Impedance is the measure of the total opposition to current flow in an alternating current (AC) circuit. It is represented by the ratio of voltage to current, and its unit is the ohm (Ω), the same as for electrical resistance. In a direct current (DC) circuit under steady-state conditions, an ideal inductor (coil) acts as a short circuit (offering no opposition to current), while an ideal capacitor acts as an open circuit, completely blocking the flow of current.
In AC circuits, components such as inductors (coils) and capacitors also oppose the flow of current, in addition to resistors. This opposition from inductors and capacitors is called "reactance." For an inductor, a changing voltage induces a magnetic field that opposes the change in current. This property is known as inductive reactance, which is a component of the total impedance.
In summary, impedance (symbolized by Z) is the complex sum of two components: resistance (R) and reactance (X). Resistance is the opposition to current that is independent of frequency. Reactance, found in inductors and capacitors, is the opposition to current that varies with frequency. Therefore, the overall impedance of a circuit is also frequency-dependent.
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