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AC and Hot Side
Name: Thomas J. G.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 2001-2002
Question:
I am currently (no pun intended) teaching a unit on
alternating current and wiring to my stagecraft class and came across an
inconsistency that I cannot immediately explain.
Perhaps you can clarify this for me.
I understand that with alternating current, because of the polar switch
(read switch in direction flow), the electron motion is more of a vibration
than an actual flow (as it is with direct current), hence there is little
net "flow" of the electron medium as we commonly picture it. Instead the
vibrating medium acts
as a transmission medium for the actual electrical energy which speeds
along as a very fast field around and through the electron current.
But if the direction of current flow switches direction 120 times each
second, how can there be a "hot" wire which is maintained at 120 volts of
potential and a "neutral" wire
that is maintained at 0 volts potential? Is seems reasonably that if the
flow switches direction, that the hot and neutral wires would also switch
voltage potential as well.
I'm sure I'm missing some simple mechanism here, or perhaps my concept of
the AC mechanism is faulty. Can you help?
Replies:
Hi Thomas,
No. I believe you have a lot of it is correct. But I will clarify a few
points for you.
Nearly all U.S. residential power service drops consists of actually two
"HOTS", also called phases and one 'return' "NEUTRAL". In the final voltage
transformation nearest your house, the secondary winding of the transformer
has what is called a CENTER TAP [also the NEUTRAL]. So at one end you are
at node "A". At the other end is node "B" and in the middle is node "N".
Go to this link for a thorough treatment of the RMS concept:
http://www.sciencenet.org.uk/database/Physics/ACCircuits/p00373b.html
Between the two extreme nodes (A, B) the voltage (rms) ~ 240 Vrms.
Between each phase ... (AN, and BN) each voltage is ~ 120 Vrms.
In fact, if you wanted to plot the exact voltage value of your outlet in
your kitchen (assuming a stated 120 V outlet). The voltage with respect to
time would be of the following formula:
Van, bn(t) = (120 / 0.707) * sin ( w * t)
Van, bn(t) ~ 170 sin ( w * t)
The "hot" and "neutral" lines voltages do fluctuate with respect to each
other, but since they tie in at specific points on the transformer it is
very important that they be wired into an outlet as prescribed.
** The "neutral" line is reference point in which the circuit is completed**
So when you need 120 Vrms (microwave, TV, stereo, etc...) you use the
voltage across A to N or B to N (based on how that particular outlet was
wired to your breaker box.
When you need to run a clothes dryer or other heavy equipment in the garage
you will probably need to plug it into a ~ 240 Vrms receptacle. In which
case, you use the two HOTS (A, B) neutral is not used. OF COURSE GROUNDING
IS USED IN BOTH APPLICATIONS.
Also, you are correct in that there is no steady (NON - FLUCTUATING)
electrical current. In fact, it is impossible to transmit power through
transformers without varying the field (AC).
Regards,
-Darin Wagner
Hi, Thomas !!
Well, if I have understood your question, let us suppose
you have a wire under the action of an AC and another
neutral. Here you say :
" But if the direction of current flow switches direction 120 times each
second, how can there be a "hot" wire which is maintained at 120 volts of
potential and a "neutral" wire that is maintained at 0 volts potential? "
I believe you wanted to say 60 or 50 times each second, isnt it ? This is
the frequency. But that does not matter, because the question concerns
about the potential difference. The neutral has no flow or vibration
of electrons and the "hot" wire presents a vibration. Well, when
both are connected by a resistance, this vibration will be transmitted
to the neutral wire so that you will have a AC circuit and this vibration
will be sent to the whole circuit.
If you interrupt the circuit, the vibration will again remain with the
wire under the influence of alternating voltage and the neutral wire
will not - again - suffer the vibration of electrons (= zero potential).
On the other side, if the neutral wire were also under the influence of
an alternating voltage and BOTH wires were connected through
a resistance, then the vibration would be the difference in positions
of the voltage waves.
A little bit more : let us suppose you have THREE wires, under the
influence of an alternating voltage, and let us suppose they were
connected in a junction "A". Depending on the synchronization
of the voltage waves, the voltage in "A" could be even ZERO.
To be neutral means to have no electron vibration. But, AFTER the
connection, the electrons will vibrate in the wire, with the same
frequency of the "hot" wire.
regards
Alcir Grohmann
Thomas,
It is actually a question of terminology used by AC electricians. The 120
volt value is not a constant. 120 volts is the root-mean-square value.
This is often indicated on devices by the abbreviation "rms". Graph voltage
as a function of time (negative as often as positive). Now square the graph
(all non-zero elements are no positive). Average the graph over time. Take
the square root of the average. This will come out to 120 volts for the
signal from the wall.
The 0 volt side is held at the same potential as the Earth. All "ground"
connections are connected to the Earth, usually through the building's water
supply. This provides a common reference voltage for all electrical
equipment. As a result, touching the vacuum cleaner at the same time as the
television does not electrocute you.
The 120 volt side oscillates between positive and negative values, while the
ground remains constant.
Note: Some older buildings do not have the constant side held at 0 volts.
The third prong is necessary to set the ground correctly. This is why
cutting off the third prong of a plug can be a very dangerous thing, as can
using an adapter without connecting it correctly.
Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
The hot wire alternately pushes electrons into, and pulls them out of, the
ground wire. The ground wire is maintained at ground potential by being
physically connected to earth.
Tim Mooney
The relative potential of the two wires does switch 120 times a
second. But the
potential, relative to ground, is applied to only the hot wire. The neutral
wire is kept at ground potential. If you touch it no electrons will flow from
you to it, or vice versa.
The hot wire is switching between +170 V and -170 V (the AC signal is
sinusoidal
with a root-mean-square value of approximately 120). Thus, if you touch it,
half the time electrons will flow from the hot wire, through you to
ground, and
the other half of the time electrons will flow from the ground, through you,
into the wire. Either way, it wouldn't be pleasant!
Greg Bradburn
Your analysis is quite correct in that each wire is at a higher potential
for half the time as the current flows back and forth, reversing direction
120 times per second.
The "simple mechanism" you are missing is that one wire can be "hot" and
the neutral wire can be at 0 volts relative to something. All voltages
are relative to some other potential (which can also be changing.)
The neutral wire is at 0 volts relative to the ground because it is
connected to the earth. The "hot" wire oscillates between being positive
and being negative relative to the ground and, of course, relative to the
neutral wire.
This is done, of course, so the more exposed parts of electrical apparatus
can be kept at the same potential as the earth. Then touching those
parts and, for example, a metal water pipe will not hurt you because they
are always at the same potential.
Best, Dick Plano
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