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?

 

[GregLocock]

"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.

 

[electricpete]

"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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[electricpete]

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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[cvg]

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.

 

[MikeHalloran]

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

 

[JStephen]

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.

 

[electricpete]

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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[electricpete]

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 ;-)

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[electricpete]

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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[debodine]

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

 

[zekeman]

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.

 

[electricpete]

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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[yates]

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.

 

[MikeyP]

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

 

[prost]

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.

 

[electricpete]

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

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[electricpete]

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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[dEARQ]

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.

 

[Compositepro]

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.