Newsgroups: sci.electronics


Sigh.  Why is it that whenever a question like this is raised, the first
thing we do is trot out the equations?  SUre, the equations are *correct*,
but I find that they really aren't very useful in getting a "feel" for how
these things work.

So here's the *non*-mathematical explanation, since you've already been
given the equations:

Every time you move charge - meaning you have an electrical current - a
magnetic field is created AROUND the path of the moving charge.  It takes
energy to create this field, meaning that this ALL BY ITSELF makes for an
opposition to starting a charge in motion.  (Note, though, that once the
field is established, it takes no more energy - meaning that a constant
current sees no opposition.)  On the other hand, suppose you have some
charge zipping along (again, this is what we mean by "current"), and you want
to stop it.  The field is already established, and if you try to stop the
charge, the energy in the field will "come back out" - the field will
collapse, and in such a way so as to try to keep the charge moving (the
current "flowing") until the field's energy is removed.  A similar phenomenon
occurs when a moving wire cuts the field lines of a magnet - a current is
"induced" into the wire, since a changing field (from the wire's point of
view) moves charge.  All of this is lumped under the phenomenon we call
"inductance", which is one of the three basic properties (along with resistance
and capacitance) that one runs into in electromagnetics.  (Capacitance is
very similar to inductance, but has to do with the effect that *electric*
fields - as opposed to magnetic - have on charge.)

To increase this "field effect", you can do something very simple - wind
a conductor into a coil shape, so that the fields produced by adjacent
turns reinforce.  This makes the component that we call the "inductor",
although it's important to note that from the above, ANY conductor MUST
have non-zero inductance, if it has any physical length at all - the
establishment of a magnetic field by a moving charge is a basic property
of the universe!  You can also increase the intensity of the field for a
given current (and so increase the inductance seen by the current) by
establishing the field in a material which has a higher "permeability" than
free space.  Permeability is simply a measure of the strength of the field
established by a given current, and some materials (such as iron) show very 
high permeabilities in comparison with ordinary space or air.  (Most materials
have the same permeability as space, and are said to have a "relative
permeability" of one.)  This is why putting an iron core in an inductor
increases the inductance dramatically.

Now, consider the action of this device on an alternating current.  When the
current is increasing, some energy must go into establishing the field
in the inductor.  Similarly, if you try to DECREASE the current, the energy
must come BACK OUT of the field.  In either case, there is opposition to
the change in current.  The faster you try to change the current (i.e., the
greater the frequency of the AC), the greater this opposition is, and so
we say that "inductive reactance" varies directly with frequency.  Note also
that, when the field is collapsing, the inductor can be viewed as a source
of energy.  This means that as you try to decrease the current through an
inductor, you will see an electric force (a voltage) appearing across it,
such that this potential is "trying to keep the current going."  Playing
with inductors and an AC source and a scope is a great way to see these
effects in action, and you'll soon see for yourself that the voltage "created"
across the inductor in this case LEADS the current through it by 90 degrees.
(In other words, you'll see a voltage peak when the current is exactly at
zero, with the current about to increase toward its own peak.)  A little
thought about the actions described above should show you why this has to be
so.

And not a bit of math in the whole thing! :-)


Bob Myers  KC0EW   Hewlett-Packard Co.      |Opinions expressed here are not
                   User Interface Tech. Div.|those of my employer or any other
myers@fc.hp.com    Fort Collins, Colorado   |sentient life-form on this planet.
Newsgroups: sci.electronics