Electromagnetic physics question

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terri
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Re: Electromagnetic physics question

Post by terri » Thu Dec 23, 2004 12:03 pm

Now, now, settle down.<p>Gee whiz, this is turning into a quasi-religious discussion. Or religion-like. Or something evoking high passion.<p>By the way, a bad neutral going into your outside service connection will create an imbalance between the two sides of the "220" lines such that a heavy load on one 110 V side will cause the voltage on the other "110V" side to swing way up since it now "sees" the other side of the 220 service line through the heavy load, instead of completely returning through the neutral line.<p>You will notice this by a brightening of the lights on the lightly-loaded side and frequent bulb burnout on that side. This effect is slightly complicated by the fact that the neutral line is also tied to ground, so the effects of a lost neutral outside the house may not be noticed for a while, depending on how good the house ground is "earthed." No theory necessary to prove this --it's an observational fact.<p>Gee. Let's see what happens now.<p>[ December 23, 2004: Message edited by: terri ]</p>
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terri
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Re: Electromagnetic physics question

Post by terri » Thu Dec 23, 2004 8:48 pm

Perfectbite:<p>"What was/is the "Edison effect"? Quite a while ago a posting to the forum asked about the 'blackening' that occurred in light bulbs. Are these phenomena related?"<p>They are two different things, although they occur in similar circumstances.<p>The Edison effect is due to the "boiling off" of electrons from a hot surface, such as the filament in a cathode ray tube "electron gun" or the filament in a vacuum tube (valve). The "boiling off" phenomenon is called "thermionic emission."<p>If another element is placed in the vacuum near this hot surface, a small current will be observed between the hot surface and the extra element. The flow of this current is called the "Edison Effect."<p>If this other element is made deliberately positive from an outside voltage source, an even stronger curent will flow --but since the electrons can only flow from the hot surface to the positive plate, we have what is called a "diode." Other posters above have talked about the diode or valve.<p>The blackening of surfaces in a bulb is usually due simply to the hot surface actually ejecting particles from that hot surface (not just electrons) which end up on the inside surface. Vacuum deposition techniques ("vacuum plating," "sputtering," etc.) take advantage of this phenomenon.<p>But in the case of the Tungsten-filament bulb, there is also a phenomenon called the "water cycle" which also causes blackening.<p>If there is the slightest amount of water in a "vacuum-filled" tunsgten-filament bulb, it will form a low-temperature-boiling complex compound with the tungsten. This low-boiling complex compound then boils off the filament and wanders around until it touches a relatively cool surface.<p>And it so happens that when this water-tungsten complex compound becomes cooled, it decomposes into water and tungsten again. The metallic tungsten is therefore deposited on the inside (cool) surface of the light bulb, and the water molecule is once again free to wander around inside the bulb until it touches the hot tungsten filament again, where it can "steal" some more tungsten. This water cycle repeats and repeats until the inside surface of the bulb is coated with metallic tungsten.<p>Nowadays, this tungsten-water cycle is suppressed because of the addition of other gases to the light bulb.<p>Thus: the Edison effect and the "blackening" happen to occur in similar circumstances, that is, a hot surface in a vacuum, but they are not the same thing.<p>Look at it this way: The Edison effect is due to boiling off of electrons from the hot surface, and the blackening is due to boiling off of actual material from the hot surface.<p>SEE ALSO:<p>thermionic emission
space charge
vapor pressure of metals
vacuum deposition
sputtering
vacuum tube or valve
electron gun
gettering<p>[ December 23, 2004: Message edited by: terri ]</p>
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perfectbite
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Re: Electromagnetic physics question

Post by perfectbite » Thu Dec 23, 2004 9:24 pm

Thank You Terri.

