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A plane EM wave travelling along z-direction is described byE=E0 sin(kz-ωt)ˆi and B=B0 sin(kz-ωt)ˆj. Show that:

(i) the average energy density of the wave is given by Uav=14ε0E20+14B20μ0

(ii) the time averaged intensity of the wave is given by Iav=12cε0E20.

Hint: The average energy density of the wave is distributed equally between the electric field and the magnetic field.
(I) Step 1: The electromagnetic wave carries energy which is due to electric field vector and magnetic field vector. In an electromagnetic wave, E and B vary from point to point and from moment to moment. Let E and B be their time averages.
The energy density due to electric field E is
UE=12ε0E2
The energy density due to magnetic field B is:
UB=1 B22 μ0
Total average energy density of electromagnetic wave:
Uav=UE=12ε0E2+12B2μ0
Step 2: Let the EM wave be propagating along z-direction. The electric field vector and magnetic field vector be represented by
E=E0sin(kz-ωt)
B=B0sin(kz-ωt)
The time average value of E2 over complete cycle=E202
The time average value of B2 over complete cycle=B202
Uav=12ε0E202+12μ0(B202)
=14ε0E20+B204μ0
(ii) Step 3: We know that E0=cB0 and c=1μ0ε0

    UB=UE
Hence,  Uav=14ε0E02+14B024μ0
   =14ε0E02+14ε0E02
  =12ε0E02=12B02μ0
Time average intensity of the wave:
Iav=UavC=12ε0E02c=12ε0E02