Question

In electromagnetic wave, the average energy density is associated

• A. with electric field only
• B. with magnetic field only
• C. equally with electric and magnetic fields
• D. None of these

Answer 1

Answer: (C)

equally with electric and magnetic fields

$Answer:-$ C

The energy in an electromagnetic wave is tied up in the electric and magnetic fields. In general, the energy per unit volume in an electric field is given by:

energy density in electric field=$\frac{1}{2}{ϵ}_0{E}^2$

In a magnetic field, the energy per unit volume is:

energy density in magnetic field=$\frac{1B^2}{2{\mu }_0}$

An electromagnetic wave has both electric and magnetic fields, so the total energy density associated with an electromagnetic wave is:

$u=$$\frac{1}{2}{ϵ}_0{E}^2$+$\frac{1B^2}{2{\mu }_0}$

It turns out that for an electromagnetic wave, the energy associated with the electric field is equal to the energy associated with the magnetic field, so the energy density can be written in terms of just one or the other:

$u=$$\frac{1}{2}{ϵ}_0{E}^2$$=$$\frac{1B^2}{2{\mu }_0}$

This also implies that in an electromagnetic wave, E = cB.