question_answer 37)

    Show that average value of radiant flux density 'S' over a single period ?T' is given by .

Answer:

                  We know that radiant flux density S (called Poynting vector) is given by                        "                   Let the em wave be propagating along -i-axis. The electric field vector of em wave be along y-axis and magnetic field vector be along z-axis. Therefore,                            Average value of the magnitude of radiant flux density over complete cycle is                  dt   or                                           =