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During the propagation of an electromagnetic wave through a medium:

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

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

Solution

In an electromagnetic wave the electric energy density u_E = (1/2)epsilon*E² and magnetic energy density u_B = B²/(2*mu) are equal at every point and instant, because E = vB with v = 1/sqrt(mu*epsilon). Substituting gives u_E = u_B; the total energy is shared equally between the two fields.

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