Water Analogies For Teaching Electricity 1

[dEARQ]

How far can I go in using water as an analogy for teaching Electrical circuits in High School?
V = IR. Does Pressure = Volume/second multiplied by resistance of the constrictor (units unknown to me)?
While teaching Physics I was able to compare series and parallel circuits with actual plastic tubing, etc., but I did not have access to a flow rate meter, etc.
Can the analogy to electricity be accommodated by similar mathematical formulas - direct formulas that are easy for high school students?

 

[electricpete]

That's an interesting twist - the pipe is a variable resistor.

dearq - I'm not sure what you meant by tangent. But it reminds me of something. Did you ever wonder what is the relationship between tangent of an angle and tangent of a curve? I never could see the connection.

;-)

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[MikeHalloran]

Don't wait for a flowmeter. Use a bucket and a stopwatch.

Much more fun/mess, and more educational.




Mike Halloran
Pembroke Pines, FL, USA

 

[Warpspeed]

I really don't see any difficulty of hooking up a simple demonstration electrical circuit, just as you would your demonstration water circuit.

This is an ampmeter, the current goes in this end and comes out the other. The reading shows current FLOW in amps.

This is a voltmeter, it measures the potential difference between this wire, and that wire. The reading shows POTENTIAL DIFFERENCE in volts.

Here is a resistor, and so on....

You lay the whole thing out in such a way that students grasp the concept of a real electrical circuit with current flow and voltage differences.

Even very young kids grow up surrounded by electrical gadgets, I am sure the concepts would not be too abstract or alien for even fairly young students to follow these days.

 

[Cockroach]

I really disagree with GregLocock. Infact, modelling similtanity is exactly that, looking for correspondence between a problem and another well documented phenonema in life.

Water and electricity only mix mathematically. I use this deductive reasoning a lot for pipe applications, and am often amazed at how well the correspondence is. Check out the textbook, Classical Dynamics of Particle and Systems, 2nd Edition, Marion, Academic Press 1970, [Library of Congress Card No. 78-107545]. Absolute peoetry.

I was drawn to the analogy between traffic, roadways in civil engineering, and electrical circuts. I guess piping would be another way of modelling commuter flow with ordinary differential equations.

Sorry Greg, really have to disagree with you on this one.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[dEARQ]

MikeHalloran: Thank you. We do the red hose will empty a bucket in x seconds, the black in y seconds and calculate both working together experiment. That is a cognitive organizer. I also teach math so maybe I can work out that "square law" law.
electricpete: I got the highest calculus(I) grade in my college class, but I tried EE and flunked out. And that was many, many years ago. I am an old codger now and while getting here I have continued to attempt a few more EE type courses to no avail.
If memory serves me correctly, the tangent of a curve is the "rise" of the slope of the curve at an instance divided by the "run" of the slope. The connection is that the line defining the slope of the curve at an instance forms an angle with the x-axis whose tangent is the same value of "rise over run."

Let me ask all you guys this if you don't mind.
Spz a pump has one or two constrictors in series (like two resistors in series with a battery). My question is, is the water under pressure as it returns from the last constrictor to the battery, or is it just going back to the pump under its own momentum? I mean, I guess the pump is kind of sucking it back, but in the frame of reference of the system so to speak, it seems in my mind to be returning under its own momentum. I would guess there is no pressure against the walls of the pipe as it returns. And is this similar to the way electric circuits work?
Ok?
Ok! Fine, then let me try even another question.
With psi meters I guess I could at least check that out and compare constrictors in series with constrictors in parallel for total or equivalent resistance. Do you agree?

Maybe some of you have already at least alluded to the answers to these quesitons, but I was thinking someone might have some precise experimental data for at least some hypothetical support.
Thanks again guys.

 

[chicopee]

dEARQ- Go on line and check out the subject on McIlroy Network Analyzer. It is an electrical analog computer which I became acquainted in the early '70's when I took a course on fluids. This analog computer consists of light bulbs, rheostats, pots and wiring from which one can simulate the effects on water distribution systems of municipalities when let's say the fire department is to need water for fire fighting. It is an excellent device showing the relationships between water flow, pressure drop, piping resistance and the output being the liminous intensities of the light bulbs. As suggested, there is an analogy.

 

[MikeHalloran]

As has been pointed out, Kirchoff's Laws do apply to liquid circuits, so you can sum the flows at a node and the pressure differences around a loop.

Liquid resistances in parallel and series can be computed, but keeping everything straight will drive you crazy without a spreadsheet. I was actually doing it as part of my work on boats, and found it convenient to work with Cv, units of gpm/sqrt(psi), and its metric inverse, 'X', units of mbar/(m^3/hr^2), because one adds arithmetically for series resistances and one adds arithmetically for parallel resistances. I forget which right now. I just copied a spreadsheet block to make the conversions in both directions; because of the square law, the order of operations is important.

Pressure differences due to elevation differences have no parallel in electrical circuits because electrons have so little mass. If you're actually going to do the math for a liquid circuit and correlate it to pressure measurements, keep the entire circuit at the same elevation.

Electrical circuits also have no real equivalent to a vacuum, e.g. trying to lift water more than 32 feet.

Trying to analyze a closed loop liquid circuit without a reservoir open to the atmosphere will drive you crazy. I have actually built such circuits for product development, and they drove me crazy. A boat circuit is much easier to deal with, because all flows begin and end at the sea.





Mike Halloran
Pembroke Pines, FL, USA

 

[Cockroach]

I would treat pressure differences due to elevation like capacitance in an electrical circut.

Essentially you are adding "potential" to the circut, are you not? I say capacitance because of the ability to discharge, much like elevation adding energy to the system but draining as fluid is lost to lower points in the loop.

Just a thought.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[GregLocock]

Ow. I thought that pressure head would be voltage.

I like the bit about Kirchoff's laws for current, that is a /good/ analogy.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[electricpete]

The capacitor analogy would be a tall tank of water.

lower case letters for mechanical and upper case letters for electrical.

Pressure expressed as height of a column of water h represents the voltage V.

Area a of the tank represents capacitance C

volume v = a * h represents charge Q=C*V

flow rate q represents current I

q = d/dt(v)= a * d/dt(h)
corresponds to
I = d/dt(Q)= C * d/dt(V)


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[GregLocock]

OK, got it, you are storing volume at an increasing potential.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[tlee123]

Expanding on Mike Halloran's idea of measuring flow with a bucket and a stopwatch:

Why not use hypodermic tubing to regulate the flow? Flow thru this tubing should be laminar (make sure Reynold's number <1000) so the flow will be linearly proportional with pressure (no squared term as with turbulent flow). You could have the flow out of the tubing go into a graduated beaker for easy measuring.

Also, the resistance to flow is linear with changes in length. You can cut the tubing in half to increase the flow rate by a factor of two, for example. The hard part would be removing the ID burr on the tube.

Also, because your flow rates will be low, the hydrostatic pressure due to the height of the water column won't change much.

I haven't run any numbers but it seems do-able and it would act more like a true electrical circuit.

The tubing is readily available through Smallparts.

Tom