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Unformatted text preview: 20 Poynting theorem and monochromatic waves The magnitude of Poynting vector S = E H represents the amount of power transported often called energy flux by electromagnetic fields E and H over a unit area transverse to the E H direction. This interpretation of the Poynting vector is obtained from a conservation law extracted from Maxwells equations (see margin) as follows: D = B = 0 E =- B t H = J + D t . 1. Dot multiply Faradays law by H , dot multiply Amperes law by E , ( E =- B t ) H ( H = J + D t ) E and take their difference: H E- E H =- D t E- B t H- J E . ( E H )- t ( 1 2 E E + 1 2 H H ) 2. After re-arrangements shown above, the result can be written as 1 t ( 1 2 E E + 1 2 H H ) + ( E H ) + J E = 0 . Poynting theorem derived above is a conservation law just like the continuity equation t + J = 0 : Poynting theorem The first term on the left, t ( 1 2 E E + 1 2 H H ) , is time rate of change of total electric and magnetic energy den- sity....
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This note was uploaded on 06/20/2011 for the course ECE 329 taught by Professor Kim during the Spring '08 term at University of Illinois, Urbana Champaign.
- Spring '08