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CHAP. 8]



nonzero, the function is a damped cosine. Only negative values of  are considered. If  and ! are zero, the result is a constant. And finally, with ! ¼ 0 and  nonzero, the result is an exponential decay function. In Table 8-1, several functions are given with corresponding values of s for the expression Aest . Table 8-1 f ðtÞ 5t

10e 5 cos ð500t þ 308Þ 2e3t cos ð100t  458Þ 100:0



5 þ j 0 þ j500 3 þ j100 0 þ j0

10 5 2 100.0

When Fig. 8-10 is examined for various values of s, the three cases are evident. If  ¼ 0, there is no damping and the result is a cosine function with maximum values of Vm (not shown). If ! ¼ 0, the function is an exponential decay with an initial value Vm . And finally, with both ! and  nonzero, the damped cosine is the result.



A driving voltage of the form v ¼ Vm est applied to a passive network will result in branch currents and voltages across the elements, each having the same time dependence est ; e.g., Ia e j est , and Vb e j est . Consequently, only the magnitudes of currents and voltages and the phase angles need be determined (this will also be the case in sinusoidal circuit analysis in Chapter 9). We are thus led to consider the network in the s-domain (see Fig. 8-11).

Fig. 8-11

A series RL circuit with an applied voltage v ¼ Vm e j est will result in a current i ¼ Im e j est ¼ Im est , which, substituted in the nodal equation Ri þ L

di ¼ Vm e j est dt

will result in RIm est ¼ sLIm est ¼ Vm e j est

from which

Im ¼

Vm e j R þ sL

Note that in the s-domain the impedance of the series RL circuit is R þ sL. fore has an s-domain impedance sL.

The inductance there-