RESISTANCE, REACTANCE AND IMPEDANCE

A Primer
Douglas Brooks, PhD
UltraCAD Design, Inc.
PART 2, REACTANCE
This is part 2 of a 3-part series on resistance, reactance and impedance. Most of us are familiar
with resistance --- at least we think we are. Those will find Part 1 pretty basic. But few of us
really understand reactance and its relationship to resistance. Part 2 gets a lot more interesting!
And while most of us sort of understand what impedance is, few really understand the
relationship between it and the other two properties. This 3-part series will tie them all together.
Reactance:
Resistance and reactance are dramatically different. Resistance does not depend on frequency,
reactance does. Resistance does not cause a phase shift, reactance causes a 90 degree phase
shift between voltage and current. Resistance causes the loss of (i.e. dissipates) power,
reactance does not. Pure (ideal) reactance returns all energy that it stores in its field.
Reactance is what we get with capacitors and inductors. But capacitors and inductors are
almost exactly opposite in their effects. Both "impede" current. But capacitance impedes current
at low frequencies, inductance at high frequencies. Capacitance causes voltage to lag current
by 90 degrees (thus we say there is a -90 degree phase shift) while inductance causes voltage
to lead current by 90 degrees (thus we say that there is a +90 degree phase shift.) Capacitors
store energy in the electric field between their plates, inductors store energy in the magnetic
field around the wire.
The three passive components in electronics, resistors, capacitors, and inductors act as though
they are on a spectrum. Capacitors and inductors are at opposite ends of the spectrum,
resistors are at the exact center of the spectrum. (See Brooks, Note 1.)

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