Question

During the propagation of electromagnetic waves in a medium,

• A. electric energy density is double of the magnetic energy density
• B. electric energy density is half of the magnetic energy density
• C. electric energy density is equal to the magnetic energy density
• D. Both electric and magnetic energy densities are zero

Hint:

Energy is equally divided between the electric and magnetic fields. The Electric energy density is equivalent to the magnetic energy density.

Answer 1

The Correct Option is C

The propagation of electromagnetic waves in a medium involves both electric and magnetic fields. An important concept to understand here is the energy densities of these fields. The energy density refers to the amount of energy stored per unit volume in the electric and magnetic fields.

In the context of electromagnetic waves:

• The electric energy density (u_e) is given by the formula:
  u_e = \frac{1}{2} \varepsilon E^2
  where \varepsilon is the permittivity of the medium and E is the electric field strength.
• The magnetic energy density (u_m) is given by the formula:
  u_m = \frac{1}{2} \mu H^2
  where \mu is the permeability of the medium and H is the magnetic field strength.

In an electromagnetic wave, both these energy densities are equal on average. This is due to the inherent properties of electromagnetic waves where the energy is equally distributed between the electric and magnetic fields during propagation. Therefore, we have:

u_e = u_m

This means the electric energy density is equal to the magnetic energy density. Hence, the correct answer to the question is that the electric energy density is equal to the magnetic energy density.

Let's rule out the other options:

• Electric energy density is double of the magnetic energy density: Incorrect, as explained. Both densities are equal during wave propagation.
• Electric energy density is half of the magnetic energy density: Incorrect, as electric and magnetic energy densities are equal.
• Both electric and magnetic energy densities are zero: Incorrect, because energy density is non-zero in an electromagnetic wave. If they were zero, it would imply no energy propagation, contradicting the nature of electromagnetic waves.