Impedance is a physical quantity that represents the performance of a component or the electrical performance of a section of circuit. The ratio of the peak value (or effective value) Um of the voltage across a section of passive circuit in an AC circuit to the peak value (or effective value) Im of the current passing through the circuit is called impedance, which is represented by z and the unit is ohm (Ω). When U is constant, the larger z is, the smaller I is, and the impedance has a limiting effect on current.
In electric current, the effect of an object on the current is called resistance. Except for superconductors, all substances in the world have electrical resistance, only the difference in resistance value. Substances with low resistance are called good conductors, such as metals; substances with extremely high resistance are called insulators, such as wood and plastic. There is also a conductor between the two called semiconductors, and superconductors are substances whose resistance value is equal to zero, but it requires a sufficiently low temperature and a sufficiently weak magnetic field for its resistivity to be zero.
In direct current and alternating current, resistance has an obstructive effect on both types of currents; as common components, in addition to resistance, there are capacitance and inductance, both of which do not have an obstructive effect on alternating current and direct current like resistance. Capacitors are "blocking direct and alternating current", that is, they have a blocking effect on direct current, that is, direct current cannot pass, but alternating current can pass, and as the capacitance value increases or alternating current increases, the less obstructive effect of the capacitor on alternating current is. Obstruction can be understood as "resistance", but it is not equivalent to resistance. This is a kind of reactance, which is the same unit as resistance, collectively called "impedance". Of course, to be precise, "impedance" should have three parts, except for these two, it is "inductive reactance". Inductance is the hindrance of inductance to current. Unlike capacitance, inductance has no hindrance to direct current (if rigorously studied, it is also hindered in the transient state of a few milliseconds before the energization reaches saturation). It has an obstructive effect. The unit of inductive reactance is the same as the unit of capacitive reactance and resistance.
