE
A wave propagates whose electric field is given by $\vec{E}=69~\sin(\omega t-kx)\hat{j}$. Find the direction of the magnetic field.
(A) $\hat{k}$
(B) $-\hat{k}$
(C) $\frac{\hat{i}+\hat{j}}{\sqrt{2}}$
(D) $\frac{\hat{i}-\hat{j}}{\sqrt{2}}$
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Step-by-Step Solution

  1. The direction of propagation of the wave is given by the direction of $\vec{E} \times \vec{B}$.
  2. The electric field is along the $\hat{j}$ direction, and the wave propagates along the $\hat{i}$ direction (from the term $kx$ in the argument of the sine function).
  3. Therefore, $\hat{i} = \hat{j} \times \hat{B}$.
  4. To satisfy this equation, $\hat{B}$ must be in the $-\hat{k}$ direction, since $\hat{j} \times (-\hat{k}) = \hat{i}$.

Correct Answer: B

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AI Suggestion: Option B

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because it requires applying the relationship between the directions of electric field, magnetic field, and wave propagation.
Knowledge Dimension: PROCEDURAL
Justification: The question requires applying a known procedure (right-hand rule or the relationship between E, B, and the direction of propagation) to determine the direction of the magnetic field.
Syllabus Audit: In the context of JEE, this is classified as COMPETENCY. It tests the student's ability to apply the concepts of electromagnetic waves to determine the direction of the magnetic field, which is a competency-based application.

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