Polarity, in terms of AC power systems, isn't the same as polarity in DC
systems.  You've got the right idea for a DC circuit.  For an AC circuit, at
least in home wiring, which is single phase 120V (in North America and a few
other areas), your circuit has three wires: hot, neutral and (usually)
ground.  A lot of electrical equipment is pretty intolerant of which way
it's connected to the circuit.  Some things, especially electronic
equipment, are sensitive.  For years now, audio enthusiasts have claimed
that they can tell the difference when a piece of their gear is connected to
the AC circuit backwards.  Physically, you can measure the difference
between the voltage levels on the hot and neutral side.  If you connect a
voltmeter between the chassis ground and the circuit ground then power up
the device, check the voltage with the plug in the socket one way and then
the other way.  You'll see different readings.  The lower of the two
readings indicates the correct polarity.  In practice, there is no absolute
ground potential, so you'll likely see two different voltages for the hot
and neutral sides of the AC circuit.  The aim is to make sure that all of
the devices that you use are connected to the same voltages with reference
to ground so that you don't end up with ground loops, or a piece of
equipment operating at a different voltage potential, causing the difference
to be transmitted over the ground wire.

Basically, the idea behind polarity is that you want the hot lead of your
device connected to the hot lead of the circuit and the neutral lead of the
device connected to the neutral lead of the circuit.  Ground, of course, is
ground.  If your wiring's polarity is reversed, it would probably behoove
you to call an electrician and get it fixed.

Incidentally, we use AC today because it's easy to generate and it has the
benefit of working with transformers, something that DC can't do.  As the
current "reverses" on the AC line, another current is induced in the
transformer by the changing magnetic field of the coils.  Thus, low voltage,
high current power from the generators could be transformed into high
voltage, low current power for transmission, meaning that the resistance
that was part of the transmission wires would have less effect.  A byproduct
of that is that cheaper, higher resistance wire (like aluminum instead of
copper) can be used.

Thomas Edison would be vindicated today, though, if he could see that the
majority of electricity that travels from Canada to Mexico is DC.  With
modern electronic switching technology, it turns out that Nikola Tesla might
have been wrong!

That's all probably more than you wanted to know ;-)

Drew Dunn
The NOSPIN Group

----- Original Message -----
From: Kyle Elmblade <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, August 24, 2000 11:07 AM
Subject: Re: [PCBUILD] A question for our resident EEs.


> I'm sorry, but I have to ask the question.  When I was going to
electronics
> school they taught us that A/C stands for "Alternating Current".  This
means
> that the current starts out at zero (theoretically) goes positive to 120V,
> back down to zero, and the negative to 120V.  So, regardless of which tine
> plugs into which side of the socket, it will be positive on the up cycle,
> and negative on the down cycle, in direct opposite proportion to the other
> tine.
>
> 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
> ("Direct Current") because there is a definite positive side and a
definite
> negative side (whether power flows from positive to negative, or negative
to
> positive, is a debate between the physicist and the electricians that will
> continue for a long time).  The transformer doesn't care which way the
plug
> is put in because the diode bridge that converts the A/C to D/C is made
> specifically to clip all the negative power out regardless of which side
> it's flowing from at the time.

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