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A plane electromagnetic wave propagates through free space along the x-axis. At a certain instant and point, the electric field (directed along y) has magnitude 9.3 V/m. What is the magnitude of the magnetic field at that instant?

1. 3.1 x 10⁻⁸ T
2. 3 x 10⁻⁵ T
3. 3.1 x 10⁻⁶ T
4. 9.3 x 10⁻⁶ T

Correct answer: 3.1 x 10⁻⁸ T

Solution

In a plane EM wave in free space the magnitudes are related by E = c*B, so B = E/c. With E = 9.3 V/m and c = 3e8 m/s, B = 9.3/3e8 = 3.1e-8 T.

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