I was tempted to call this subject "the
three Rs of electrical theory", because it's one of the most
standard subjects that every student probably learns in
their first EE class; However, as you may have noticed,
"Impedance" doesn't start with a letter R, so I couldn't do
that. :( Nonetheless, these three forces, which are closely
linked, are indeed important to the beginning
electricity/electronics student. The good news is that they
are relatively simple to understand, and can be dealt with
using a few fairly easy and standard math formulas.
Resistance is a force that tends to resist the flow of
electrical current. Resistance is usually created
deliberately by a resistor, a device used to create
resistance in a circuit. Resistance is pretty
straightforward: The more resistive a resistor is (i.e. the
more ohms it's rated for), the more it restricts the flow of
electricity through it. Perhaps the best news of all is that
you only need to concern yourself with resistance if you're
working with DC (direct current); The slightly more
complicated concepts of reactance and impedance only exist
in the world of AC (alternating current).
Reactance, unlike resistance, is usually undesirable in a
circuit. Whereas resistance is created by a resistor to
achieve some effect, reactance is an unfortunate by-product
of certain electrical components. There are two basic types
of reactance: Capacitive reactance and inductive reactance.
Appropriately enough, capacitive reactance is created by
capacitors, while inductive reactance is created by
inductors. Using either of these device types in an AC
circuit will introduce some reactance. Like resistance,
reactance is expressed in ohms, and it behaves in much the
same way as resistance, in the sense that it tends to
restrict the flow of current through a circuit.