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Water Analogies For Teaching Electricity

Water Analogies For Teaching Electricity

Water Analogies For Teaching Electricity

(OP)
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?

RE: Water Analogies For Teaching Electricity

"Does Pressure = Volume/second multiplied by resistance of the constrictor (units unknown to me)?
"

Yes

Can I rant?

Why is it that electrical dudes seem to want to use analogies all the time? If the analogy is perfect, then the concept is already understood, if the analogy is faulty then you have implanted a misleading idea into some of your students.

The one that really bugs me is the people who labour long and hard to turn spring/mass/damper systems into their electrical analogs, and vice versa.

end of rant

The resistance of hydraulic systems is somewhat similar to the resistance of electrical systems, and your simple analogy does work. Except that I was taught about electrical resistance when I was 14, yet did not learn in any non-intuitive fashion about the resistance of pipes until first year at uni, as they are non-linear. I'm a bit surprised to hear that a hydraulic experiment is easier to set up than a low voltage breadboard.

Cheers

Greg Locock

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

RE: Water Analogies For Teaching Electricity

"The one that really bugs me is the people who labour long and hard to turn spring/mass/damper systems into their electrical analogs, and vice versa."

That is my favorite.  Let velocity play the role of voltage.  Force plays the role of current.  Mobility =Velocity/Force plays the role of Impedance = Voltage/Current.  A spring k has Impedance = 1/k (acts like a resistor).   A damper c has impedance j*w/c (acts like and inductor).  A mass m has impedance = 1/(j*w*m) (acts like a capacitor).... and one terminal of that capacitor is always connected to ground.

Boom you're done.  Any lumped damped mass-spring vibration system is turned into a lumped R/L/C system and can be solved very easily by an EE using circuit analsyis.  The advantage of transforming it into a circuit is to use the advanced tools developed by EE's for circuit analysis which are generally superior to the stone-age tools developed by mechanical engineers.

No, I was just joking about the last part. But it is easier for me as an electrical engineer to turn it into an electrical problem and solve it from there using tools which were drummed into my head for a large portion of 4 years until they became almost second-nature.

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RE: Water Analogies For Teaching Electricity

The only annoying thing is that the term "impedance" in mechanical terminology represents force / velocity which is exactly upside-down from what the quantity analogous to electrical impedance is.   It doesn't change the analogy but just throws a wrench in the communication.

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RE: Water Analogies For Teaching Electricity

dEARQ

actually pressure = volume/second times resistance is not correct.  For example, in hydraulics you may have static pressure in a system even though the flow rate is zero. Static pressure is related to the difference in elevation between point A and point B (sometimes this is the depth of water).  In addition, the frictional resistance is a function of the square of the velocity of flow - it is not a linear relationship.

RE: Water Analogies For Teaching Electricity

The only standard unit of fluid resistance in liquid flow is Cv, which is _unfortunately_ defined something like "the flow in GPM required to produce a pressure difference of 1 psig".  I say unfortunate because the definition as stated masks the fact that the units of Cv are GPM/sqrt(psid).  Which is to say that, yes, an analog of Ohm's law applies, but it's a square law law.

I personally think it's a mistake and a disservice to use a hydraulic analogy to teach electricity, because students come away with the impression that fluid flow is as easy to model as electricity flow, and it's not.  Maybe it made sense in the early 20th century, when a high school kid was a lot more likely to have done some plumbing than to have burned up a resistor.  Now, they're much more likely to have flushed the smoke out of an IC than to have figured out how a toilet flushes.



Mike Halloran
Pembroke Pines, FL, USA

RE: Water Analogies For Teaching Electricity

Greg- it was a bit before my time- but seems the point of turning spring/mass/damper systems into their electrical analogues was that you could then model the thing via analog computer and view results on an oscilloscope.  Of course, that kind of thing sort of went out of style about the time I was born.

Electric Pete- the velocity=voltage analogy is one I remember hearing in college.  It works out fine on the math end- but is very un-intuitive.  My brain says that pressure=voltage, flow rate = amperage, etc., and it's just simpler to understand that way.

dearQ- I suggest the approach that should be taken is to equate the electrical terms to the piping, but not in a proportional sense.  More pressure = more flow, and that's easy to understand.  But twice as much pressure, twice as much flow is not a given.  So you use the analogy to help them understand that more voltage = more current, then switch to the electrical side of it to discuss the actual mathematical relationship.

