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chrisbaird
chrisbaird is offline
#4
Mar5-12, 10:46 AM
P: 617

Is there a way to know how the amplitude of a wave will evolve?


Yes, this is essentially a statement of conservation of energy. If a wave spreads out, its energy density must decrease. The J dot E term accounts for the fact that energy can also be lost from the fields to accelerating charges that create currents.