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For an electromagnetic wave traveling through a material medium, which statement about the electric and magnetic energy densities is correct?

1. The electric energy density is twice the magnetic energy density.
2. The electric energy density is one-half of the magnetic energy density.
3. The electric energy density equals the magnetic energy density.
4. Both the electric and magnetic energy densities are zero.

Correct answer: The electric energy density equals the magnetic energy density.

Solution

For an electromagnetic wave the instantaneous electric and magnetic energy densities are equal: u_E = (1/2)e0 E^2 = u_B = B^2/(2 mu0). So the electric energy density equals the magnetic energy density.

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