Posted on 05/28/2009 11:30:09 AM PDT by Squidpup
Thanks - I only saw the first movie.
Thanks for the superb explanation.
One thing, though. You note, “Under clear skies there is a voltage gradient from ‘ground’ level of several hundred volts per meter of altitude.” Why? And if I’m six feet tall, or approximately two meters, why don’t I short out when I stand up?
Good question, it's true our bodies are made up of largely salt water and thus reasonably good conductors of electricity, however we are encased with a skin with much better insulating properties (in the range of millions of ohms).
If you consider a car battery for a moment, it supplies 12 volts and has an internal resistance of a few hundredths of an ohm. Ohm's law tells us that if you connect a 12 ohm resistor to the battery terminals, a current of one amp will flow. If however you connect the terminals with an errant crescent wrench, the current is limited only by the internal resistance and so perhaps a thousand amps will flow through the short circuit.
The point being that the short circuit current (which causes the damage to your body, not the voltage) is limited by the internal impedance of the power supply. When you are dealing with lead/acid batteries that is quite low so very large currents can flow. The same is true of the commercial power grid. However the voltage gradient I spoke of is actually a form of static electricity and the "source" has an impedance in the millions of ohms. The voltage can be harvested with a helium balloon and a copper wire of a couple of hundred feet, the higher you go the greater the voltage but there is almost no current available to do anything useful with it (except make some impressive sparks). To actually do something you need power which is the product of voltage multiplied by the current flowing.
Most electrical rotating machinery (generators, alternators, motors, &c.) are based on magnetic forces which are fundamentally created by current flow. It is possible to create analogs of these machines which function thru electrostatic forces (those that cause flyaway hair on dry days), those forces being created by voltage differences. I've seen electrostatic motors that spin endlessly, powered only by a wire sticking twenty feet in the air. As a novelty, they are fascinating, in practical terms they are of little use as they produce just enough power to overcome their own bearing friction.
In summary then you are protected by the resistance of human skin and the inherent internal impedance of the static power in the earths atmosphere. The game changes during a thunderstorm as a different sort of static generator comes into play (the difference between rubbing an amber rod with a piece of silk and a Van de Graaff generator) with tens of thousands of horsepower driving the process. The available voltages actually break the air molecules down to a plasma which is a very good conductor and allow huge currents to flow.
Regards,
GtG
PS You can "by-pass" you skin resistance and actually "taste" electricity by licking the terminals of a nine volt battery. Your taste buds are stimulated by the modest current flow. Do NOT try that with 120 VAC unless you are bored with life.
Oh, tasting a 9V battery is one of my old-timey ways of testing for battery life. There is no mistaking a fresh battery! ...ow!...
Thanks again for the great tutorial. One more question: where does that voltage gradient come from, and with air motions, people walking through it, and other disturbances, why doesn’t it equilibrate?
You have hit the heart of the matter with that one! I don't know and I suspect neither does anyone else. By analogy, we know the earth has a geomagnetic field. The explanations all seem to say it's because we have an iron core, although they never really say why that in and of itself would create a magnet. If you were to heat an iron sample, as the temperature rises at some point iron loses all of its magnetic properties. That happens at a dull red heat, this phenomenon is know as the Curie point. The core is molten, and well above the Curie point so how does it then generate a magnetic field?
Never the less the magnetic field exists. I'm not sure if it is actually the source of the E field but I know as fact that where there is a moving magnetic field (the spinning earth) there is an electric field as well. Electricity and magnetism are two faces of the same coin and do not exist independently. So I'll wave my hands in the air and say it's there because it is irrefutably there and was put there by whoever wrote the rules of the game.
As to why it doesn't become homogeneous, there several reasons:
It is continuously being refreshed. Another analogy here, it's rather like a lake in the mountains that has an outlet that leads to the sea. Why doesn't all the water wind up in the ocean? Because the water evaporates from the ocean and fall as rain in the mountains and keeps refilling the lake.
Also, the air is a dielectric which does not easily conduct an electric current, so the field does not immediately collapse to "ground" potential. Humidity and rain do affect the field and and cause it to weaken whereas dry air allows it to build.
It has a gradient because we measure relative to the "ground" literally. The earth itself becomes our zero reference point. The air above it contains a distributed "space charge" of free electrons (which repel each other because they are all of the same polarity) the charge is uniformly distributed and as we increase in altitude, we have more and more of the "space Charge" between us and the reference.
Anyway, that's my story and I;m sticking to it!
Regards,
GtG
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