## Electromagnetic physics question

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ian
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### Electromagnetic physics question

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
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### Re: Electromagnetic physics question

terri wd0edw

Chris Smith
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### Re: Electromagnetic physics question

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
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### Re: Electromagnetic physics question

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
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### Re: Electromagnetic physics question

<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
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### Re: Electromagnetic physics question

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
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### Re: Electromagnetic physics question

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
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### Re: Electromagnetic physics question

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>
Dale Y

rshayes
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### Re: Electromagnetic physics question

jwax
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### Re: Electromagnetic physics question

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
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### Re: Electromagnetic physics question

Chris Smith
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### Re: Electromagnetic physics question

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
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### Re: Electromagnetic physics question

Bernius1
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### Re: Electromagnetic physics question

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 ??
Can't we end all posts with a comical quip?

jwax
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### Re: Electromagnetic physics question

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.

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