All sparky on the western front
This article argues against the usual water analogy in teaching electronics. The author wants us to use some modern nonsense instead. The basic problem is that the author doesn't understand how the water analogy is used!

The waterfall analogy — where the height, flow rate and number of rocky obstacles in a waterfall equate to voltage, current and resistance — has no relevance beyond simple battery-based circuits.

Worse still, it — and a lot of our language around circuits — feeds into the idea that moving electrons themselves carry energy from one part of a circuit to another. It's a lie.

The truth is way more spectacular: the energy doesn't travel through the wires at all — it shoots through the space around them, at the speed of light. (Way to bury the lead, science!).

The thing is, there's a conceptual model that explains what's happening in a simple battery/bulb circuit and that works equally well for high-end AC circuits and everything in them. The same concept that actual physicists use (see below). So kids don't need to learn a DC 'waterfall' model of current that they'll have to abandon if they go on to learn about anything more advanced than a torch.

Stupid. Real electronics teachers don't use the water model exclusively; it's just an initial transfer from ordinary life to the world of fields. By the time we get to transistors and capacitors and inductors, we're using fields AND flow. Author doesn't seem to know this.

And the proposed new model has its own problems. If you're dealing solely with the fields around the conductors, how do you visualize the pinching that happens inside the conductor of a FET? I don't see the advantage.

Beyond that, the proposed 'physics model' assumes distinct positive and negative charges, which are not really there in practical circuits. The waterfall picture DOESN'T assume opposite types of charge; it works solely with a difference of potential energy between two ends of a resistor or two ends of a battery. Electrons flow 'down' from the more negatively charged end to the less negatively charged end.

[Sidenote: this two-field model closely resembles Georg Ohm's 1827 model, which was based on an analogy with heat and cold.]

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While pondering these two models of electricity, focusing tightly on pot diff, I suddenly looked 'backwards' and realized something new (to me!) about waterfalls. More broadly a new intuition about gravitational potential energy.

You don't need Niagara to observe a waterfall. Your kitchen sink will do just fine. You can FEEL and SEE F=MA with a faucet and a dish.



Even within this one-foot waterfall, you can SEE the acceleration and FEEL the force. Drops accelerate and break up from top to bottom, and you can feel more force at the bottom of the diff. If you want to rinse a dish faster, hold it lower. [Artistic note: I made the dish grow larger to symbolize the increased pressure. Poor symbol, but couldn't think of a better way to indicate it.]

Here's the new (to me!) intuition. Previously I had thought of pot energy in this context as a sort of subjective or arbitrary imposition. Because gravity is effectively the same at both heights, you're not really changing anything by moving the dish up and down. This felt completely unsatisfying.

Now I see. The mass of the earth is irrelevant. Only the potential difference matters. It's just the same as two ends of a resistor. The difference in voltage between the two ends of a resistor is set by everything in the circuit, the combination of all source pushes, reactances and resistances at this moment. It doesn't depend on some big globe-sized "universal voltage planet". No such thing. And it's the same with gravitational potential.

The giant mass of the earth is not the 'special' end of the pot diff. It's just a local lower end point, same as the positive (really less negative) pole of a battery.

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In modern electrical circuitry you can't see the pot diff, but it wasn't always that way. Lady Danbo is testing her code skills here on a Signal Corps SCR-74 transmitter, used in WW1. This was an extremely primitive gadget, basically just a Model T spark coil with a key attached. No oscillator, no tuning, no antenna impedance matching. You were putting out a signal on all frequencies at once, with antenna length somewhat restricting or preferring a broad band.



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Now let's stretch the analogy beyond its breaking point. In terms of economics as a verb, can we form an economic equivalent of pot diff? Not cleanly or objectively. You can't really map econ onto f=ma or v=ir. But the spark gap does offer a sloppy analogy. In econ, potential is blackmail force. Money that has an army of soldiers or an army of lawyers or one Sharpton or one Goldman behind it has vastly more ability to overcome the resistance of the air. Consider a medical transaction with a hospital on one side of the gap and me on the other side. As an uncool old white dude, I have less than zero force. Price comes at me rigidly, with open-circuit output impedance; no way to pull down the price. The hospital can charge me $10000000 or $10000000000000000 and I have to pay. But if I'm represented by an insurance company, my side has a high blackmail force. The insurance company can withdraw all of its patronage if the hospital doesn't charge a normal and reasonable price. Result: Spark bridges the gap. Hospital's output impedance is nearly shorted. Price pulls down to $100.

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Artistic note for Poser types: the spark-gap transmitter is now released in this set.

Labels: Danbo, Experiential education

  ¶ 2:31 AM