Electric Circuits, 9th Edition P5. Designed for use in a one or two-semester Introductory Circuit Analysis or Circuit Theory Course taught in Electrical or Computer Engineering Departments. Electric Circuits 9/e is the most widely used introductory circuits textbook of the past 25 years. As this book has evolved over the years to meet the changing learning styles of students, importantly, the underlying teaching approaches and philosophies remain unchanged. | 16 Circuit Variables We can now state the rule for interpreting the algebraic sign of power Interpreting algebraic sign of power If the power is positive that is if p 0 power is being delivered to the circuit inside the box. If the power is negative that is if p 0 power is being extracted from the circuit inside the box. For example suppose that we have selected the polarity references shown in Fig. b . Assume further that our calculations for the current and voltage yield the following numerical results i 4 A and v -10 V. Then the power associated with the terminal pair 1 2 is p - -10 4 40 W. Thus the circuit inside the box is absorbing 40 W. To take this analysis one step further assume that a colleague is solving the same problem but has chosen the reference polarities shown in Fig. c . The resulting numerical values are i -4 A. v 10 V and p 40 W. Note that interpreting these results in terms of this reference system gives the same conclusions that we previously obtained namely that the circuit inside the box is absorbing 40 W. In fact any of the reference systems in Fig. yields this same result. Example illustrates the relationship between voltage current power and energy for an ideal basic circuit element and the use of the passive sign convention. Example Relating Voltage Current Power and Energy Assume that the voltage at the terminals of the element in Fig. whose current was defined in Assessment Problem is v 0 0 v 10c 5 Wi kV t 0. a Calculate the power supplied to the element at 1 ms. b Calculate the total energy in joules delivered to the circuit element. b From the definition of power given in Eq. . the expression for energy is w f p x dx Jo To find the total energy delivered integrate the expresssion for power from zero to infinity. Therefore total 200 000c 200 000c 1 x j __ -10 000 -20c - 20c 0 20 20 J. Solution a Since the current is entering the terminal of the voltage drop defined for the element in Fig.
