JEE Main · 2014 · 06 Apr 2014 · Offline · EM Waves

During the propagation of electromagnetic waves in a medium:

(A)Electric energy density is equal to the magnetic energy density.✓ Correct
(B)Both electric and magnetic energy densities are zero.
(C)Electric energy density is double of the magnetic energy density.
(D)Electric energy density is half of the magnetic energy density.

Answer: (A)

Solution

In a travelling electromagnetic wave the electric and magnetic fields rise and fall together. Their energy densities are

u_{E} = \frac{1}{2} ε_{0} E^{2}

and

u_{B} = \frac{B ^{2}}{2 μ _{0}}

, and because

E = c B

with

c = \frac{1}{ε _{0} μ _{0}}

, substituting shows

u_{E} = u_{B}

at every instant. The electric and magnetic energy densities are equal.

JEE Main · 2014 · 06 Apr 2014 · Offline · Single correct option

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