General > General Technical Chat
Electrons are round!
gnuarm:
--- Quote from: Simon on August 14, 2023, 04:40:43 pm ---
--- Quote from: aetherist on August 14, 2023, 07:30:56 am ---Ok, i have the correct results. I got my results from the link provided by TimFox.
A 1.0 mm Cu wire 1000 m long has a resistance of 35 ohm
A 10.0 mm Cu wire 1000 m long has a resistance of 3.6 ohm.
A 10.0 mm wire has a Xsectional area 100 times that of a 1.0 mm wire.
Hence the resistance of a 10.0 mm wire should be 0.35 ohm based on Xsection area.
And the resistance of a 10.0 mm wire should be 3.5 ohm based on circumference.
So, my circumference theory has an "error" of 0.1 ohm.
But your area theory has an error of 3.15 ohm.
--- End quote ---
good lord, heaven knows how I correctly worked out harness lengths in my previous career particularly when I had to calculate the length of cable required to supply a load that was more sensitive than most to voltage drop on a 24V system. I must have been on magic mushrooms at the time and was obviously helped by the fairies..... Or I used my understanding of current and correct physics.
--- End quote ---
I once worked at a place where there were rumors of a guy who not only was stupid, but had no idea just how stupid he was. This was a beltway bandit, where we would either build systems for the government, or sometimes simply provide direct support.
The story of this guy goes that he was in a meeting where they were picking the location of new antennas to be installed. Different people gave a few opinions. Then this guy, we'll call him Floyd, chimes in that we should put the antenna at the bottom of the hill. Everyone had a laugh, then Floyd continued. He said the antenna should be a the bottom of the hill, so that they would collect the electrons as they ran down the hill. Another good laugh. But, Floyd wasn't laughing! He was being serious!
Another time, Floyd and some other guys were doing a facility survey to prepare for installation of equipment in a room. It's basically measuring the space, making sure there was enough AC, electricity, etc. One of the team members was calculating the floor area, length times width. Floyd said he was doing it wrong. It's length divided by width. The guy asked him what he was talking about. Floyd's reasoning was, "How many quarters in a dollar? How many quarters in two dollars? See?"
It's hard to argue with someone like that.
TimFox:
It's like playing chess with a pigeon: First, he flaps his wings and knocks over the pieces, then he takes a dump on the board, finally he flies home and brags that he won the game.
coppercone2:
EPAIII:
Actually, since electrons are best described by quantum theory which uses what is called a wave function that describes the probability of finding any individual electron at any given point in the universe and since that implies that each individual electron can actually and truly be at any given point in the universe when that wave function is made to collapse, then the shape of an electron, if it can indeed be said to have any shape at all, would be the shape of the universe itself.
And since we don't actually know what the shape of the universe itself is, we do not know what the shape of an electron is. Not really!
Nominal Animal:
--- Quote from: TimFox on August 14, 2023, 11:32:32 pm ---I haven't done the calculations yet but I guess that the erroneous wire table I linked was in terms of mm2, the common wire size in metric-speaking countries, rather than mm diameter, since I have had technical conversations with foreign engineers who used "mils" or "mm" as an abbreviation for area.
--- End quote ---
It matches the \$R(\ell, A) = \ell \rho / A\$ exactly, when "gauge" is area in square millimeters, \$\ell\$ is length in meters, and resistivity were \$\rho = 35.2 \text{ n}\Omega \cdot m\$.
It seems the table has several errors, not just using "gauge" and "mm" for cross-sectional area. For example, the resistivity of copper is around \$\rho = 17 \text{ n}\Omega \cdot m\$ (depends on the copper alloy and structure, hardened/non-annealed copper tends to have a slightly higher resistivity/lower conductivity), so it looks like the author erroneously halved the area, as in "\$A = \pi r^2 / 2\$" –– which is a common error when confusing radius and diameter \$D = 2 r\$ and thus \$A = \pi r^2 = \pi (D/2)^2 = \pi D^2/4\$.
Understandable errors, but horribly annoying when taken as correct. I, too, were bamboozled for a second, before I checked some of the stated resistances (which obviously did not match the known correct and measured values). And the edits to this post were because I was again bamboozled when investigating how the factor of 2 error came to be.
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