Practice portal › Alternating Current › Power in AC Circuits
Asked in GUJCET 2024 · Wattless current
Idea: the power averaged over a cycle is P=VᵣₘₛIᵣₘₛ cos φ, so a current is wattless only when cos φ=0, that is φ=90°.
A phase difference of 90° needs zero resistance, because tan φ=X/R becomes infinite only when R=0.
An ideal inductor on its own does exactly this: the current lags the voltage by 90°.
Physically it stores energy in its magnetic field for a quarter cycle and hands all of it back in the next, so nothing is consumed over a full cycle.
So only L is connected across the a.c. supply.
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