In a plane electromagnetic wave, the electric field oscillates with amplitude  20Vm−1, then the energy density of magnetic field is (in J/m3 )

Average energy density of magnetic field  =B2rms2μ0=(B0/2)22μ0=14B02μ0But  B0=E0CAverage energy density of electric field=  14×E02C2μ0But    C=1μ0∈0 ∴E0=1C2μ0 Hence,  Average energy density ofmagnetic field = 14×∈0E02=14×8.55×10−12×(20)2                                                                                                              = 8.55×10−10J/m3