What is Impedance? A 5 Minute Introduction

[Greg Bernhardt]

Definition/Summary
The impedance of a load (a combination of components) in an AC current is a complex number $Z\ =\ R+jX$ where $R$ is the resistance of the load and $X$ is its reactance.
It can also be written in polar form: $Z\ =\ |Z|e^{j\phi}$, or as the phasor $|Z|\angle\phi$.
Impedance is the AC equivalent of resistance: it is used in the AC version of Ohm’s Law: $V_{complex} =\ I_{complex}Z$
(or $(V_{max}/I_{max})\angle\phi =\ Z,\text{ where }\phi$ is the phase difference by which the voltage leads the current), and it obeys the same series or parallel combination laws as resistance does.
Impedance depends on frequency (except for pure resistances).
Impedance is measured in ohms ($\Omega$).
Equations
For a load across which the voltage leads the current by a phase angle $\phi$:
$$Z\ =\ |Z|cos\phi + j|Z|sin\phi\ =\ R+jX$$
(in polar form: $Z\ =\ |Z|e^{j\phi}$)...

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[AndyW]

Impedance is a measure of the opposition to an alternating current (AC) flow in a circuit. It is represented by a complex number Z, which has a real part (resistance, R) and an imaginary part (reactance, X). Impedance can also be expressed in polar form as |Z|e^{j\phi}, where |Z| is the magnitude of the impedance and \phi is the phase angle.

In an AC circuit, the voltage and current are also represented by complex numbers V_{complex} and I_{complex} respectively. The relationship between them is given by Ohm's Law for AC circuits: V_{complex} =\ I_{complex}Z. This can also be written as (V_{max}/I_{max})\angle\phi =\ Z, where \phi is the phase difference between the voltage and current.

Just like resistance, impedance follows the same laws for series and parallel combinations. However, unlike resistance, impedance is frequency-dependent. This means that the value of impedance changes with the frequency of the AC current.

Impedance is measured in ohms (\Omega), just like resistance. It is an important parameter in AC circuits as it determines the amount of current that will flow through a circuit for a given voltage. It is also used in the calculation of power in AC circuits.

In summary, impedance is a complex number that represents the opposition to AC current flow in a circuit. It is composed of resistance and reactance and is measured in ohms. It follows the same laws as resistance and is frequency-dependent.