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NEET
Exam
NEET
Subject
Physics
Chapter
Electromagnetic Waves

NEET · Physics · Electromagnetic Waves

A plane electromagnetic wave with a magnetic field amplitude of 2 ×...

A plane electromagnetic wave with a magnetic field amplitude of 2×10−7 T2 \times 10^{-7} \text{ T} falls normally on a non-reflecting surface of area 2π m22\pi \text{ m}^2. The total energy delivered to the surface in 1 minute1 \text{ minute} is:

(Take c=3×108 m s−1c = 3 \times 10^8 \text{ m s}^{-1} and μ0=4π×10−7 T m A−1\mu_0 = 4\pi \times 10^{-7} \text{ T m A}^{-1})

Options

  1. A3600 J3600 \text{ J}
  2. B30 J30 \text{ J}
  3. C1800 J1800 \text{ J}
  4. D900 J900 \text{ J}

Correct answer

C. 1800 J1800 \text{ J}

Step-by-step solution

The intensity II of an electromagnetic wave in terms of its magnetic field amplitude B0B_0 is given by:

I=B02c2μ0I = \dfrac{B_0^2 c}{2\mu_0}

Substitute the given values:
I=(2×10−7)2×(3×108)2×(4π×10−7)I = \dfrac{(2 \times 10^{-7})^2 \times (3 \times 10^8)}{2 \times (4\pi \times 10^{-7})}

I=4×10−14×3×1088π×10−7I = \dfrac{4 \times 10^{-14} \times 3 \times 10^8}{8\pi \times 10^{-7}}

I=12×10−68π×10−7=1208π=15π W m−2I = \dfrac{12 \times 10^{-6}}{8\pi \times 10^{-7}} = \dfrac{120}{8\pi} = \dfrac{15}{\pi} \text{ W m}^{-2}

The total energy EE delivered to the surface is:
E=I×A×tE = I \times A \times t

Given the area A=2π m2A = 2\pi \text{ m}^2 and time t=1 minute=60 st = 1 \text{ minute} = 60 \text{ s}:
E=(15π)×(2π)×60E = \left(\dfrac{15}{\pi}\right) \times (2\pi) \times 60

E=30×60=1800 JE = 30 \times 60 = 1800 \text{ J}

Answer: 1800 J1800 \text{ J}
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