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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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