RE: Water Analogies For Teaching Electricity

There is a book whose first edition is published in 205 called "Vibration and Shock Handbook" edited by DeSilva(ISBN 0-8493-1580-8).

It includes impedance analysis of lumped mechanical damped mass/spring systems.  So while that approach was useful for analogue computers, it is not yet out of vogue.

I do think there are some pretty good intuitive strides to make in some cases by using the circuit analogy for mass spring systems.  Circuit analysis is a somewhat graphical and intuitive process.  Treating the mass/spring system as a circuit will make you to analyse the flow of forces (currents) through the system.  It is a different approach than just writing out the equations and much more intuitive for my money.

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RE: Water Analogies For Teaching Electricity

Ummm correction. 205 would have been old.  That book was published in 2005.

By the way, do you think that the Vib and Shock Handbook is hoping to capitalize on name recognition among dislexic engineers who are familiar with the Shock and Vib Handbook winky smile

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RE: Water Analogies For Teaching Electricity

The vibration m/k/c analogy to electrical r/l/c circuits works very well (imo) because both are assumed linear.  Although better suited to teaching an EE vibration than to teaching a mechanically-inclined person circuits (I wouldn't recommend that).

As pointed out above, there is nothing linear about relationship between flow and DP in fluid systems.

Qualitatively there may be something to be gained by comparing electrical circuits to a non-existent idealized fluid system defined by DP = R * Flow. DP plays the role of voltage, R plays the role of resistance, flow plays the role of current.   You can demonstrate KCL (sum of flows into a tee = sum of flows out of a Tee).  (Note we must have an incompressible fluid).  You can demonstrate KVL (sum of DP's around a loop is always 0 when proper polarity applied).  You can do power calculations.  You can do voltage dividiers, current dividers, Norton Equivalent....whatever you want because all the tools you need were established once you proclaimed that DP = R * Flow corresponds to V=R*I.

There might be some marginal benefit in teaching EE this way. As was mentioned elsewhere,  you would be doing a disservice to their understanding of fluid systems by creating a fluid flow model that doesn't exist in the real world.

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RE: Water Analogies For Teaching Electricity

I have explained electrical basics to non-electrical persons successfully with a very simple water analogy.  (Disclosure, I am NOT a professor nor qualified teacher of any sort, just been around for years).  However, I keep the analogy very simple and I  don't use the water analogy beyond what I list here.

I ask them to visualize a water tower filled with water, having a pipe that allows the water to drain out of the bottom of the tower.  Since almost everyone can understand a water bucket with a hole in it, most people can easily visualize this tower/pipe setup.  

I tell them that the pressure of the water in the tower forcing water to drain down through the pipe represents the electromotive force (voltage).  The amount of water that goes through the pipe in one second is the current flow (amperes).  The limitation on the amount of water that can move through the pipe in one second due to the size of the pipe is the resistance (ohms).

That is the how I use hydraulic principles to teach electrical basics.  It works for me, anyway.

Generally to go beyond this, as noted by wiser heads than mine in this forum, is more difficult to justify.

debodine

RE: Water Analogies For Teaching Electricity

Whoever said that there is an analogy between pressure differential and fluid flow vs voltage differential and current iss wrong. It is a wellknown fact that for liquids the pressure differential is proportional to the square of the flow and therefore ther is no fluid analogy possible there.
There are plenty of mechanical/electrical  analogies out there relate springs, masses, forces, velocities to capacitances,inductances resistance and current, but not fluids.

RE: Water Analogies For Teaching Electricity

I don't think anyone said that.  I said "Qualitatively there may be something to be gained by comparing electrical circuits to a non-existent idealized fluid system defined by DP = R * Flow.....As was mentioned elsewhere,  you would be doing a disservice to their understanding of fluid systems by creating a fluid flow model that doesn't exist in the real world."

My point was that this is the analogy required if you are teaching circuits and there is some intuitive appeal but it doesn't correctly reflect fluids.   I thought it was clear from my comments and the others in this thread.  But you are welcome to repeat it as many times as you like.

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RE: Water Analogies For Teaching Electricity

Zekeman has it on the ball. No analogy between voltage/current/resistance and pressure drop/flow/diaphragm coeefficent is possible since the first is a linear law and the second a square law. Also diapragm coefficient or its equivalent in the case of bends etc. varies with flow-rate.
The spind/mass/ damper analogy is much better and is a classic since they obey the same law. Analog computers were built to predict behavior of such mechanical systems, based on the analogy.

