water analogy- a simple calculator

A few years ago, a guy from CCD design has tried for four hours to convince me and a few colleagues of mine that some kind of water analogy was the best thing around to teach the behavior of the MOS transistor. The problem is that I am much more used to electronics than water piping, so basically I needed to understand the water equivalent by coming back to the electronic circuit :-) I am not able to say if a beginner can find those kind of analogies useful (or just funny), but usually I do not see things that way.

George Herold:

Wait! Who sys that water is similar to electrons?

I was talking about electric current, that flows from a high reservoir potential to *ground* potential, throung an acqueduct and hose that have a fixed resistance and a variable thumb

Electrons, by the way, go the other direction. :D

One of the problems I had with the water analogy that was given to me is that the guy I described in my previous message represented the flow of electrons with the flow of water. Maybe it was a nice thing for him, but it was utterly unpleasant for me since all signs were reversed when taking into account the currents...

So my understanding of FET's is that they are just conduction channels, that get cut off as voltage (pressure) is applied. There can be all sorts of pinch-off effects and complications as you try and close the gate too fast. But the water analogy looks pretty good for FETs.

I've never measured the pressure drop along a tube of known size, flow velocity and...(and the fluid equivalent of resistivity)

George H.

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Oh, I thought that was the analogy. The flow of electrons looks like the flow of water. At some low enough velocity...I think it must work?

George H.

Yeah, that's a problem. Once you learn about semiconductors and holes, you can just remeber that in our system the holes flow in the positve direction. (that was a joke) No analogy is perfect. It's better to think about the real electrons, once the analogy is understood.

George H.

A transformer: High flow at low pressure is transformed into low flow at high pressure.

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You can't fix stupid. You can't even put a band-aid on it, because it's
Teflon coated.

It's like a valve that is controlled by a low flow of water. They are used in irrigation work.

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You can't fix stupid. You can't even put a band-aid on it, because it's
Teflon coated.

A true anology uses the same equations, with the same solutions. A mass-spring-dashpot mechanical system is mathematically congruent to an R-L-C circuit. A garden hose and a finger aren't congruent to any circuit I know of.

The water flow analogy only works at low Reynolds' numbers. Liquids are horribly nonlinear.

It's easier to just directly explain the physics, and the math, of electricity. There's very little electronics you can do without the math.

John

Michael A. Terrell:

Humm. Maybe John is right.

John Larkin:

That's the best one for explaining AC circuits. But what about DC circuits?

Gosh, I'm sorry that my explanation was not clear enough.

Sure, the first thing one thinks about when talking about pipes and water is turbulence, in particular when they're used as an example.

Yes, I've seen the results in my interviews. A couple of months after finishing school they're back to blank.

This platitude should validate the previous mirror climb?

Agreed in principle, but there can be serious debate over the proper analogy even in this basic RLC case. Consider that with a series connection, current is analogous to force in that it is identical in all elements, while the voltage drop across each element is analogous to relative velocity. That doesn't fit well with the voltage-as-pressure concept.

Mechanical engineers use different analogies as needed, and can make the math work out even when there isn't much intuitive connection (to this poor EE, at least!).

Best regards,

Bob Masta DAQARTA v6.00 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, FREE Signal Generator Pitch Track, Pitch-to-MIDI Science with your sound card!

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Really?
Got a reference?

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View in Courier: 
                                 WIRE  NEEDLE POINT
     +-----------------------+  /     /
  +--|+ HIGH VOLTAGE SUPPLY -|=====---
  |  +-----------------------+ 
 GND

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But congruence isn't necessary for an analogy to be valid.

For example, a 3-4-5 right triangle is like a 9-16-25 right triangle,
and they're analogous to each other, but they'r not congruent

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on.

That's a nice analogy too. It's nice to be able to go back and forth 'cause sometimes your brain is better as seeing things with a different perspective. And by looking at where the analogies fail, you can learn more about both systems.

Oh if the mechanical system has a damping term that doesn't go as the velocity then you can get different behaviour. (Just like the water analogy.)

Scroll down here till you get to damping, down near the bottom.

You can see friction damping, damping ~ velocity in the middle pic and v^2 damping... only the middle one matches an RCL circuit.

The frictional damping is kinda interesting. The thing just stops at some point and that point is *not* (in general) the equilibrium position.

George H.

Why do MEs need analogies? They can see and feel their stuff!

John

Right. I can't think of an electrical analogy to friction.

The old SU vacuum-piston carburetor, and the diaphragm carbs on my old Honda motorcycles, provably had too much static friction to work. But add a lot of engine vibration, and shazam, zero friction.

A weak analogy is ADC dithering.

John

HA!

A transformer could be a water-powered motor driving a pump. For non-steady-state, a driven piston pushing another piston of a different size.

It's easier to just explain the electricity. That way, you don't have to explain two systems, when you're trying to teach one.

One interesting analogy is a synchronous buck switcher to a pair of gears. That sort of works. You can apply conservation of energy in both ideal cases and predict behavior without having to go into gory detail.

The holy grail of automotive design is the continuously-variable transmission. The electronic equivalent is easy. Eat your hearts out, MEs!

John