CanadysPeak » April 27th, 2014, 1:47 am wrote:Truth is, most analogies don't work because voltage is energy per Coulomb, and that is a case not seen elsewhere in ordinary life, or at least I think.
neuro » Sun Apr 27, 2014 7:23 am wrote:CanadysPeak » April 27th, 2014, 1:47 am wrote:Truth is, most analogies don't work because voltage is energy per Coulomb, and that is a case not seen elsewhere in ordinary life, or at least I think.
Actually, Canadys, any "potential" is Energy per unit.
Pressure is energy per unit volume.
Chemical potential (diffusional), i.e. (RT/F).log(concentration), is energy per particle, and for ions in solution it works exactly as electric potential (see Nernst equation).
Height from ground is energy per mass unit (given gravity).
And for ideal systems - i.e. neglecting frictions, particle-particle interactions, considering stationary conditions (no accelerations), etc - all these yield flow equations similar to Ohm's law.
Dave's conclusion above about all resistance being "not additive, the smallest aperture (Highest Resistance) is virtually the only controlling factor" is probably true for pipes, because the resistance along the pipe is much smaller (totally irrelevant) with respect to the resistance of the smaller aperture.
If you put a 1 kOhm resistance in series with a 1 MOhm, you don't see any additivity, even though that's electric stuff, unless you have a very sensitive gauge. And if you add 20 feet copper wire you also do not see any difference.
However, when you consider COMPARABLE resistances, they are additive in hydraulic systems as well, and there is no reason why they would not be.
CanadysPeak » April 27th, 2014, 1:33 pm wrote:In electricity, it seems important (at least to me) that one understand voltage as being defined by the gradient of the electric field, and it be path independent.
To me, electricity seems so easy that I can't imagine needing help to conceptualize it.
neuro wrote:If you put a 1 kOhm resistance in series with a 1 MOhm, you don't see any additivity, even though that's electric stuff, unless you have a very sensitive gauge.
CanadysPeak wrote:One of the saving graces is that most students pay no attention to what we say to them.
Dave_Oblad » April 27th, 2014, 9:49 pm wrote:My point earlier was simply that because water can't be compressed (more or less) the pressure is distributed evenly in my example (C) for chambers X,Y,Z. It doesn't require a pressure gradient to flow through the individual chambers. Water is pushed through (at the Speed of Stick) to replace missing water and the main pressure gradient will only be found at the extreme small and local aperture between Inside and Outside the System. If you radially widen that exit aperture then the next smallest aperture becomes dominant in a series pipe situation.
Obvious Leo » Sun Apr 27, 2014 11:37 pm wrote:CanadysPeak wrote:One of the saving graces is that most students pay no attention to what we say to them.
Your cynicism is only surpassed by your experience of the real world, my friend. Where would we be without education, which is only marginally better than its alternative? Heaven forbid we should allow the hoi polloi to do their own thinking.
Regards Leo
neuro wrote:Why do you keep saying there is no gradient? If no pressure gradient exists between two points there will be no water flow.
Donald E. Simanek, Emeritus Prof. of Physics, Lock Haven University of Pennsylvania wrote:In your example of three constricted pipes, I ask this: If you had one system with three narrow pipe segments of length x butted together in series (equivalent to one narrow pipe of length 3x), and another system with only one segment of length x (the systems otherwise identical), would they have the same flow rate? If there's no significant dissipative processes like laminar flow and viscosity, the answer should be "yes" for the pressure difference between reservoir and output is the same in both systems. Now if the three segments were separated as in your diagram, should that matter? No.
Dave_Oblad wrote:Atmosphere saturation in water allows some compressibility.
Dave_Oblad wrote:Hope this helps,
Wiki wrote:The bulk modulus of water is 2.2 GPa. The low compressibility of non-gases, and of water in particular, leads to their often being assumed as incompressible. The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume.
Dave_Oblad » Thu May 01, 2014 8:49 pm wrote:Hi Canady,
While you are technically correct.. water is generally considered non-compressible.Wiki wrote:The bulk modulus of water is 2.2 GPa. The low compressibility of non-gases, and of water in particular, leads to their often being assumed as incompressible. The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume.
For the pressure levels we are dealing with in my examples, it's fine to consider water as incompressible.
Water downhill is not a pressure gradient exactly. A pressure gradient is what I would expect to find in a swimming pool, the deeper I go. Each additional foot deeper I descend, the greater the pressure becomes. This would be a true Pressure Gradient.
From the two statements above.. suppose I was to glue a bathroom scale to a wall and hand you a 10 foot steel rod. I ask you to hold one end of the rod and place the other end of the rod against the now vertically inclined scale and push on the rod. I asked you to apply 10 pounds of pressure at your end of the rod, which is horizontal to the ground and the pressure plate of the scale it perpendicular to the rod, so you can see the weight of the pressure you are applying. You apply 10 pounds of pressure to the rod and see the scale read 10 pounds. Now... where is the pressure gradient in this example? Do you believe you have to apply 20 pounds of pressure on your end of the rod.. to see the scale show 10 pounds?
Of course not. There is NO gradient along that Rod. This is the same as the water in the horizontal pipe.
The Gradient only exists at the physical interface between the pressure of the water in a pipe and the pressure outside the pipe (atmospheric pressure). If the exit aperture from the water pipe is a lot smaller than the diameter of the water pipe, then virtually all the Gradient is located in that very small area around the aperture. You won't see a pressure gradient along a horizontal pipe.
