> My question is this:  Why does polarity matter, and why is such a big deal
> made out of plugs that supposedly force a particular polarity by
> allowing it
> to be plugged in only one way?  Polarity IS a consideration with D/C power

     You are correct, electrically AC is non polaric, usually it does not
matter which wire is which from an ELECTRICAL point of view, however from a
safety point of view it is VERY important.
        In a single phase power system (what comes out of the wall in most parts of
the world) there are two electrically active wires. Here in North America
there is the "neutral" wire (usually coloured white) and the "hot" wire
(usually coloured black).
        Now here is why polarity matters: where the neutral wire comes into your
home it is connected to ground, and therefore it is very near ground
potential. The hot wire is "floating" about ground. That is why touching the
neutral wire usually doesn't matter, it is very near to ground potential and
therefore the voltage between your feet touching the ground and the neutral
wire is pretty close to zero volts. The hot wire OTOH is 120V RMS above
ground, your feet are connected to the ground so that the voltage difference
between you and the hot wire is 120V, quite a deadly thing. This is why
which wire is which matters, if you touched the white wire you might get a
tingle, the black wire would hurt or even kill you.
        I have heard of some countries (I believe some of Europe is like this) that
use a "floating" system, both the neutral and hot wires are floating about
ground. Why? Because this results in no circuit between the ground, you and
the wire you touch. Unless you touch BOTH wires you will most likely not be
hurt. These systems usually have a mandatory third "ground" wire as part of
the socket.

> of you who wonder why they don't send D/C over regular power lines, the
> answer is simple.  Latency.  It's hard enough to get 120V to your house as
> it is (transformer stations are used to reboost the power for
> areas that are
> a long ways from the power plant).  Getting D/C to your door
> would be nearly
> impossible without a large number of repeater stations to maintain power
> levels concurrently through the lines.

     I believe the main reason DC is not used for distribution is because
transformers don't work with DC, it is VERY difficult to change the
potential of a DC current, it usually involves changing the DC current to
AC, stepping it up (or down) and then changing it BACK to DC! Quite an
inefficient process. A transformer OTOH operates quite close to 100%
efficiency. Now why does changing the voltage matter? Why don't we just send
120V from the power station to our home? I won't go too far into the math
(although it is simple) but suffice it to say that a transmission line is
NOT perfect, some power is lost in it. Very simply the amount of power
dissipated in a wire is the current going through it squared times the
resistance of the wire. So, the more current that flows through it, the more
power is wasted heating the line. An easy way to combat this is to use a
transformer which does not increase the amount of power (since it is
passive) but it does change the ratio of voltage to current. That way you
can increase the amount of voltage and at the same time decrease the amount
of current flowing, thereby decreasing the power wasted by heating the line.
        My my, that was much longer than I thought it would be, if you want me to
explain more feel free to email me directly. Hope this helped, TTYL

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