The rms value of the electric field of the light from the sun is `720 N//C` The total energy density of the electromagnetic wave is

Options

A. `3.3xx10^-3Jm^-3`
B. `4.58xx10^-6Jm^-3`
C. `6.37xx10^-9Jm^-3`
D. `81.35xx10^(-12)Jm^-3`

Answer

b

Explanation

Total average energy density of e.m wave is
`u=u_E+u_B=1/2in_0 E_(rms)^2+1/2 (B_(rms)^2)/(mu_0)`
`=1/2in_0 E_(rms)^2+1/(2mu_0) (E_(rms)^2)/(c^2) ( :. B_(rms)=(E_(rms))/c)`
`=1/2in_0 E_(rms)^2+(mu_0 in_0)/(2mu_0) (E_(rms)^2) ( :' c=1/(mu_0in_0))`
`=1/2in_0 E_(rms)^2+1/2in_0 E_(rms)^2=in_0 E_(rms)^2`
`=(8.85xx10^(-12))xx(720)^2=4.58xx10^-6Jm^-3`