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MrAl
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Re: Electromagnetic physics question

Post by MrAl » Thu Dec 23, 2004 10:59 pm

Hi again,<p>RonH and Stephen:
Yes, there's a "mention" of efficiency, but what
they are talking about is efficiency over line
length, not voltage rating.<p>Dont you realize that 40% higher efficiency is
so much of an increase that every single argument
for DC over AC would start out:<p>"DC is always favorable over AC because of the
tremendous advantage in efficiency due to the
much better voltage rating untilization of the
components specifications."
40 percent is QUITE an advantage to say the least.
I couldnt see so many people overlooking this
supposed fact.<p>Realize also that the test voltage for a 330kv line (AC or DC) is much higher than that
anyway, and that there will always be surges on
the line AC or DC, which the line still has to
take without damage. Just because we have a DC
line it doesnt mean there will always be a
constant voltage on it.<p>There's also direct mention of the rating of the
insulators, which are rated in RMS voltage for
a given distance, meaning that when you buy one
you look for the RMS voltage, not the peak.
Come to think of it, i've never seen a wire
insulation rated in peak voltage either.<p>One other point is that the AC line peak to
ground is much less than the line to line peak.
This might mean the AC lines have to be spaced
further apart, but the distance across the insulator
can be even less than DC if you look at it that
way.<p>
Take care,
Al
LEDs vs Bulbs, LEDs are winning.

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Re: Electromagnetic physics question

Post by rshayes » Fri Dec 24, 2004 3:54 am

Hello Mr Al:<p>The increased capacity of a DC line is a fact, and it is not overlooked. The problem is that using the extra 40 percent of capacity is very expensive if it can be done at all.<p>A DC system requires a rectifier at the transmitting end and an inverter at the receiving end that are not needed for an AC line. In 1880, these functions would have been done with motor driven commutators. At somewhere around 1000 volts, comutators become difficult to operate. Sixty five years later, the Radiation Lab Series noted that "Dynamotors have been built with voltages as high as 2000 but this is inadvisable because of difficulties with commutation and insulation". (Page 418, Components Handbook, MIT Radiation Lab Series, vol 17) Even in 1880, it would have been possible to use AC voltages much higher than this without a significant extra cost in insulators. Thus in 1880, the AC line would have been preferred no matter what the length was. Tesla's AC system was superior to Edison's DC system.<p>Mercury vapor rectifiers were used to some extent in the 1920's, and by the 1940's were developed to the point where they could handle thousands of amperes up to 4000 volts. Ignitrons were triggerable, like SCR's, and were used as rectifiers and inverters for such things as electric locomotives.<p>According to the ABB web site, the Pacific Intertie originally used mercury vapor devices, but it has been, or is being, converted to banks of SCR's. I expect that either one of these devices would be very expensive at the 500 KV level. On a short transmission line, it would be cheaper to simply use 40 percent more copper than to install the switching equipment. As the line gets longer, the cost of the extra copper becomes higher than the extra cost of the terminal equipment, and the DC line is more attractive. Even today, I suspect that the DC line is only practical from an economic point of view if it is longer than about 500 miles. In this respect, Edison was right in the long run, but he didn't have the technology to implement a high voltage DC line in 1880.

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Re: Electromagnetic physics question

Post by Mike6158 » Fri Dec 24, 2004 6:23 am

Interesting thread. Personally I think much more of Tesla than I do of Edison but both men impacted history... no doubt about that.<p>Maybe I missed it but I didn't see any significant mention of reactance and it's effects.<p>This was a great explanation-<p> <blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr> Same goes for magnetic fields, they form a Ripple in the pond, that goes from shore to shore with just a pebbles worth of effort.<hr></blockquote> Now all you need is the reflected wave coming back from the shore and viola... we can "see" the effects of reactance. BTW- Somewhere in my stash of books, at least I think that's where I read it, there is a great story about Tesla shutting down the entire city of Colorado Springs (briefly mentioned in this thread). He used the curvature of the earth as a parabolic reflector... I don't remember exactly what he was doing. Just that he drove the grid's ground potential to a significantly positive value :) and raised mortal hell with the cities gen sets. :D
Speaking of positve ground- Nobody mentions the transformer connections in the discussion of house wiring / floating neutral. Wye-Wye, Delta-Delta, Wye-Delta, and Delta-Wye. I don't know about residential transformers but in the industrial world I've seen all four of these used. We prefer Delta-Wye these days but some of our older facilites use Delta-Delta because of it's ground fault tolerance. The problem is tolerance does not equal safety. The last project that I worked on used a Wyw-Wye connected feeder transformer. Much to my dismay, we shut down our gens (delta-delta connected) and went on purchased power. There was some initial concern that the the wye-wye connected system would give us problems with our variable frequency drives due to the possibility of significant 3rd order harmonics but (a) there was nothing that we could do about the primary side and (b) we were NOT going to connect the secondary side in a delta configuration. It all worked out. Well that's enough rambling for the morning.<p>Merry almost Christmas ya'll
"If the nucleus of a sodium atom were the size of a golf ball, the outermost electrons would lie 2 miles away. Atoms, like galaxies, are cathedrals of cavernous space. Matter is energy."

