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Electric Circuits 8th Edition 54

Electric Circuits 8th Edition 54 - late the product of the...

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30 Circuit Etements We use ideal resistors in circuit analysis to model the behavior of physical devices. Using the qualifier ideal rem'inds us that the resistor model makes several simplifying assumptions about the behavior of actual resistive devices.lhe most impo ant ofthese simplifying assump- tions is that the iesistance of the ideal resistor is constant and its value aloes not vary over time. Most actual resistive devices do rot have constant resistance. and their resistance does vary over time. The ideal rcsistor model can be used to represent a physical device whose resistance doesn't vary much from some constant value over the time period of interest in the circuit analysis.In this book we assume that the simplifyhg assump- tions about resistanc€devices are valid, ard we thus use ideal resistors in circuit analysis. We may calculate the powel at the termhals of a resistor in several ways. The first approach is to use tle defining equation and simply calcu-
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Unformatted text preview: late the product of the terminal voltage and curent. For the refeience sys-tems shoM in Fis.2.6. we x'dte \2.6) wheno = iRand (2.7) when 1' = -iR. A second method of expressing the power at the terminals ol a resis-tor expresses power in terms of the currcnt arld the resistance. Substitutirg Eq. 2.1 into Eq. 2.6,we obtain Power in a resistor in terms of current b' P = i 2 R Likewise, substituting Eq.2.2 into Eq.2.7,we have p = -1)i : -(-i R)i = i2R. (2.e) p = u i = ( i R ) i p (2.8) Equations 2.8 and 2.9 are identical and demonstrate clearly that, regard_ less of voltage polarity and current direction, the powel at the terminals of a resistor is positive. Therefore, a resistor absorbs power from the circuit. A tlird method of expressing the power at the teminals of a iesistor is in terms of the voltage and resistance The expression is independent of the Dolaritv references. so R Power in a resistor in tenns of vottage F (2.10)...
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