Throughout human civilization, allegory, analogy, and metaphors have been essential tools for passing complex lessons from one generation to the next. With just a bit of creative license, an artistic comparison can liven the otherwise mundane task of rote memorization and instead offer what newcomers respond to positively: something familiar. And because of this, analogies can be just as captivating as learning tools as they are convenient.
On the other hand, isn’t it possible that analogies and metaphors share disadvantages? Especially, on counter-intuitive matters? A common example might include publications that depict an atom’s electrons like satellites orbiting a planet. It’s recognizable, sure. But if it doesn’t reflect reality, then why does it belong in a learning setting?
In this field (electrical engineering), one of the more persistent analogies is the hydraulic analogy of electric circuits. The hydraulic analogy likens common electrical phenomena like voltage, current, and resistance to pressure, flow rate, and restriction. The tradition predates the discovery of the electron and the positively-charged atomic nucleus. Even in the early 21st century, electrical schematics are traditionally drawn with conventional charge flowing from the top of a page toward the bottom, as if they were drawn down by gravity.
Of course, practitioners understand this is just an analogy and a visual design choice. But the limits of the hydraulic analogy become more obvious for students dabbling in circuit theory and electromagnetism; the study of why circuits misbehave.
For example, decades ago a student in an electronics community I frequented in the early 2000’s had questions regarding regenerative and super-regenerative radio receivers. Both topics seemed overly ambitious because they require knowledge of stability criteria; but fellow members more experienced than myself obliged to try and answer his questions anyway.
However, the discussion quickly broke down as the user insisted on working away at the problem from a “capacitors = charge reservoir” hydraulic-analogy mindset. In his view, a capacitor was a charge-storing element only, almost like an elastic bladder that could store pressurized water for later release. However, this model does not account for first-order capacitor’s properties such as the ability to cause phase shifts or block low-frequency signals (concepts that are critical to understanding regeneration). Ironically, these capacitors properties do in fact emerge from a capacitor’s ability to couple voltage to an energy-storing field between the capacitor’s own plates. But because the inexperienced user was only informed by the hydraulic analogy, both he and the more experienced users were essentially working in different languages. Needless to say, the conversation went nowhere slowly.
Sometimes, an awkward explanation can be worse than no explanation at all. Certain applications of capacitors such as charging one up before using it to ignite a xenon flash bulb obviously fit the hydraulic analogy quite well. Others don’t.
For example: in safety engineering, why is it the two isolated halves of a transformer are galvanically isolated, whereas the two isolated halves of a capacitor are not? The answers lie in the 4 Maxwell equations and the field-based nature of electromagnetism.
The limitations of the hydraulic analogy become even more apparent in the domain of ultra high frequency (UHF) circuit design, where wavelengths shrink down toward the size of the components themselves, and capacitive effects become more pronounced.
For example, many UHF and high-speed digital designs employ meanders. Snake-like conductive paths meant to delay a signals arrival. Intuitively, one might look at a meander and assume the time delay added is proportional to the length of the meander. And in the low-speed domain where the meander is essentially a lumped element, that’s usually true. In the high-speed world of consumer electronics, cutting-edge communications and video equipment; a meander’s time delay is a function of not only its length, but also its capacitive properties. In fact, some portion of the signal can “skip” over the meanders entirely—coupling through electric fields between the gaps.
I for one say there’s a considerable risk that the learner may inadvertently take the comparison at face value, and walk away with an oversimplified mental model of how electricity really behaves. If my hunch is correct, then at best: this represents “knowledge” that must be unlearned or un-taught at a later date…representing wasted time and effort. At worst: these misconceptions can balloon into common misconceptions that withstand the test of time.
In part 2, we’ll examine why field theory (not plumbing) is ultimately the language of modern electrical engineering. And why this approach is more indispensable than in previous electronic generations.
