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Re: AC polarity



Chris, et al., wall receptacles in homes have three holes: a long, a short,
and a round. The round hole is connected to a green (or green with yellow
stripe) wire called the ground (or bond). All the green wires in the house
are connected to a buss in the service entrance (or breaker) box. The long
holes are connected to white (or gray) wires and are connected to a
different buss in the service entrance (or breaker) box called the neutral.
The short hole is connected to some other colored wire (usually red or
black) which is connected to a circuit breaker.

There may be several breaker boxes, but it/they are connected to only one
service entrance box. In *that* box the white buss and the green buss are
bonded (connected) and then bonded to a long rod in the ground or to the
water system if there is no plastic between the connection and the ground.
This is the "ground" -- to which all other potentials (voltages) are measured.

Somewhere on a utility pole there is a transformer with a 230V center tapped
secondary. The center tap is connected to "ground" in the service entrance
box, The other two conductors are connected one to the red and one to the
black buss in the service entrance box.

Thus a circuit across black/white is 117V -- likewise across red/white.
These two potentials have a phase difference of 180o. The stove is
connected across red/black for 230V operation.

Now to the question: Some people refer to these two phases -- the red and
the black -- as "polarities" ie red phase and black phase. Some people
refer to the red (or black) and *white* as "polarities" ie "neutral" and "hot".

In a correctly wired home, you can only get a shock from the red or the
black wires as all the others are "grounded" ("bonded") and as are all the
pipes and appliance bodies. (Except for some hand/table appliances, which
have no bond/ground.)

Best wishes

Jim Green




At 11:17 PM 7/5/96 -0500, you wrote:
Hi all!
I've got what (I hope) is a fairly simple question. Forgive me, for
physics is only my minor. :-) What does polarity MEAN when talking about
AC devices plugged into a wall outlet? At first I thought maybe it was a
phase thing, but I don't think that would be possible without some kind of
syncronization. Maybe a bias voltage across the two "prongs"?

Thanks for the help!
Chris

-- Chris Clayton
misterc@uiuc.edu http://www.students.uiuc.edu/~misterc
Senior+, Teaching of Chemistry University of Illinois - Urbana/Champaign
MSN Chemistry Forum Tutor: ChrisC_Asst@msn.com

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