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It is generally more convenient to consider the time-average version of (2.65), which reads
where denotes the time-average operation. In the case of time-harmonic fields, this equation can be further manipulated as follows. First, it is straightforward to show that and . Second, the terms and may be expressed in terms of the electric and magnetic susceptibility tensors by using and , where the electric and magnetic conductivity tensors are related to the susceptibility tensors as
(2.68a)
(2.68b)
which leads, after replacing the instantaneous field vectors by their phaser counterparts, to