Understanding the Basics (and more) of Impedance 
Impedance Explained  Learn About Impedance Here!
AC Alternating Current Ohm's law power formulas. Generally, if you are master electrician, troubleshooter, or engineer, you may prefer this Ohms law chart, with a unique resistor band color chart. Think Impedance  Need Z formulas? (Please see our TechNotes link for our DC Ohm's law chart)
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Symbol Z: A measure of the total opposition to current flow in an alternating current circuit, made up of two components, ohmic resistance and reactance, and usually represented in complex notation as Z = R + IX, where R is the ohmic resistance and X is the reactance. (For an AC circuit Ohm's law can be written V=IZ where V and I are the oscillating phasor voltage and current respectively and Z is the complex impedance for the frequency of oscillation.)

Opposition to flow of alternating current. Impedance consists of resistance plus reactance (capacitive or inductive). Measured in Ohms.
(TechNote: We will try to explain impedance here in several different ways. What may "click" for you, may not "click" for another. Please excuse our redundancy  we hope you find this helpful. Also see our Impedance Matching link above.)
AC Alternating Current Theory: Ohm's law defines the relationships between (AP) apparent power, (ET) voltage total, (IT) current total, and (Z) impedance.
Alternating Current  as in the electrical outlets in our homes, the flow (current) of electricity is always changing direction (60 times every second in the U.S., 50 times every second in other countries), through the loads such as lamps, water heaters, televisions, and all the things we plug into the wall outlets. These items also have resistance, but, because of the alternating current, we have to change our mathematics starting with R or RESISTANCE being replaced by a thing called IMPEDANCE or the letter Z. Now, if we think about water flowing in the river again, but this time with the flow (current) changing direction 50 or 60 times in every second, we can imagine some very big differences in the DC and AC theories. The AC theory is mostly used in journeymanship levels through upper engineering people.
For practical purposes, an electrical current always meets some opposition when flowing through a circuit, even if that circuit is merely a straight copper wire. The opposition to alternating current is called impedance because it impedes, or hinders, the flow of electrons. Impedance is represented by the letter Z in equations, and it's measured in ohms. Impedance is closely related to resistance, but the two are not identical. Impedance is how much a device resists the flow of an AC signal, such as audio. Impedance is similar to resistance which is how much a device resists the flow of a DC signal.
Once you know the resistance and reactance of a circuit, the impedance is actually the overall opposition to current presented by the circuit. The impedance of a circuit is also expressed in ohms; unfortunately, you cannot simply add the resistance and the reactance to get the impedance.
The total impedance of a circuit is the square root of the sum of the squares of the resistance and reactance. That is, impedance (symbolized by the letter Z) is the square root of (resistance squared plus reactance squared).
Impedance in electricity, measured in ohms of the degree to which an electric circuit resists the flow of electric current when a voltage is impressed across its terminals. Impedance is expressed as the ratio of the voltage impressed across a pair of terminals to the current flow between those terminals. When a circuit is supplied with steady direct current, the impedance equals the total resistance of the circuit. The resistance depends upon the number of electrons that are free to become part of the current and upon the difficulty that the electrons have in moving through the circuit.
When a circuit is supplied with alternating current, the impedance is affected by the inductance and capacitance in the circuit. When supplied with alternating current, elements of the circuit that contain inductance or capacitance build up voltages that act in opposition to the flow of current. This opposition is called reactance, and it must be combined with the resistance to find the impedance. The reactance produced by inductance is proportional to the frequency of the alternating current. The reactance produced by capacitance is inversely proportional to the frequency of the alternating current. In order for a source of electricity that has an internal impedance to transfer maximum power to a device that also has an impedance, the two impedances must be matched. For example, in the simple case of pure resistances, the resistance of the source must also equal the resistance of the device. Impedance matching is important in any electrical or electronic system in which power transfer must be maximized.
