Electromagnetic physics question

[ian]

I know steady DC current creates an electromagnetic field. Alternating current produces an alternating electromagnetic field. My question is.......are they both the same thing?
I know it's impossible to measure a DC magnetic field on an antennae 100 miles away, but does that DC field impinge on my instruments, washed out by noise, nonetheless? Different frequencies have different characteristics and travel in different ways, and can be detected according to their properties. But at the source is the magnetic field the same whether it be DC, alternating, high frequency, etc. etc? And does that magnetic field reach out whether it be DC, AC or whatever?
Ian

[terri]

What's the exact nature of the problem you're observing?<p>Quick and dirty: <p>(1) Yes, magnetic fields, of whatever origin, theoretically go out to infinity, but as you've mentioned, at some distance, they're swamped by other noise.<p>(2) The earth's magnetic poles are shifting, and the last I heard, it was at 78 degrees N and 104 degrees west, southwest of Nunavut, Canada's Ellef Ringnes Island. Are perhaps your observations due to the strong (but slowly shifting) pole near where you are?<p>(3) High impedance instruments, such as >10 megohm digital voltmeters pick up all kinds of things if the leads aren't connected to anything --including static from just waving the leads around in dry air, and induced voltages from power leads, etc. That's one reason I keep an old 1000 ohms-per-volt D'Arsonval VOM around... when I get a reading on it, I know it's a "real" reading, although the low impedance of the meter introduces some loading errors. But the high impedance meters of today are almost electrometers, and some are capable of even picking up the variation in voltage with altitude on naked wire stuck up in the air.<p>(4) You might even be picking up magnetic fields from the aurora.<p>(5) In general, the electro-magnetic effects of a tranmitting antenna follow the inverse square law, and at 100 miles, it is unlikely that this is a probable cause.<p>(6)) Unless you're near a body of water or wierd mineral deposits, it's unlikely that you're experiencing the antenna's "ground wave" at that distance unless perhaps it is an ultra-low-frequency transmitter. (Maybe the Russians are experimenting again --or still.)<p>(7) There are thermocouple effects caused by simply placing the test probes on dissimilar metals which sometimes show up on high-impedance meters to confound one's readings. Again, another reason to keep a low-impedance mechanical volt-ohm-meter (VOM) around.<p>Need more info.<p>[ December 18, 2004: Message edited by: terri ]</p>

[Chris Smith]

The reason AC magnetic fields affect things at a distance, where as the DC ones don’t seem to effect much except at a close range, is the AC is like a string and impedance is involved. <p>AC is a push and pull affair and no matter how long the sting is, the other end of the string moves both ways continuously back and forth applying its effort.<p> DC on the other hand, only pulls one way, once, and thus as the strength loses its pull over distance it gets weaker and weaker. Like trying to drag a long rope over the ground. Too much Resistance. <p>However, AC fields and Impedance, seems to glide effortlessly over long distances just like AC transmission lines, VS Edison’s DC power lines and straight resistance.<p> 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. <p>The faster the Frequency, the less movement, the lesser the impedance, and the further it can travel with the same amount of effort.<p>[ December 18, 2004: Message edited by: Chris Smith ]</p>

[jwax]

ian- Your question, if I read you right, basically has been around since man stumbled upon lodestone and its attraction to iron! In that simple case of a steady state magnetic field, yes, it's the same magnetic component whether it is alternating or not. The "field" is better defined as a "region of distinctive effects", in that it affects specific things, but much more noticeably locally.
Same "stuff" that makes up a component of gamma rays, radio waves, AC power, solenoids, and compass energy!
John

[cato]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Chris Smith:


However, AC fields and Impedance, seems to glide effortlessly over long distances just like AC transmission lines, VS Edison’s DC power lines and straight resistance.
[ December 18, 2004: Message edited by: Chris Smith ]<hr></blockquote><p>The reason AC power transmission won out over DC is because AC can be stepped down by a transformer. Therefore, the transmission voltage can be raised very high, and the transmission current can be reduce. With lower transmission current, there is reduced I squared R power loss....Stricly an ohms law thing...nothing to do with effortless gliding...

[Chris Smith]

DC can also be stepped up. But It never was done, and for a good reason. <p>It does not share the same characteristics as AC, as in Simple DC RESISTANCE. <p> You can build a DC generator to produce ten thousand volts, instead of 120, but this was never a viable option compared to AC impedance. <p>The Higher the Frequency, the lower the resistance, something that doesn’t happen with a DC component. <p>DC frequency is rated as ONE, while just 60 HZ as in Ac changes everything. <p>Impedance works to reduce the Resistance values not solely based on Voltage as you suggest, but higher frequencies in both Electro or magnetic wave fronts do reduce the resistance value for many other reasons, all having to do with physics.<p> The electron at high frequency, does not take the same path, nor does it exhibit the same characteristics as a DC current.

