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Date: 11-5-2021
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Reactance does not consume power
A coil or capacitor cannot dissipate power. The only thing that such a component can do is store energy and then give it back to the circuit a fraction of a cycle later. In real life, the dielectrics or wires in coils or capacitors dissipate some power as heat, but ideal components would not do this.
A capacitor, as you have learned, stores energy as an electric field. An inductor stores energy as a magnetic field. A reactance causes ac current to shift in phase, so that it is no longer exactly in step with the voltage. In a circuit with inductive reactance, the current lags the voltage by up to 90 degrees, or one-quarter cycle. In a circuit with capacitive reactance, the current leads the voltage by up to 90 degrees.
In a resistance-reactance circuit, true power is dissipated only in the resistive components. The reactive components cause the VA power to be exaggerated compared with the true power.
Why does reactance cause this discrepancy between apparent (VA) power and true power? In a circuit that is purely resistive, the voltage and current march right along in step with each other, and therefore, they combine in the most efficient possible way (Fig. 1A). But in a circuit containing reactance, the voltage and current don’t work together as well (Fig. 1B) because of their phase difference. Therefore, the actual energy expenditure, or true power, is not as great as the product of the voltage and the current.
Fig. 1: At A, current (I) and voltage (E) are in phase in a nonreactive ac circuit. At B, I and E are not in phase when reactance is present.
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