General: Units of Impedance

This week’s General Class question comes from sub-element 5 (Electrical Principles) group A (impedance) [G5A10]

What unit is used to measure impedance?

A. Volt
B. Ohm
C. Ampere
D. Watt

Impedance is the measure of opposition to current flow in a circuit when an AC voltage is applied.  With that in mind, we can automatically eliminate A) Volt and B) Ampere (current) as contenders.  Similarly, since the D) Watt (Power) is the mathematical product of voltage times current, we can eliminate it.

Impedance is the AC (Alternating current) corollary of DC (Direct current) resistance, which is measured with the B) Ohm.  The two properties are not the same at all in practical terms, but mathematically serve the same purpose.  For example, a resistor-capacitor (RC) circuit or resistor-inductor (RL) circuit with a DC voltage applied, only displays the resistance of the resistor in the circuit.  With an AC voltage applied, the waveform of the output will change according to the impedance also, which is affected by the value of the capacitor or inductor.

Impedance is a very important concept in radio, since you’ll use it to measure everything from feed line and antennas, to tuner circuits and speakers.

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