Impedance (measured in Ohms) is the measure of the resistance to current that a circuit presents over a given voltage, or the ratio between the rms values of current and voltage. In DC circuits, impedance is the same as resistance.
Impedance is a complex number, with resistance being the real part and reactance being the imaginary part. In general, the formula for impedance is
With R being the resistance, j the imaginary unit (not i to avoid confusion with current), XL being the inductive reactance, and XC being the capacitive reactance.
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