Ya out there Red?

Ain't the world strange? I get to teach mathematics and computer science at an Ivy League school. And yet we disagree. Guess that's what makes a horse race.

Sure. Since you believe, why not write down the equations so that we can see the analogy between water flow and electricity? Equations are comparatively precise, and we won't have to weasel too much what the words mean, I'll bet.

[stuff snipped]

Ray, I think we're not even looking at the same *book*.

Back to the water tower. I've got a large tank, atop a 10 meter tower. And a BIG pipe from the tank to the ground level at which I'll be working. There's 1,000 kg of water in the tank. The potential energy stored in the tank is one million g (where g ~ 9.8 m/s^2), which (rounding off a bit) gets me 98,000 joules.

At the bottom of the BIG pipe, the water pressure will be rho g h, where rho = 1000 kg/m^3 is the density of water g ~ 10 m/s^2 is gravitational acceleration h = 10m is height.

That comes out to a pressure P1 = 1000*10*10 Pascals, or 100 kiloPascals.

If we plumb a 1 meter piece of (idealized) PVC pipe into the bottom of the BIG pipe, and leave the other end open to the air, the pressure on the open end will be pretty nearly zero (air pressure really is negligible at this scale). So the pressure at the other end of the pipe will be P2 = 0 Pascals.

What rate will water flow through the PVC pipe? Well, it all depends on the diameter. The flow rate (I admit it...the flow rate in an idealized model of a pipe!) is proportional to the pressure difference between the ends; the constant of proportionality is the "resistance to flow" of the pipe. So the equation looks like this:

flowrate = flowResistance * (P1 - P2)

OK. Hold that thought.

For an electrical circuit, consisting of a single resistor, with voltages V1 and V2 at its ends, we have

current = resistance * (V1 - V2)

Do we still agree? Great.

Those two equations are REALLY, REALLY, analogous. And the analogies are

(1) electron flow (current) water flow; (2) electrical resistance resistance to flow in the pipe (3) voltage pressure

Two things you'll note here:

  • current is NOT analogous to pressure, but to flow.

  • voltage is analogous to pressure.

Because the pipe happens to have constant resistance, that means that voltage, which is analogous to pressure, is also a constant multiple of the current. But that particular coincidence is a consequence of the simple situation, which the more general analogy (voltage analogous to water pressure) works even in more interesting cases, where the resistance varies.

Just to hammer the point home: go back and double the tank's size on the water tower. Put 2000kg into that tank that's 10m high. Observe that the equations for pressure and flowrate, and the associated computation, don't change at all. Pause to look at what you said earlier:

"The total water regardless of height is the voltage analogy."

Now explain to me where "the total water" has anything to do with the flow-and-current analogy. I just doubled the total amount of water, and nothing in the relevant equations changed.

Ray says: "The pressure represents the CONSTANT current, not relating to the total current which is analagous to the flow itself."

If I read this last statement correctly, your analogy would be

Mass (or volume) of water in the tank Voltage Pressure at bottom of feed pipe "the constant current" Flow "the total current"

Let's look at that circuit again.

V1 ---/\/\/\/----- V0

with resistance R ohms. Let's call the current through the resistor "i". I *think* that you're saying "i" corresponds to pressure in the feed pipe. which I have called P1.

As for "the total current", I'm not really certain what you mean. You might mean the total charge transport over some period (integral i(t) dt, taken over a fixed interval [a,b], which, because i is constant, just turns out to be i*(b-a) ); that'd be measured in amp-hours rather than amps, though, so it wouldn't be a current. I can't believe an EE would mean that. But I can't figure out what else it COULD mean.

Now...what equation involving voltage, current, and resistance, corresponds to WHAT equation involving "total water", "pressure, and flow"?

I guess I'm stumped. I've got an analogy that's good down to the level of equations. It happens to agree with the analogies presented by lots of other people (Google "voltage corresponds to pressure"), whereas yours seems to agree with no one else's (Googling "voltage corresponds to total water" leads to a bunch of pointers that say...voltage is analogous to pressure!). But hey, if it works for you, you should go with it.

As for others ... I'll just say that I find Ray's explanation (a) unusual, (b) not as helpful as other explanations in letting me predict circuit behavior. Even though he's an EE and a teacher, I'm sure he'll agree that there are many fine textbooks whose explanations you can rely on, even if they differ with his explanations in some particulars. I commend them to you as you seek to determine which explanation to believe.

--John

Reply to
John F. Hughes
Loading thread data ...

"John F. Hughes" wrote

This sounds like the old bit where a guy is sitting with one foot in a bucket of ice water and the other foot in a bucket of water at 150 degrees. On average, his feet are comfortable!

Reply to
Morgans

On Thu, 1 Feb 2007 13:52:50 +0000 (UTC), I said, "Pick a card, any card" and "John F. Hughes" instead replied:

Now we're both wasting time. You asked how a constant in electronics could be different than a constant in physics and I explained the differences and why. I used analogy to avoid confusing the casual reader. I really didn't believe you'd need any lessons beyond the analogy. As of now, you may continue this debate with only yourself. The answer you sought was given. Now you're just finding points to contend. Have fun. You win. I give up.

-- Ray

Reply to
Ray Haddad

I built a test bed to play with the pulsed taper after the bulk charge. I will let you know how it works out if you want. To know for sure I will have to calculate the area under the curve for a CV cycle compared to the area of the pulses for a CC pulsed system, BUT I'll bet they come close. With the pulsed system you get a make an easier power control system. You just stick with a CC power input and then you use pulse width to taper. In fact, Maxim offers 2 basic IC's to do that very same thing, but you have to supply the constant current wall wart, and I don't recall seeing too many of those.

Jim

Reply to
James Beck

I don't know what the kind of NDAs we have, so I'll stick with generics, but if you have bought an aftermarket BEC, glow driver, or servo mixer/reverser, it might be one of mine. I doubt you have one of my battery chargers unless you are in the gaming industry, automotive, or work with "real" UAVs. We are currently doing a VERY small power conversion project for one of the automakers. We do so much gaming stuff, there is no way you can go into an arcade or casino and not be surrounded by my stuff. There are so many players in the hobby market for things like chargers and speed controls, it is just not worth the money to get involved unless you really come up with something new and unusual or really have a big end buyer. I can say that a couple of the guys in my club have and use some of the prototype chargers that I gave them.

Jim

Reply to
James Beck

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.