The impedance of a direct current path is called a direct current resistance, which is the inherent static resistance of the component when a direct current is applied. The direct current resistance is governed by Ohm's law R=U/I.
The impedance of the AC path is called the AC resistance. AC resistance generally refers to impedance. In circuits with resistors, inductors, and capacitors, the obstruction effect on AC power is called impedance. Impedance is commonly represented by Z and is a complex number. The real part is called resistance (R), and the imaginary part is called reactance. The obstruction effect of capacitance on AC power in the circuit is called capacitance (xc), and the obstruction effect of inductance on AC power in the circuit is called inductance (xL), The resistance caused by capacitors and inductors to alternating current in a circuit is collectively referred to as reactance. The unit of impedance is ohms, and the calculation of impedance needs to be done using vectors, i.e. Z=√ [R ^ 2+(xL-xc) ^ 2].
For pure resistive devices, the impedance form they exhibit in both AC and DC circuits is the same. But for inductive components connected in series in a circuit, its DC resistance is equal to 0 if the internal resistance of the coil is ignored. However, it exhibits a large AC resistance that prevents changes in current.
Similarly, for capacitors connected in parallel at both ends of the power supply or in series in the circuit, their impedance characteristics are equivalent to short circuits for AC in the frequency range of the capacitor. Therefore, the AC resistance of the capacitor is 0, but for DC circuits, the impedance of the capacitor is quite large. For capacitors connected in parallel in the circuit, their impedance can be considered infinite after being fully charged, and of course, their small leakage current can be ignored.
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