From d8f73419515489cd4c42753fdca6e807cf35806d Mon Sep 17 00:00:00 2001 From: Andrew Kesterson Date: Thu, 4 Jun 2026 18:43:28 -0400 Subject: [PATCH] WIP --- 05-photoresistors/README.md | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/05-photoresistors/README.md b/05-photoresistors/README.md index ea1ed26..d3527cf 100644 --- a/05-photoresistors/README.md +++ b/05-photoresistors/README.md @@ -20,9 +20,9 @@ The answer is because, as it turns out, if you take the measurement downstream o For an actual electrical engineer, or someone with a better grasp of electronics fundamentals, this may be a real "DUH-HUH" moment. So forgive me as I shake the blinders from my eyes. -The water analogy from many simpler electronics tutorials lead me to use words like "upstream" and "downstream", but those terms are a bit misleading. Yes current flows, and it flows like water, but an electrical circuit is not actually like a river or a water hose. When a river flows around a sharp bend in topography, the bend reduces the flow of the river because the floor of the riverbed and the earth on the riverbank introduce friction against the water and reduce its velocity. Once the water flows past this bend, it may be completely straight and fully unimpeded until it reaches its eventual destination in the ocean. There are two interesting questions here: +The water analogy from many simpler electronics tutorials lead me to use words like "upstream" and "downstream", but those terms are a bit misleading. Yes current flows, and it flows like water, but an electrical circuit is not actually like a river or a water hose. When a river flows around a sharp bend in topography, the bend reduces the flow of the river because the floor of the riverbed and the earth on the riverbank introduce friction against the water and reduce its velocity. Once the water flows past this bend, it may be completely straight and fully unimpeded until it reaches its eventual destination in the ocean. There are two interesting questions here. -First question. At what point downstream from that bend does the river become the ocean? Or, for our circuits, at what point does the circuit cease to be supply voltage and become ground reference voltage? +1. At what point downstream from that bend does the river become the ocean? Or, for our circuits, at what point does the circuit cease to be supply voltage and become ground reference voltage? At some point after the bend in the river, the river will eventually reach the ocean. The ocean and the river are made up of different types of water. The river is fresh water; the ocean is salt water. There is an interface between the two at which point the water is neither river nor ocean, it is some mix of both. You go just a little upstream from that interface, it is the freshwater river. You go a little more downstream, it is the saltwater ocean. Electrical circuits do not behave this way. For an electrical circuit, it would be more like, as soon as the topography of the bend is complete, the freshwater river immediately ceases to be detectable and it is instantly the ocean. Even if the real body of the ocean is hundreds of miles away, at the instant where the pathway leaves the topographical bend connected to the upstream freshwater source, it becomes the river. This is because, in an electrical circuit, the concept of the hundreds of miles of riverbed between the bend and the ocean are irrelevant; the only thing that matters is the potential difference between point connection points. As soon as the downstream leg of our resistor R1 becomes attached to the rest of the circuit that eventally leads (unimpeded by another components) to ground, that downstream leg of R1 **becomes ground itself**.