# 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

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 , uE = uB)

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

= (8.85 × 10-12)× (720)2

= 4.58 × 10-6 Jm-3