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Re: Electromagnetic physics question

Post by ian » Fri Dec 24, 2004 3:42 pm

Well here I am again after starting this monster thread to point out 2 items overlooked in this voluminous argument.
1) DC or AC, what's the consensus of electromagnetic effects 100 miles away? Does a simple magnet have as much an effect as a 100MGHZ antennae? <p>2) No mention of Tesla's insanity in his later years. Sucking in investors to make his power transmission device which never worked. His acceptance speech of some award during WW2 where he laid out the invention of a "heat ray" strategically placed to stop the german forces.

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Chris Smith
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Re: Electromagnetic physics question

Post by Chris Smith » Fri Dec 24, 2004 3:50 pm

Tesla’s Insanity, trumps Edison’s generic mind any day.<p>The best thing that can be said for Edison, was that he was a good republican, or robber Barron.<p> Greed motivated his life, not altruism and certainly not Science.

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Re: Electromagnetic physics question

Post by rshayes » Sat Dec 25, 2004 4:36 am

When the fields are don't change (created by stationary and fixed charges or currents), the influence at a distance depends on the magnitude of the field and the physical dimensions of the charge or current creating the field. In the case of a bar magnet, the field will be strongest at the poles of the magnet, moderate at distances about equal to the length of the magnet, and only a few percent at distances greater than four or five times the length of the magnet. A strong magnet 10 miles long might produce some field 100 miles away, but the field at 200 or 300 miles will probably be very weak.<p>The same happens for charge. At distances greater than a few times the spacing of the charges, the field gets weak compared to the original field.<p>The energy density tends to decrease as the cube of the distance (or possibly faster), since most of it remains in the original volume of space.<p>These are static solutions to Maxwell's equations. The fields are fixed at one position in space and the energy in the field is stored in the local area.<p>A change in a magnetic field creates an electric field. The reverse is also true, a change in an electric field creates a magnetic field. These changes also move away from the original location at the speed of light. A decreasing magnetic field will cause an increasing electric field. When the magnetic field reaches zero, all of the energy stored in the magnetic field will be transferred to the new electric field. The electric field then begins to decrease and to create a new magnetic field of the opposite polarity from the original magnetic field. When the electric field reaches zero, this magnetic field will be storing the original energy, but in the opposite polarity and in a different position. This process continues, and with each alternation, the energy moves farther away from the original area. This is the radio wave solution to Maxwell's equations.<p>In effect, the original energy is on the surface of a sphere which is expanding at the speed of light. The amount of energy received by a detection device depends only on the sensitive area of the detection device and on the total energy radiated, but not on the dimensions of the transmitting device. In general, the energy density is the inverse square of distance.<p>If the wave is forced to stay within a smaller volume, the attenuation may be less. For example, if the ionesphere of the earth and the ground ares reflective at the frequency of the wave, it will move in a band of constant height an increasing radius. In this case, the energy density is inversely proportional to the distance. This will be modified by some losses to the ground and ionesphere, since neither is a perfect reflector.<p>In the static case, the energy density at 100 miles will be about 1/1000 the energy density at 10 miles. An unconstrained radio wave will have an energy density at 100 miles that is about 1/100 the energy density at 10 miles. The constrained radio wave may have a energy density at 100 miles that is 1/10 of the energy density at 10 miles.<p>At 100 miles, the radio wave is potentially 10 to 100 times easier to detect than the static field. At 1000 miles, the radio wave may be 100 to 10,000 times easier to detect.<p>The atmosphere can form ducts that constrain the waves even more. It is possible, but rare, for a radio signal on a 1000 mile path to be stronger than the signal on a 100 mile path. The 27 MHz Citizens Band sometimes has these conditions during a sunspot maximum. When this happens, a signal of less than 5 watts can be received 2000 miles away.

