Q: The r.m.s value of electric field of light coming from the sun is 720 N/C . The average total energy density of the electromagnetic wave is

(a) 3.3 × 10^{-3} Jm^{-3}

(b) 4.58 × 10^{-6} Jm^{-3}

(c) 6.37 × 10^{-9} Jm^{-3}

(d) 81.35 × 10^{-12} Jm^{-3}

Sol: Total average energy density of EM wave is

$\large u = u_E + u_B = 2 u_E $ (since , u_{E} = u_{B})

$\large u = 2 (\frac{1}{2}\epsilon_0 E_{rms}^2) = \epsilon_0 E_{rms}^2 $

= (8.85 × 10^{-12})× (720)^{2}

= 4.58 × 10^{-6} Jm^{-3}