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The amplitude of the magnetic field part of a harmonic electromagnetic wave in vacuum is B₀ = 720 nT. What is the amplitude of the electric field part of the wave?

Asked in GSEB Board July 2023 · E0 = cB0 and E/B = c

• (1) 216 N C⁻¹
• (2) 172 N C⁻¹
• (3) 2.19 N C⁻¹
• (4) 153 N C⁻¹

Step-by-step solution

Why the other options are wrong

• (2) No route reaches 172 N C⁻¹: it is neither cB₀ nor cB₀ divided by any factor the problem offers. Sanity check: cB₀ = (3×10⁸)(720×10⁻⁹) = 216 N C⁻¹.
• (3) 2.19 N C⁻¹ is about a hundredth of the right answer, which smells of a mishandled prefix -- but even reading n as 10⁻¹¹ gives 2.16, so no clean slip lands here. Sanity check: nT means 10⁻⁹ T, and cB₀ = 216 N C⁻¹.
• (4) 153 N C⁻¹ is (216)/(√2), the root-mean-square electric field. The question asks for the amplitude, which is √2 times larger.

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