[russlk]

The major difference between a DC electromagnetic field and an AC one is that the AC field can be focused and beamed to a remote location. Energy can be transmitted via AC fields, but not with DC (in a wireless system).

[dyarker]

Back to the M. Faraday experiment guys. Two parallel wires, one with a battery and switch, the other with a galvanometer (centered needle low current meter). Switch ON, pulse of current in one direction, then no current. Switch OFF, a pulse of current in other direction, then no current.<p>A steady magnetic field does not induce a current in a conductor. Changing (moving) lines of force of magnetic field "cutting" across a conductor induces a current. Coiling the conductor allows the lines to "cut" the conductor many times within the strong area of the field.<p>A high voltage DC can be generated. Though the wire insulation in the generator becomes prohibitive. Transformers are more efficient; the insulation doesn't move. AC distribution lines are current/voltage phase corrected, so I^2R is the same. Then at the user end, the problem of "stepping down" DC to safe voltages again.<p>Tesla and Westinghouse where right, Edison was wrong. Tesla thought higher frequency and distribution without wires. He was wrong on that point. Might have got it work, but we'd all have cancer by now (maybe).<p>50Hz/60Hz can be generated by reasonable generator RPM, and allow reasonably sized and efficient transformers to be built; and is far far from skin effect problems of high frequency on the transmission lines.<p>C U A L -<p>[ December 19, 2004: Message edited by: Dale Y ]</p>

[rshayes]

Edison wasn't all that wrong. A transmission line can carry about 40 percent more power with DC than with AC. One amp rms times 1 volt RMS will deliver 1 watt into a 1 ohm resistive load. If the load is reactive, the power delivered will be less. One amp DC times 1.414 volts DC will deliver 1.414 watts to a 1.414 ohm load. The RMS currents are the same, so the IR losses are the same. The peak voltage is the same. The DC line will carry 41.4 percent more power with better efficiency.<p>The difference is in the terminal equipment. Transformers are needed to step the voltage up and down for either DC or AC lines. The DC line also requires high voltage rectifiers and filters on the transmitting end and high voltage switches on the receiving end in addition to the transformers. These devices were not available to Edison. Even now, the added expense makes a DC line more costly except for long transmission lines.<p>Tesla had vague ideas about transmitting power by exciting resonant modes of the Earth. The Tesla coil was based on resonance. The installation that he built as an experiment was done before World War I, and before most electrical engineers had clear ideas about transmission lines and cavity resonators. Heaviside had worked out the theory of transmission lines several years before, but Tesla may not have been familiar with that work, since it was mainly applied to the analysis of undersea telegraph cables. The information on cavity resonators was extensively developed in relation to waveguides during World War II. I have not seen any information that Tesla was familiar with this field either.<p>The simplest resonant pattern for the Earth is probably where the circumference is about one wavelength. This will be around 7 Hz. Making an efficient antenna for 7 Hz would be difficult, to say the least. Most of the power would be lost in ground resistance. The transmitting site and a point 180 degrees away would be hgih voltage points. A line around the Earth midway between them would have zero voltage. Coupling in this region would have to be inductive. There would be resonant modes for the Earth at higher frequencies, but the location of voltage maxima and minima would be much more complex.<p>Note that the length of the power distribution network in the United States is about one wavelength long at 60 Hz. This may cause problems even in a wired system.<p>A DC distribution system would be more efficient and could be built all the way down to the individual house. The cost of converting the present system would be enormous, and would not produce enough benefit to be worthwhile.

[jwax]

steven- Watt? DC transmission on wires is more efficient than AC transmission on wires? News to me. General Electric wants to talk with you. So if I apply 120 VDC to my toaster, it'll get hotter than if I use 120 VAC? You could patent that!
John

[Chris Smith]

Steven:<p> Your history is woefully missing. Tesla invented the poly phase motor and 60 Hz in America, and understood more abouct electricity than all the electrical scientist of the day. <p>Edison, was a boss, who hired slaves to solve his engineering problems, and had very little understanding of electricity. <p>Tesla, invented the Radio almost twenty years before Marconi patented it, and Marconi couldnt patent it, with out the parts Tesla invented and patented almost a decade earlier.<p>That’s why just recently Tesla was awarded rightly so, the coveted award of Inventing the Radio, before Marconi, some 50 years late. <p>Tesla, in 1892 had already built a Remote control, which is a radio, and had one installed in a toy boat and operating, before the turn of the century. <p>DC offers nothing over AC, as the power factors never come into play because DC resistance Robs all the power factors that you mention. <p>All you have to do is look up AC 60 HZ VS AC 600 Hz, and you will find a Impedance value that keeps getting better, something that DC can never do. <p>And that is why AC was chosen over DC, not rectifiers, or any other reason of the day. <p>HV DC could have driven smaller generators via a motor if Edison had his way, and those motor driven generators could have powered a line, about one quarter mile at 120 volts before pooping out.

