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modphysnoob
E cross H= Power Curl(E cross H)=H (curl E)- E(Curl H) H ( -dB/dt ) -E( J+ dD/dt) for some reason they say J=0 , I don't get why?
J depends on what material is used. In free space there are no electric charges, and hence J equals to zero.
(same answer as Anonymous1921) If you use this equation with reference to an electromagnetic wave travelling in a vacuum, there is not current there, so J = 0