text: It applies directly to DC and only sort of to AC.\n\nThe problem with AC is the power. If you drive a perfect inductor or capacitor, you will have voltage and current, but no power.  In real life, the load of an AC circuit has both reactance (capacitive or inductive) and resistance, so there will be power drawn, but in a more complex manner than simply P=IE.\n\nThe load is complex also, the inductance and capacitance are 180 degrees out of phase with each other and 90 degrees out of phase with a real (resistive) load. So when you add them you need to use trigonometry.\n\nDon't even ask about RF circuits. It is another order of magnitude higher in difficulty.