air impedance

in article c9rbpt$2m3k$ snipped-for-privacy@justice.itsc.cuhk.edu.hk, Wayne at ares snipped-for-privacy@hotmail.com wrote on 6/4/04 7:38 PM:

Hi,

I want to measure the impedance or conductance of air. How can I achieve and what is the possible value of the impedance of the air? Thank you very much!

Wayne

For uniform plane waves, it is 377 ohms.

Bill

Re: air impedance This is not air impedance. 377 it's only the relationship between E and H fields in vacuum. Impedance of air, related to a ohm's law, it's the relationship betwheen current flow and electrical field. Thus, for measuring air impedance, you need to generate a current flow between two electrodes, applying a relatively high continous voltage, and measure the current.

Regards, Jorge (sorry for my poor english)

"Repeating Rifle" escribió en el mensaje news:BCE6BB24.19D58% snipped-for-privacy@sbcglobal.net... in article c9rbpt$2m3k$ snipped-for-privacy@justice.itsc.cuhk.edu.hk, Wayne at ares snipped-for-privacy@hotmail.com wrote on 6/4/04 7:38 PM:

Hi,

I want to measure the impedance or conductance of air. How can I achieve and what is the possible value of the impedance of the air? Thank you very much!

Wayne

For uniform plane waves, it is 377 ohms.

Bill

Am I missing something? The impedance of air is essentially infinite until its dielectric strength is exceeded and it ionizes. The impedance is then close to zero. Regards,

John Phillips

"I never perfected an invention that I did not think about in terms of the service it might give others."

Thomas Alva Edison

The answer to this questions deserves a degree of formality and we need to look to electrostatic theory and the equations of Mr. Maxwell to provide the correct answer.

The impedance of air is NOT infinite... radio waves propagate through a vacuum quite readily at the speed of light and this is normally not an ionized medium. The propagation speed through air is just a bit less by a miniscule fraction.

I haven't got my electrostatics book handy, but the overall impedance for air or for free space can be calculated just like it can be for a transmission line. It has to do with the factors µ (the permeability) and e (the Permittivity).

Permeability, also called magnetic permeability, is a constant of proportionality that exists between magnetic induction and magnetic field intensity. This constant is equal to approximately 1.257 x 10-6 henry per meter (H/m) in free space (a vacuum). In other materials it can be much different, often substantially greater than the free-space value, which is symbolized µo.

In engineering applications, permeability is often expressed in relative, rather than in absolute, terms. If µo represents the permeability of free space (that is, 1.257 x 10-6 H/m) and µ represents the permeability of the substance in question (also specified in henrys per meter), then the relative permeability, µr, is given by:

Permittivity, also called electric permittivity, is a constant of proportionality that exists between electric displacement and electric field intensity. This constant is equal to approximately 8.85 x 10-12 farad per meter (F/m) in free space (a vacuum). In other materials it can be much different, often substantially greater than the free-space value, which is symbolized eo.

In engineering applications, permittivity is often expressed in relative, rather than in absolute, terms. If eo represents the permittivity of free space (that is, 8.85 x 10-12 F/m) and e represents the permittivity of the substance in question (also specified in farads per meter), then the relative permittivity, also called the dielectric constant er, is given by:

er = e / eo = e (1.13 x 1011)

As stated above, the permeability and permittivity in air is near, but not exactly exactly equal to the free space permeability and permittivity (in a vacuum -- outer space, for example). And if you know both the permeability and the permittivity of the medium you are dealing with (there are published tables), you can calculate the impedance of that medium very accurately.

Beachcomber

Looking back at the original question, and after I posted my answer to the impedance and air, I just realized that the poster may be asking something else, poorly worded as it is.

Air breaks down (ionizes) at about 2000 volts per centimeter of gap, more or less depending on humidity, pressure, temperature, and so forth. Perhaps this is what you mean by the "conductance" of air?

