Against the hydraulic analogy

The most popular tool for teaching electronics makes the trade harder to learn.

If you ever picked up an introductory book about electronics, you were probably exposed to a teaching aid known as the hydraulic analogy. It is a reimagining of electronic circuits as a series of tubes; in this model, electricity is akin to water, batteries are pumps, and switches behave like valves.

Pssst, kid… want to learn circuit design?

I learned the basics the same way, but I’ve come to profoundly dislike this approach. I think it held me back for years. To be clear, analogies and simplifications are useful — but they need to hold water, requiring only incremental tweaks as you progress. And from this perspective, the hydraulic analogy springs far too many leaks. Further, because the analogy is so seductive, many authors and students don’t know when to let go.

The hydraulic model is iffy by the time you get to inductors and capacitors, but the issues become hard to ignore by the time you’re discussing semiconductors. The components are central to all modern electronics, so there’s not much you can do until you grasp their operation. Yet, there’s no plausible plumber analogy for how they behave. One otherwise solid book tries to explain a junction field-effect transistor (JFET) the following way:

An illustration from “Practical Electronics for Inventors” by Paul Scherz.

An accordion-shaped valve! But hold on: in a typical JFET, aren’t the source and drain terminals supposed to be interchangeable? And don’t these transistors allow some gate current to flow when the voltage becomes positive?… I guess we could add some check valves here and there.

Next up, MOSFETs. Look around on Wikipedia and you’ll come across the following design:

The hydraulic MOSFET. Cc-sa, by Wikipedia user Vanessaezekowitz.

It’s beautiful — we even have a check valve to simulate the body diode. But why is the flow of “water” through the transistor (drain current) relatively insensitive to pressure (drain-source voltage)? Who knows, kiddo. Good luck making sense of amplifiers.

If it sounds like nitpicking, it gets worse; presented without comment is a well-publicized reinvention of a bipolar junction transistor (BJT) as a pull-chain toilet with a bucket instead of a handle:

The flush transistor. Artwork by Giovanni Organtini.

The problem with these models isn’t that they aren’t intuitive: it’s that the analogy is horrifically incorrect and can’t be fixed by tacking improbable plumbing fixtures on top. Worse, when the illusion falls apart, you can’t just backtrack a bit or add a footnote here and there; the only path forward is to flush it all down and start anew.

In memoriam, by author.

👉 For other articles on electronics, click here. For a collection of pipe-free articles on electronics, click here. You might enjoy an explanation of currents, voltages, and impedances; a discussion of transistors; a primer on signal amplification; or a deep dive into op-amp dynamics.