However.. I suppose one could make a model of series resistance if we inclined the pipe. Then I would have no choice but agree we would see a pressure gradient down the length of the water pipe, because now it is an inclined tank (per se). In this case extra pipe would add extra resistance in terms of pressure and the added pipe sections would be additive. So as a model, perhaps this works ok, sort of. (except for the back-wash effect.. but hey.. no model is perfect...lol)
Regards,
Dave :^)
Dave_Oblad » Thu May 01, 2014 11:32 pm wrote:Hi Canady,
Ok.. technically everything is compressible.. even Diamond. But I made my point and an expert agreed.. so that's good enough for me. Anyway, while we are on the subject, here is an interesting problem.
But first some back-story. After High school I went to Devry Institute to get a BS Degree in Electronics. I did real well 1st semester.. straight "A"s. I was TA in almost every class, helping slower students. College was keeping me from be drafted as my number was up. 2nd Semester was going ok until I butted heads with my analytical Math Professor.
He was teaching methods to solve complex problems but I kept looking for and finding short-cuts. I always got the right answer, but seldom used the right method. But almost none of the other classmates ever even got the right answer, despite their using the correct methods. His grading was:
A) Right Method and Right Answer
B) Right Method but Wrong Answer
C) Wrong Method but Right Answer
D) Wrong Method and Wrong Answer
E) No Answer
So I coasted along with a B- average grade. Then one day some professor offered this problem to the school. It was posted in every classroom, anybody could try to solve it. So I noted the problem, saw how to solve it and solved it.. in about 2 minutes. I wanted to be first to solve it, so I was working on it when the Professor noticed me doing it. He said: "Haw! You solve that and I'll give you an "A" for the course." So I doubled my efforts and did all the Math long-hand, no slide rule. Figured if I didn't solve it first, I may still win the contest by accuracy. At the end of class I raced to the Office and submitted my answer. Turns out I was first after all. Then was told we had 1 month to solve it... Crap!
So about 50 students and 10 Professors (including my nemesis) submitted Entries. Can you guess who won? Me of course. I got a moment in the spot-light and a small article in the local newspaper. My big mistake was flaunting my victory to my Math Professor. He said to me privately, that he would see to it I would never graduate from that school. He used every trick he could to reduce my grade. In the end, about half of the class failed, including me. I had a B- Average but required a solid B.
I complained to the Dean, but he said sorry, not his place to second guess the Professor. I confronted the Professor and asked about his promise.. he said tough, I refused to learn the correct methodology. I pointed out that his reasoning was irrational. Would he rather trust his life to someone that always used the correct Method but never got the right answer or me, who always got the right answer, regardless of Method. That just made him madder and I got expelled. Just as well, I would never be able to pass his course with him in charge, no matter how far I bent over. So without a Degree and no College Deferment, I got drafted and sent to Vietnam.. almost got killed.. but made it home safe. Pursued Electronics anyway, from the bottom via OJT. Wasn't easy, but Business cared more about Productivity than Honors, once you get past the hiring prejudices.
A few years later, when I had made Senior R&D Engineer, I had the opportunity to select new-hires. None of the students from Devry (from that time) could solve some rather Basic Math Problems. But they knew the Dot Pitch in an Epson LA30 Printer. Go Figure?
So, here is the Problem that caused me so much grief:
Catch ya later my friend....
Best Regards,
Dave :^)
Dave_Oblad » Fri May 02, 2014 3:42 am wrote:Oh Canady...
Forgot to mention.. not only was I first to hand in the answer.. mine was the only correct answer submitted.
Meaning I beat my Math Professor in his own field. That was rather key to his negative treatment of me. He was the first Academic Bully I had ever met. I've been bullied all my life. Physical in School, Academic in College, and Rank in Military and Business. I felt compelled to prove myself to everyone since that Professor.
So, perhaps he did me a favor.
But I changed when my wife passed away in 2003. It broke me spiritually. Now I am just trying to coast to the end. I am done fighting. Although this place has awakened some of that competitive spirit again.. so I am kinda liking it here.
Best wishes ol man,
Dave :^)
neuro » Fri May 02, 2014 5:05 pm wrote:hi Dave
I feel obliged to submit my guess, though I do not know what R.T. stays for:
imagining you are asking what is the overall resistance between the two points at the left of the figure, my result is 7/6 Ohm.
total current is V.6/7
from the left to the right, you have
V/2 A across the first 2 Ohm resistor
V/7 A across the second
V/14 A across the third (V/28 coming from each 1+1 Ohm path)
and V/7 A across the fourth
for a total of V.12/14 = V.6/7
more in detal:
V.5/28 A across each 1 Ohm and 3 Ohm directly connected
of which V/28 goes trough the more distant 1(3) Ohm
and, together with the mirroring path, carries
V/14 total current across the third 2-Ohm
and V/7 across the 2 Ohm
========================
anyway, I'd like you (or your friend expert) to answer a question, about the pipes above:
Is having a half a centimeter dameter aperture at the end of a 10 feet long 4" pipe the same as having a 10 feet long, half a centimeter diameter pipe?
If you quite convincedly respond YES, then I feel content, and imagine that viscosity and blood vessel elasticity account for additive seres resistance and parallel conductance in the circulatory system.
However, if you say NO, then something is wrong in your reasoning.
Neuro wrote:Is having a half a centimeter dameter aperture at the end of a 10 feet long 4" pipe the same as having a 10 feet long, half a centimeter diameter pipe?
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