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Date:         Sun, 5 Apr 2009 18:35:00 -0300
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Learning Electricity Part Two
Comments: To: Gary Bawden <goldfieldgary@gmail.com>
In-Reply-To:  <67f8ac6c0904051351s6ea31a5vccb484d45fd6874c@mail.gmail.com >
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 05:51 PM 4/5/2009, Gary Bawden wrote: >Under what >circumstances would electricity seem analogous to a compressible >fluid, as opposed to non-compressible?

Electrons hate each other (this is called the pathetic fallacy). Electrons have no emotions whatsoever so far as we can tell, but they *act* as though they hate each other, and they push each other away with, for their size, great force. If I remember correctly what I saw in wikipedia last night, two theoretical point sources at a meter distance, each representing a coulomb of charge, would repel each other with a force of about 9,000 tons.

So if you take a piece of wire where all the electrons and protons are neatly balanced but the electrons are held pretty loosely to an individual atom and shove an electron in one end, very very quickly (in copper, about 95% of the speed of light*) an electron would try to pop out the other end.

*which is curious, because if you painted one electron blue and followed it through the wire it would only move at about 300 feet per second. But the current of electrons moves very fast indeed, unless you're building a very fast computer, in which case you have to keep making it smaller and smaller because the bloody electrons move so bleeding slowly (a few nanoseconds per meter, that is).

Back to our wire...if you give the outgoing electron somewhere to go you can insert the incoming one easily, but if not you have to shove a little, and now all the electrons in the wire will feel a little crowded -- It's pathetic, I tell you. The second one will take more pressure, and the third more yet. If you crowd enough in, the electrons jammed together in the pointiest bit will be physically squeezed out the point and jump to someplace with a lower potential, i.e. pressure. This is of course called a spark, which under the right conditions can lead to a very different beast called an *arc*. In dry air at sea level, the amount of potential difference necessary to jump a spark one inch between spherical electrodes of one-inch diameter is about 25,000 volts.

The property of being able to hold electrons in a compressed state is called capacitance, and its unit is the farad. One farad is the amount of capacitance (capacity) able to hold one coulomb of electrons at a pressure of one volt. In the real world that is a very big unit; so it's usually more convenient to work in micro- or pico-farads. In recent years people also use nanofarads in preference to fractional microfarads, but I prefer the old way. The Navy manual undoubtedly represented a capacitor as similar to a spring, but it's really more like a rigid container into which electrons can be crowded under pressure.

A single wire has very little capacity (called self-capacity), and we can entirely ignore it for most automotive purposes. "Normal" capacitors are made of two conducting sheets separated by an insulator, such that electrons crowded into one of the sheets can exert pressure on those in the other sheet, but cannot themselves physically pass through the insulator (if too much potential exists between the two sheets so that electrons actually rupture and pass through the insulator, the resulting component is technically known as a wire).

Typical insulators are air (for very small, usually adjustable capacitors), paper, mica, ceramics, or polystyrene or polyester or polycarbonate film, stacked in multiple layers or rolled together; and typical values produced by these methods range from a few picofarads for air and small ceramics up to as much as a microfarad or more for some rolled-film "caps." I once owned a home-made high-voltage capacitor made of twenty or so small window panes alternating with sheets of aluminum foil, the whole submerged in oil in a wooden box. For some reason my imagination let me down, and I failed to kill myself with it (I was fifteen or so).

Larger values are made by a different process, and are called "electrolytic" capacitors. These contain (usually aluminum) thin sheets separated by thin paper soaked with an electrolyte solution. The insulator is an extremely thin layer of oxide formed on one of the plates; because it so thin, electrolytics can achieve high capacity in small physical size. One snag is that they are directional, and if hooked up backwards tend to explode. Another is that they have both a shelf- and a working-life. Twenty or so years ago, capacitors built by this technique achieved the previously undreamed of value of one or more *farads* in a container half an inch across by less than that high. Such devices can be used to replace batteries in some applications like memory backup.

In non-electronic automobiles about the only place that capacitance played an important part was in the Kettering (points-type) ignition system and the radio. In our beasts you can take away the ignition and add the ECU, clock, temp gauge, wiper delay relay, coolant-level warning controller etc.

Aren't you sorry you asked? :-)

>I read those WWII US Navy basic >electricity books, probably about the same time you did, but I recall >them using water as a comparison. It's still stuck in my mind that >way...

I tried learning stuff from an electrician's mate manual, but I hated the sucker with a passion because it never ever ever said "why." And I've never been any good at memorizing. It was tantalizing.

-- David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/ '89 Po' White Star "Scamp"


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