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Chapter 6. Reactance

As I have indicated, there are only three types of passive components: resistors, capacitors, and inductors. Each of these impedes the flow of current, but each does so in a different way. We categorize them these three ways:

  • Resistance (designated by the symbol R) is the impedance to current flow that is presented by a pure (ideal) resistor. Resistance is measured in ohms. Resistance is directly related to the size of the resistor, but it is not a function of frequency and there is no phase shift associated with this impedance to current flow.

  • Reactance (designated by the symbol X) is the impedance to current flow that is presented by an ideal inductor or capacitor. Reactance is measured in ohms. Reactance is related to the size of the component but is also related to the frequency of the signal. There is a 90-degree (only) phase shift associated with reactance.[1]

    [1] Note that this statement says there is a 90-degree phase shift (plus or minus) associated with reactance. It is exactly+/–90 degrees and nothing else. It is important to understand that reactance only has a 90-degree phase shift, and no other value of phase shift. Reactance, for example, cannot have a 45-degree phase shift under any conditions. (It is only impedance, discussed in the next chapter, that can have any value of phase shift between +/–90 degrees.)

  • Impedance (designated by the symbol Z) is what we get when we combine resistance and reactance (see Chapter 7). Impedance is measured in ohms. It is a function of frequency and there is normally a phase shift associated with impedance. That phase shift can range between +90 degrees and –90 degrees.[2]

    [2] It is not wrong to describe resistance as impedance, but more properly resistance is only the “real” part of impedance. It is not wrong to describe reactance as impedance, but more properly it is only the “imaginary” (shifted 90 degrees) component of impedance.


  

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