Beachcomber

in article snipped-for-privacy@4ax.com, John Phillips at snipped-for-privacy@hotmail.com wrote on 6/5/04 6:00 AM:

You are OK. It is others that are missing something. Air does not follow Ohm's law. At low voltages, air between electrodes acts as an ionization chamber.

Bill

in article snipped-for-privacy@netnews.comcast.net, Beachcomber at snipped-for-privacy@nospam.xyz wrote on 6/5/04 9:14 AM:

Wording a problem correctly is important. Doing so by itself goes a long way toward solving the problem. Unfortunately, many engineers and scientists as well consider use of proper English to be an unworthy skill. After all, even children can learn how to speak grammatically.

ill

On Sat, 05 Jun 2004 16:36:02 GMT Repeating Rifle wrote: | in article snipped-for-privacy@4ax.com, John Phillips at | snipped-for-privacy@hotmail.com wrote on 6/5/04 6:00 AM: | |> Am I missing something? The impedance of air is essentially infinite |> until its dielectric strength is exceeded and it ionizes. The |> impedance is then close to zero. | | You are OK. It is others that are missing something. Air does not follow | Ohm's law. At low voltages, air between electrodes acts as an ionization | chamber.

My little 1/8 watt 100 ohm resistor doesn't follow Ohms law, either, after a very short time with 240 volts.

Please explain why EMF has anything to do with the impedance of air. It propagates as well in the vacuum of space as in air. Anyone who can argue that a vacuum has an impedance less than infinity needs to learn the laws of physics.

Regards,

John Phillips

"Prohibition goes beyond reason in that it attempts to control a man's appetite through legislation. A prohibition law strikes a blow at the very principles this country was founded upon."

Abraham Lincoln

Well, not quite. Impedance is not a property of a substance. Neither is resistance, to make a comparison.

If you re-phrase the original question to ask "What is the resistance of copper" the problem becomes obvious. If you ask, however, "What is the resistivity of copper," or "What is the permittivity of air" you are asking answerable questions.

Impedance and resistance are properties of electrical circuits. Until the circuit is defined impedance is undefined.

in article snipped-for-privacy@4ax.com, John Phillips at snipped-for-privacy@hotmail.com wrote on 6/5/04 3:03 PM:

This is indicative that a little knowledged is a dangerous, or at least almost useless, think. The electromagnetic impedance of air is just about the same as that of vacuum. Numnerically, it is 377 ohms. Figure it out and post again.

Bill

References please.

Regards,

John Phillips

"Prohibition goes beyond reason in that it attempts to control a man's appetite through legislation. A prohibition law strikes a blow at the very principles this country was founded upon."

Abraham Lincoln

Actually he did not perfect many inventions. Most of his inventions were made by his employees.

Ah, but one of his 'inventions' that is often overlooked is the idea of a R&D lab. It was his idea to hire a bunch of bright employees for the purpose of inventing things.

daestrom

I think Da Vincy had him beat by a few years. How do you think he turned out so many paintings?

Makes one wonder if "intellectual property" was a issue at that time.

--s falke

Oh yes believe it was. Some Jewish guy in England invented the internal combustion engine used in automobiles and he patented it. Around 1915 or so, Ford stole the idea and used them in his autos. He was sued and the court awarded the complainee about 60 million dollars (Which would be like 60 billion now) I believe it was the highest award up to it's time. Ford never forgave that Jew for sueing him and hated the Jews ever after and that is why we see FORD distributing the Protocols of Zion around the world.

Oh yes, there were lots of major suites and Edison was involved in a number of them. They sure valued intellectual properties as they do now.

Aside: What broken newsreader do you use to post with? It screws up the threading and adds a "b" in the subject.

Ain't that the truth!

Proper English aside, the confusion appears to be between 'impedance', which implies the propogation of EM waves and resistance, or more poroperly stated, resistivity. In the latter case, air doesn't conform to Ohms law, so the answer will be somewhat more difficult to come by without understanding the conditions.