Before we begin to explore the effects of resistors, inductors, and
capacitors connected together in the same AC circuits, let's briefly review some
basic terms and facts.

**Resistance** is essentially *friction* against the motion of
electrons. It is present in all conductors to some extent (except
*super*conductors!), most notably in resistors. When alternating current
goes through a resistance, a voltage drop is produced that is in-phase with the
current. Resistance is mathematically symbolized by the letter “R” and is
measured in the unit of ohms (Ω).

**Reactance** is essentially *inertia* against the motion of
electrons. It is present anywhere electric or magnetic fields are developed in
proportion to applied voltage or current, respectively; but most notably in
capacitors and inductors. When alternating current goes through a pure
reactance, a voltage drop is produced that is 90^{o} out of phase with
the current. Reactance is mathematically symbolized by the letter “X” and is
measured in the unit of ohms (Ω).

**Impedance** is a comprehensive expression of any and all forms of
opposition to electron flow, including both resistance and reactance. It is
present in all circuits, and in all components. When alternating current goes
through an impedance, a voltage drop is produced that is somewhere between
0^{o} and 90^{o} out of phase with the current. Impedance is
mathematically symbolized by the letter “Z” and is measured in the unit of ohms
(Ω), in complex form.

Perfect resistors (Figure below) possess resistance,
but not reactance. Perfect inductors and
perfect capacitors (Figure below) possess reactance but
no resistance. All components possess impedance, and because of this universal
quality, it makes sense to translate all component values (resistance,
inductance, capacitance) into common terms of impedance as the first step in
analyzing an AC circuit.

*Perfect resistor, inductor, and capacitor.*

The impedance phase angle for any component is the phase shift between
voltage across that component and current through that component. For a perfect
resistor, the voltage drop and current are *always* in phase with each
other, and so the impedance angle of a resistor is said to be 0^{o}. For
an perfect inductor, voltage drop always leads current by 90^{o}, and so
an inductor's impedance phase angle is said to be +90^{o}. For a perfect
capacitor, voltage drop always lags current by 90^{o}, and so a
capacitor's impedance phase angle is said to be -90^{o}.

Impedances in AC behave analogously to resistances in DC circuits: they add
in series, and they diminish in parallel. A revised version of Ohm's Law, based
on impedance rather than resistance, looks like this:

Kirchhoff's Laws and all network analysis methods and theorems are true for
AC circuits as well, so long as quantities are represented in complex rather
than scalar form. While this qualified equivalence may be arithmetically
challenging, it is conceptually simple and elegant. The only real difference
between DC and AC circuit calculations is in regard to *power*. Because
reactance doesn't dissipate power as resistance does, the concept of power in AC
circuits is radically different from that of DC circuits. More on this subject
in a later chapter!