This equation further emphasizes why the rms value is chosen in discussion rather than peak values. Consequently, p( t) is positive at some times and negative at others, indicating that capacitive and inductive elements produce power at some instants and absorb it at others. For a capacitor or inductor, the relative signs of i( t) and v( t) vary over a cycle due to their phase differences (see (Figure) and (Figure)). For a resistor, i( t) and v( t) are in phase and therefore always have the same sign (see (Figure)). ![]() ![]() A plot of p( t) for various circuit elements is shown in (Figure). Since the current and the voltage both depend on time in an ac circuit, the instantaneous power is also time dependent.
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