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QPage 339: Q5: HC Verma: Concepts of Physics - Volume 2 Electromagnetic Waves Page 339: Q5 Solution: The energy contained in a small volume through which an…

⚠️ Common Exam Traps & Conceptual Pitfalls

Energy density is proportional to the square of the electric or magnetic field, $upropto E^2+B^2$. If $E(t)=E_0cos(omega t)$, then $E^2(t)=E_0^2cos^2(omega t)= frac12E_0^2[1+cos(2omega t)]$, so the energy density oscillates at $2omega$ (double the wave frequency). Students often mistakenly treat $u$ as linear in $E$ and conclude the frequency is unchanged.

The energy contained in a small volume through which an electromagnetic wave is passing oscillates with

Multiple Choice Options

  • A zero frequency
  • B the frequency of the wave
  • C half the frequency of the wave
  • D double the frequency of the wave

Step-by-Step Verified Solution

**Answer:** (d) double the frequency of the wave. **Step-by-Step Explanation:** Energy density is proportional to E_ or B_, which oscillates at twice the wave frequency.

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