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Assume we have some circuit with resistance ##R##. In DC, the current ##I## linearly depends on the connected voltage ##V## by Ohm's law ##I=V/R## .

In AC, we can independently connect any voltage and current to the system by using a transformer, provided we have a power source with ##P=VI## . The resistance of the circuit doesn't even show up.

It's not clear to me why there seem to be more degrees of freedom about the connected power source in AC than in DC. Also, shouldn't DC come out as a limit from AC if the frequency goes to zero?