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VOLTAGE IS PRESSURE?
Then how many ATM equals One Volt?
W. Beaty 3/2004

If voltage is like pressure, what pressure is it?

Let's make a bunch of assumptions. Looking around on various pages of hydraulic backhoe pumps, I find that a 30 cubic in/sec flow rate pump can run at 1500 psi, transferring about 5 kilowatts. What are the typical hose diameters like? Maybe 1" inside?

So, if a 5KW DC motor runs at 240v and 20 amps, and if the wires are equal in size to the hydraulic hoses above, then what hydraulic pressure "equals" one volt of potential?

The speed of the "electron fluid" is slow, but the speed of the hydraulic fluid in the hoses is fast. Pressure will have an inverse change (since watts is proportional to fluid speed times pressure diff.: we must raise pressure while slowing the flow, so that power remains constant.) The speed of hydraulic fluid in the above example is about 30 in/sec. Using http://amasci.com/miscon/speed.html, the speed of charge carriers for 20 amps in 2cm solid copper "hoses" is around .0002"/sec.

Therefore, if a hydraulic system was flowing at .0002" per second rather than 30" per second, yet was still delivering 5 kilowatts, the pressure would have to be proportionally higher than 1500 psi.

I make it out to be...

  • 1 volt potential "equals" a hydraulic pressure of 1,000,000 psi
Very rough calculations, lots of weird assumptions, might be off by 2x or 10x.

Huh. So that's why electrons can flow so slowly in everyday circuitry. The "workng pressures" in simple electric circuits are astronomical when compared to the hoses in industrial hydraulics. And the resistance of wires is terrible, it's huge: like pumping hydraulic fluid through packed sand or powder. Even worse than that! Electric circuits are like pumping warm tar through pipes full of packed sand. If it moves fast enough for humans to notice, it would char itself from the frictional heating. Circuitry has high pressure, slow flow, and not much heating caused by excruciating slow fluid-flow in the mile-long tubes of sand, tubes called "the power grid."

And think about AC power systems. It's not just one volt anymore. Sending megawatts down an AC line is like sending 60Hz sound waves along a long column of fluid, with the sound pressure being hundreds of billions of PSI, and the fluid inside the pipes only vibrating microscopically back and forth.

Now if only hydraulics behaved like wires! In that case a hydraulic motor could still turn at a decent speed, even when supplied with extreme mega-PSI pressure and almost no flow rate. The oil could move at 1mm per hour, while the oil-motor spun at 1000RPM. With something like that, the losses in miles-long hoses would be tiny, and we could replace all of our power technology with "hydr-icity" pipes instead of "electr-icity" pipes!

PS

Separate topic.  In some pop science book (perhaps "Time Travel and Papa
Joe's Pipe,"  the author mentioned a question asked by his aging father.

The father had had lots of experience in heavy machinery, and knew just 
how many maximum watts of mechanical power could be sent down how large a 
particular driveshaft.  But the father was always confused about 
something.

How could megawatts be sent down a half-inch aluminum cable by an 
electrical generator?  Why are wires so small when compared to the 
equivalent steel drive shaft?

The above hydraulic analogy supplies the answer:  "pressures" in 
electrical circuits are stunningly huge!  One volt equals a million pounds 
per square inch!  The "electrical pipes" don't burst until you reach much 
higher pressure than that; the analogous giga-torr levels.  And even the 
simplest tiny "electricity pump" can easily produce such "pressures."  
With such things attainable, huge wattage can be sent down a long "hose" 
where the fluid in the "hose" need only crawl along imperceptibly, and the 
"hose" can be extremely narrow, yet deliever relatively huge energy-flow.

Yet at the same time, wires have enormous frictional problems, and if the 
"electron fluid" should ever be pumped at a speed humans can see (say a 
few cm per second,) the wire quickly heats up and melts!

As a hydraulic system, electric circuitry is very strange: it uses the 
analog of gigantic pressure, yet at the same time it's not practical to 
pump the "fluid" in the pipes any faster than a snail's pace.  

 





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