RE: Water Analogies For Teaching Electricity

Analogues are critical in some circumstances as anyone who has attempted to build their own speakers at home will testify. Loudspeaker specs are written as mechanical analogues with the electrical and acoustic parameters converted to equivalent masses, springs and dampers. It is purely by convention that a mechanical analogue was chosen.

I was told at university that you should use whatever domain you are happiest working in. At that time it was probably electrical. Now it would probably be mechanical (although I still think of springs in series as resistors in parallel). The choice of impedance or mobililty analogues is again pretty much up to the user except that some problems come out neater in one form or the other.

I think that getting used to different domains can only enhance an engineer's knowledge, particularly in acoustics where the concept of impedance is one with which so many students struggle. In my case an example would be transmission line theory vs. acoustic duct networks. I didn't understand either of them until I had studied both side by side.

M

--
Dr Michael F Platten

RE: Water Analogies For Teaching Electricity

I dunno; as an aerospace engineer, I found it easier to remember the resistance formula (Resistance is proportional to length, but inversely prop. to wire diameter) by thinking of electricity as a water. The analogy isn't perfect, but it certainly helps to intuitively recall, if you will, the basic principles and relationships.

RE: Water Analogies For Teaching Electricity

yates - Actually it was MikeH who said it. zeke repeated it.

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RE: Water Analogies For Teaching Electricity

I think in evaluating the usefulness of the analogy we also have to remember that the subject being taught is electricity, not fluids.

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RE: Water Analogies For Teaching Electricity

(OP)
Thank you everyone for your responses.  I am convinced that teaching electric circuits in high school by using such an analogy is not productive.  However, if I ever get the flow-rate meter, etc. in my lab I am going to try it out for myself anyway.  Finally, I encourage all of you to consider teaching at the high school level someday where there is currently a need for people who know science.

GregLocock: Why use apparently unnecessary analogies?  For creativity... H.S. students are presented with all kinds of creative ideas and hopefully they will come up with some of their own.  I will admit that it is risky for some of the smarter ones.  My 2004-2005 physics students set up these water circuits and allowed me to suggest the analogies.  I just told them that though not perfect, they are worth considering for practice thought experiments.

Also analogies are for transforming the new fundamental ideas into concrete learning foundations.  There are a lot of new formulas for the high school 11th and 12th graders and we try to stretch them as far as they will go for practice.  Even then at the end of the year some are still trying to memorize for the final...???  (High School students!)

electricpete:  I appreciate your advice and sometimes I must be reminded to quit while I'm ahead as tangents can lead us astray.

RE: Water Analogies For Teaching Electricity

I've always found the hydraulic analogy to be very useful, but I am also mechanically or visually minded. The problem in the discussion above reveals a problem in some peoples perception of the analogy which is wrong and would therefore make such an analogy wrong. Electrical resistance is equivalent to hydraulic flow resistance. However, a section of pipe is not the equivalent of a fixed resistor. At zero flow it has zero resistance. At any given flow it has a given resistance. Other than that I believe everything is consistent in the analogy; pressure=voltage, current=flow rate, resistance to electrical flow=resistance to water flow. Capacitance is equivalent to a rubber bladder (or spring loaded piston in a cylinder) and inductance is the inertia of the flowing water.
The purpose of an analogy is to think of ways to make it work and be useful and not to try and think of ways that it doesn't work. People do think differently, however, and if you do not have any better intuitive feel for plumbing than for electricity than the hyraulic analogy is useless to you.

RE: Water Analogies For Teaching Electricity

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.

winky smile

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RE: Water Analogies For Teaching Electricity

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

RE: Water Analogies For Teaching Electricity

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.

RE: Water Analogies For Teaching Electricity

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

RE: Water Analogies For Teaching Electricity

(OP)
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.

RE: Water Analogies For Teaching Electricity

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.    

RE: Water Analogies For Teaching Electricity

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

RE: Water Analogies For Teaching Electricity

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

RE: Water Analogies For Teaching Electricity

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.

RE: Water Analogies For Teaching Electricity

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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RE: Water Analogies For Teaching Electricity

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.

RE: Water Analogies For Teaching Electricity

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

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