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Re: Electromagnetic physics question

Post by Mike6158 » Sat Dec 25, 2004 5:27 am

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>The 27 MHz Citizens Band sometimes has these conditions during a sunspot maximum. When this happens, a signal of less than 5 watts can be received 2000 miles away.<hr></blockquote><p>Happy Holidays to all.<p>Actually, I've communicated, using CW, Phone, and PSK31, over 2,000 miles, either side of and during the solar maximum. At one time there was a net on 20 meters, AM, that I would occasionaly participate in. The station farthest away was in South Africa. On one occasion, the guy that worked with me to get my CW speed up so that I could work CW mobile turned his rig down to 0.25W on 20m and I copied him fine. He was, as the crow flies, around 1,500 miles from me. We established the QSO on 20m SSB and switched to CW. I made a comment about how great his signal was and how I wished that I had a 2kW amp. He was running under 150W at the time. The purpose of the lesson was to show me that a good antenna has more value than an amp. I was using a G5RV dipole and he was running a Mosley beam. I've heard of but never worked 50Mhz long distance comm. I've also heard of 2m and 70cm "skip" as some people call it but I have never worked it. Mostly just 80m, 40m, 20m, and occasionally 10m. I have since moved and at the moment I am antenna challenged.<p>So- Long distance comm isn't just for CB :) HOWEVER, that was a great explanation of how radio waves work. Are you a teacher? You have very good skills in that respect.
"If the nucleus of a sodium atom were the size of a golf ball, the outermost electrons would lie 2 miles away. Atoms, like galaxies, are cathedrals of cavernous space. Matter is energy."

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Re: Electromagnetic physics question

Post by rshayes » Sat Dec 25, 2004 6:53 am

Nope, I'm an electrical engineer.<p>I wish that I had a better ability to visualize electric and magnetic fields. There used to be a technique for sketching electric and magnetic fields with a pencil and paper but I can't get the hang of it. In one respect, Tesla had an incredible mind, in that he could intuitive visualize the operation of an induction motor. I certainly can't do that even when I know how its supposed to work.<p>In the late fifties, my family lived in Twentynine Palms, about 100 miles east of Los Angeles. There was a translator that provided television service by converting a couple of the Los Angeles stations to UHF channels and retransmitting them down into town from a mountain near town. There was somme occaisional interference on one of the channels, and at one point the interfering station wiped out the LA station entirely. This just happened to occur during the station break, and we got the call letters of the interfering station. It was in Fort Worth, well over 1000 miles away.<p>About this time, I think an amateur in Hawaii managed to make contacts with the West Coast on the 50 MHz, 144 MHz, 220 Mhz, and possibly 420 MHz bands through some form of ducting propagation.<p>I also remember an article in Popular Electronics by Don Stoner describing a two transistor transmitter that probably put out less than 100 milliwatts. I think he may have claimed one or two contacts with Australia using that transmitter.<p>Freak propagation conditions can give truly amazing results.

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Chris Smith
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Re: Electromagnetic physics question

Post by Chris Smith » Sat Dec 25, 2004 11:09 am

Perhaps the ripple in the pond, best describes the propagation of EMF and magnetic wave fronts. You can use the “slit” method used to describe the pathways of light, to see how interferences act on these wave fronts.

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