NEET 2025 · PhysicsField directionsPrevious Year Question
The electric field in a plane electromagnetic wave is given by E_z = 60 cos(5x + 1.5×10⁹ t) V/m. Then the expression for the corresponding magnetic field is (subscripts denote field direction)
- A.B_z = 60 cos(5x + 1.5×10⁹ t) T
- B.B_z = 60 sin(5x + 1.5×10⁹ t) T
- C.B_y = 2×10⁻⁷ cos(5x + 1.5×10⁹ t) T✓
- D.B_x = 2×10⁻⁷ cos(5x + 1.5×10⁹ t) T
Correct Answer
(C) B_y = 2×10⁻⁷ cos(5x + 1.5×10⁹ t) T
Solution & Explanation
Answer: (C) B_y = 2×10⁻⁷ cos(5x + 1.5×10⁹ t) T. The amplitude is B₀ = E₀/c = 60/(3×10⁸) = 2×10⁻⁷ T. The (5x + ωt) phase means the wave travels in the −x direction. With E along ẑ and propagation along −x, B must satisfy E × B ∝ −x̂. Since ẑ × ŷ = −x̂, B is along +y. B and E share the same phase function.
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