[Chris Smith]

Dale<p>Actually Tesla got it right, in colorado, before he burned down the power house. He liked to push things to the limit!<p>He transmitted 10,000 watts of power some ten miles away and lit some light bulbs,[10,000 watts] but yes, it wasnt practical and today if we tried, we would use even higher frequency and cancer may or may not be a factor?

[rshayes]

One of the major power lines in this country is the Pacific Intertie. This is actually a group of 4 power lines. All of them start on the Oregon-Washington border. Three of them are AC lines with a capacity of 1600 megawatts each. These end in the vicinity of San Francisco. The fourth is a DC transmission line with a capacity of 3100 megawatts that ends in Sylmar, near Los Angeles. It is 846 miles long (about 300 miles longer than the others) and about twice the capacity of the AC lines. If I have to hang 900 miles of copper in the air, I would want to get the most efficient use of it, and that appears to be DC transmission.<p>This isn't the largest one. There is a higher capacity DC transmission line in Brazil.<p>With practical sizes of wire, there is not much difference in loss between DC and 60 Hz. At higher frequencies, in the Khz range, the skin effect causes the AC resistance of the wire to increase over the DC resistance. Some inductors used in radios were wound with special wire to avoid this problem ("litz wire").<p>I would suggest getting history from other sources than Tesla web sites. His major field of expertise was probably AC power. He is generally credited with the invention of the AC polyphase motor, and that was a significant achievement. The Tesla coil is popular and spectacular, but I don't know of any extensive practical use that has been made of it in one hundred yars since then. The AC distribution system is based on the transformer, which Tesla did not invent.<p>His patents do not indicate any knowledge of radio. For example, No. 645,576, issued in 1900, describes a wireless power transmission scheme. It is based on using baloons to raise electrodes into a rarified atmospheric region and using high voltage to create a conductive path between them. He seems to be talking about creating a glow or arc discharge between two electrodes and using that to carry current. Give him credit for thinking big. The thought of a glow discharge 1000 miles long boggles the mind.<p>The remote controlled boat patent, No. 613,807, issued in 1898, describes a rather elaborate decoding scheme in great detail. The control transmitter is not described in detail, but seems to be a capacitor discharged through an induction coil, which is connected to an antenna. There does not appear to be any deliberate attempt to tune the transmitter. The receiver is simply an antenna connected to a coherer, again with no tuning means.<p>The coherer was developed by several people. One version was developed by Branley about 1891. An earlier version may have been used by Lodge for studying electrical effects in the atmosphere. In effect, the transmitter was simulating a miniature lightning bolt and the receiver was a device already well known for detecting electric discharges in the atmosphere.<p>Loomis demonstrated communication over several miles using atmospheric disturbances in 1865.<p>Radio waves were fairly well known by this time. Maxwell's equations were published by 1873, and radio waves can be shown to be one solution of these equations. Hertz demonstrated the generation and reception of these waves in 1888. The combination of Hertz's transmitting and receiving devices and the Branley coherer would have given Tesla a radio controlled system, but he didnt do that.<p>Marconi didn't invent radio, but he did invent and develop devices for using it to transmit over successively longer distances, ending in communication across the Atlantic Ocean.<p>Edison applied himself to a wider range of devices than Tesla, for example the light bulb, batteries, telephone and phonograph. He also observed the "Edison effect" in a light bulb. This was the basis for the Fleming valve, which was later developed by De Forest into the vacuum tube. That was the big fish that got away for Edison.<p>The best view of technical history is probably the patent office web site. The patents are prepared by the actual inventors and tell how they thought their inventions worked. Some of their explanations are absolutely amazing.

[Bernius1]

Fascinating. Thanks, Y'all. However, Re: original question; a DC wire or coil DOES have a field around it, but the lines of force have to be 'moving' to generate a voltage. In AC, the collapsing fields move the lines, so there is an apparent movement of the secondary. A simple metal detector is a coil with a DC current, which produces a change in voltage when metal passes through the field.
MY question would be; If a twisted pair cancels 'common-mode' noise, then
A) Will a twisted-pair ring of wire be less detectable by a metal detector, and
B) Would twisted-pair power lines radiate less RF and residual noise ??

[jwax]

stephen- I take offense to your statement about Tesla: "His patents do not indicate any knowledge of radio"!
Why then did the USPTO withdraw the Marconi radio patent, and grant it instead to Tesla?
I hope you are not blinded by popular history which portrays Edison as anything other than a rude, thieving, odorous, and insulting tinkerer. And yes, he didn't care who or what he was playing with, as long as he could get to market and sell it, regardless of what it was. That's why is "inventions" are so varied.
Thanks for enlightening us on the use of DC transmission. It is indeed in use for cheap, specific point-to-point trasmission where no outages are allowed, since the entire system is down when any part of it is down. Cheap because only two lines are needed, not three for poly phase.
As to efficiency, a watt of AC still equals a watt of DC.