Poynting Theorem: Why is 1st Term Ignored?

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The discussion centers on the application of the Poynting theorem in calculating power in electrical systems, specifically regarding current in wires and Joule heating. It highlights that the first term in the theorem represents the total energy stored in electromagnetic fields, while the second term indicates energy flow from these fields. In stationary states where fields are time-independent, the first term vanishes, making it unnecessary for calculations. However, when electromagnetic fields are changing, the first term must be included since its derivative is not zero. Understanding these distinctions is crucial for accurate power calculations in varying electromagnetic conditions.
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why is it that when we use the poynting theorem to calculate power of

e.g, current flowing down a wire , which gives joule heating ,

we don't make use of the 1st term, but only the 2nd term is used?

910f60a9378cc62bee1e0bd603e62733.png


doesn't the first term give the total energy stored in the EM fields?

the 2nd term gives the energy flowing out of the EM fields

the 3rd term is the workdone per unit time on the charges right?
 
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If you look at stationary states only, i.e., if the fields are time independent, the first term vanishes. Otherwise it has to be included.
 
oh! so as long as i have E or B induction(changing) fields, i have to include the first term because its d/dt is not 0?

thanks!
 

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