[HN Gopher] Against the Hydraulic Analogy
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Against the Hydraulic Analogy
Author : surprisetalk
Score : 43 points
Date : 2024-12-27 21:16 UTC (3 days ago)
(HTM) web link (lcamtuf.substack.com)
(TXT) w3m dump (lcamtuf.substack.com)
| bun_terminator wrote:
| say no to manipulated scrollbars
| wisty wrote:
| Maybe some people just aren't smart enough to understand quantum
| tunnelling and band gaps, and there's no real better analogy. And
| even if you do kind of get these, the bad analogies let you
| remember the standard behaviour so you can reason about what the
| micro details are.
| weinzierl wrote:
| The problem is in my opinion not the hydraulic analogy itself,
| but the fact that it is usually presented as a model.
|
| It has its place, but it should come with a big disclaimer at
| the front that it is not a _model_ where we assume it is
| incomplete and which does not explain everything but an
| _analogy_ that is fundamentally wrong in everything and only
| meant to help us understand the one particular aspect of the
| real thing.
| casey2 wrote:
| But it IS a model. It says that the model you use for water
| is the same as the one for electricity.
|
| I actually like it because it has no pretense that you are
| actually doing/learning science. Compare that to the
| simplified models in high school physics where you have the
| government literally lying to children about how physical
| phenomena work (e.g. Newton's cradle) and nobody cares
| despite the teacher's insistence that you "need to know" this
| misinfo.
|
| If you want to complain about that model for water being
| wrong though I'm all ears. But the model for water also being
| the model for electricity is just obviously wrong, cause they
| aren't the same thing, if both were correct then there would
| only be a single word.
| fragmede wrote:
| "all models are wrong, some are useful."
|
| -George Box
| stared wrote:
| Well, quite a few things can be described using the same
| mathematics - including switches, resistors, capacitors, and
| inductive coils. Sure, it is not everything, but well - fluid is
| fluid, electricity is electricity.
|
| For a small simulation, see http://sjbyrnes.com/1235/.
| iterance wrote:
| This seems unnecessarily negative. Teaching analogies don't need
| to perfectly resemble the system being taught. From a pedagogical
| perspective, the aim is typically to relate something unfamiliar
| to something familiar, like water flowing in a pipe, in order to
| help someone gain footing in a topic.
|
| Further connections can be developed mathematically; electric
| force propagation does obey wave equations, after all. But by
| then the analogy has served its purpose and should be droppped in
| favor of more rigorous knowledge development.
| FuriouslyAdrift wrote:
| When I was in EE at Purdue, most of my profs also railed
| against the hydraulic model. It really messes you up when it
| comes time to get into higher frequency digital circuits (CPU
| designs were just starting to brush up against quantum problems
| at the time).
| sanderjd wrote:
| Why would anyone be attempting to use this analogy to learn
| about intermediate to advanced topics like that? That's not
| what learning aid analogies are for... They are for beginners
| to gain some initial intuition in order to get over the
| immediate hump of unfamiliarity with a new subject.
| dmurray wrote:
| They wouldn't, but people would _create_ these increasingly
| tortured analogies out of the same motivations that other
| creative people would create, say, a raytracer in Brainfuck
| or a pregnancy test that can run DOOM.
| cruffle_duffle wrote:
| By the time you get into higher frequency circuits I would
| think you would already understand exactly where the
| hydraulic analogy breaks down. Most grade school students
| aren't doing any kind of extremely high frequency work, let
| alone even basic 60hz work.
| sevensor wrote:
| When I was taking undergrad EE classes, I don't recall it
| ever coming up. I think people's comfort with and intuition
| for electricity are at least as good as for water, which
| actually behaves in deeply unintuitive ways. We've been
| living with switches and toaster ovens for generations now,
| people understand resistance, voltage, and current just fine
| on their own terms. Or at least, water analogies don't help
| because people don't understand water any better.
| FuriouslyAdrift wrote:
| Once you start dealing with EM fields and even fairly
| common filter designs in the AC domain, the hydraulic
| analogy starts breaking down and becomes an impedement to
| learning... so my prof would say.
|
| I was never taught the water analogy but I saw quite a few
| students struggle trying to wrap their heads around a
| physics (EM, RF, potentials, etc) based circuit design and
| theory.
| calmbonsai wrote:
| As a fellow EE, I was amused that the plumbing model never
| came up in my engineering classes, but was introduced (and
| re-introduced) at several levels in my physics classes.
| JKCalhoun wrote:
| Agree. The hydraulic analogy is a good introduction to the
| duality that is current and voltage -- as well as, to varying
| degree, capacitance and resistance. But then let's just stop
| there.
|
| No problem.
| calmbonsai wrote:
| I completely agree. Every model is "wrong" to some degree, it's
| just a matter of applying the "least wrong" one in the proper
| context to yield the most practicable solution.
|
| Even basic circuit theory (Kirchhoff's laws) falls apart when
| the preponderance of propagating wavelengths become close to
| the characteristic length of the propagating medium.
|
| Then that "wrong" simplifying model must transition to the much
| more complex transmission line theory.
|
| By the same analogy, one doesn't use basic Bernoulli
| pressure/velocity/flow equations for plumbing when dealing with
| chaordic or turbulent flows outside of very limited Reynolds
| number regimes--one has to do numeric simulations of the more
| general Naiver-Stokes vector differential equations.
| NotYourLawyer wrote:
| Analogies aren't perfect, but they're still useful. How else are
| you gonna explain to a grade schooler the difference between
| voltage and current? This article suggests no alternative.
| fn-mote wrote:
| This.
|
| The premise of the article is that the hydraulic analogy breaks
| down for semiconductors.
