Electric Circuits 8th Edition 39

Electric Circuits 8th Edition 39 - 1.6 P owerndE neqy a...

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Equation 1.4 shows that the pow€r associated with a basic ckcuit element is simply the pmduct of the current in the element and the voltage adoss the element. Therefore, power is a quaDtity associated with a pair of tei_ minals, and we have to be able to tell from our calculation whether power is being delivered to the pair of teminals or extracted from it. This infor- mation comes ftom the corect application and interpretation of tle pas- sive sign convention. If we use the passive sign convention, Eq. 1.4 is correct if the rcference alirection for the current is in the direction of the rcference voltage drop across the terminals. Otherwise, Eq. 1.4 must be written with a minus sign. In other words, if the current reference is il the direction of a refererce voltage rise across the teminals, the expression for the power is (1.5) 1.6 Power and Eneqy '- I;:] 41.-_stl}-l , l - - : l . L - - - I *-113 1 *----f:--] ( c ) p = - d (d)p = ,i + The algebraic sign of power is based on charge movement tbiough voltage drcps and rises. As positive chargesmove ttrrough a drop in volt-
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