|
| My reaction: no kidding, a classical explanation doesn't cover
| fundamentally quantum devices.
|
| The whole premise that your model should only need small tweaks
| to evolve seems wrong. The author has probably forgotten their
| first encounter with quantum mechanics. It's not a small tweak.
| It's a bewildering change.
|
| Is it particularly wrong to be using hydraulics to explain
| MOSFETs? I'm not qualified to have an opinion on this; maybe
| someone is holding on to the analogy too long. For classical
| effects it works well.
| cruffle_duffle wrote:
| Obviously you bust out your university physics book and work
| through the equations with them! There is nothing my 9 year old
| loves more than doing derivatives!
|
| We didn't even teach her the equations for velocity or
| distance, we derived them starting from Jerk and integrated our
| way up the food chain. And none of that linear single dimension
| nonsense either, kiddo learned in both 3D and polar coordinates
| from day one; with friction too because like spherical cows,
| friction-free is a lie.
|
| Teaching grade schoolers single-dimension, frictionless physics
| really messes them up once they get into grad school and start
| doing real math.
| aziis98 wrote:
| Related video by Steve Mould about "Spintronics" [1]
| Mechanical circuits: electronics without electricity
|
| https://youtu.be/QrkiJZKJfpY
|
| [1]: https://upperstory.com/spintronics/
| zokier wrote:
| My problem with this "hydraulic" analogy (first time I've seen it
| called that) is that I feel most people don't have that good
| intuition on hydraulics either, so you are just trading one
| poorly understood subject with another almost as poorly
| understood one.
|
| Maybe 50 years ago engineering students learning electronics
| could have had more solid hydraulics background knowledge, but I
| bet these days more people start from electronics before doing
| hydraulics.
| vertnerd wrote:
| You are correct. The best way I've found to build electrical
| intuition is to build circuits and experiment with them. It
| takes time.
| moolcool wrote:
| Counterpoint: Youtuber AlphaPhoenix does a very good job
| explaining electricity using the hydraulic analogy
| https://www.youtube.com/watch?v=X_crwFuPht4
| lordnacho wrote:
| All models are wrong, but some are useful.
|
| I thought you just used hydro on simple RLC circuits, and kept it
| at that?
|
| For semiconductors there's no reason to invent a hydro equivalent
| that doesn't exist, what's the point? You want a model to be
| intuitive to the student, and a made-up thing won't be.
|
| As you go further with EE, things also tend to turn into a sort
| of logic gate puzzle rather than a continuous (voltage-current)
| problem. That's a while other analogy whose limits you also need
| to know.
| snakeyjake wrote:
| >Wikipedia doesn't stop there; here's their take on the metal-
| oxide-semiconductor field-effect transistor (MOSFET):
|
| This stood out to me because I have contributed quite a bit to
| the Wikipedia entry for MOSFET.
|
| This illustration does not appear on the Wikipedia entry for a
| MOSFET: https://en.wikipedia.org/wiki/MOSFET
|
| It only appears on the Wikipedia entry for "hydraulic analogy".
| People do not learn electronics from the Wikipedia entry for
| "hydraulic analogy".
|
| I assert that it is highly likely that not a single human being
| on earth has ever used the Wikipedia entry for "hydraulic
| analogy" to learn electronics. It is likely that several people
| have used that entry to learn about the hydraulic analogy.
|
| Then again, I learned electronics from 1988's The Way Things
| Work, which uses Wooly Mammoths, large stone edifices, and
| titanically large humans so maybe I was trained on analogy from
| an early age and learned to not become too attached to them when
| they start to break down (like how Wooly Mammoths don't exist
| anymore).
|
| edit: oh goodness I just looked it up and the hydraulic analogy
| for a circuit in The Way Things Work is utterly magnificent,
| fantabulous, funny, and educational. No mammoths, unfortunately.
| ipdashc wrote:
| > 1988's The Way Things Work
|
| I'm so glad to see this book mentioned on here! It was my
| favorite as a kid and it still holds up today IMO. I have one
| of the newer editions at home and this comment reminded me to
| go page through it a bit. Still brings a smile to my face.
| fifilura wrote:
| https://en.wikipedia.org/wiki/The_Way_Things_Work
| MengerSponge wrote:
| Exactly! It's a wonderful Lie to Children.
|
| https://en.wikipedia.org/wiki/Lie-to-children
|
| Just because the way you understand a profound truth is not the
| same way it was first introduced to you does not mean that the
| first introduction was flawed or wrong. It, more likely than
| not, means you're less of a child now than you were then.
| jaredhallen wrote:
| I found a copy of that book in excellent condition at a thrift
| store a year or so ago. Paid $1. One of the best bargains I've
| scored in a long time.
| ajnin wrote:
| I learned electronics as a kid from a kit that used a water
| analogy to describe things. The transistor was described as a
| vane across a large water canal, connected with a hinge to a
| smaller vane in a smaller water canal, which allowed it to move
| up and down. So, a small water current in the small canal could
| cause a large water current in th large canal. Did it fully and
| accurately describe the gain characteristics of a real transistor
| ? No, it didnt. But it would be ridiculous to dismiss it entirely
| as a learning tool because of it. Overall I think the hydraulic
| analogy is a very good one, it holds up well and it breaks down
| only in the face of quantum phenomenon. It allows to understand
| electricity using concepts children are familiar with. All
| children like to play with water and have some intuitive sense of
| it. By the time you care about quantum electronics you can also
| understand the limitations of the analogy.
| gmurphy wrote:
| The water flow analogies always messed me up because young
| literal me couldn't handle reconciling the abstraction with
| "actually current flows negative to positive"
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