Adventures in Engineering

The wanderings of a modern ronin.



I was listening to Chris Gammel's



The standard water-flow analogy for electronics is that electrons are water, and wires are pipes. Voltage is equivalent to water pressure (psi), and current is analogous to water flow (gallons per minute). Batteries are like pumps that supply an endless amount of water/current at a certain psi/voltage. Resistors are considered to be like very narrow sections of pipe, which restrict the flow rate of the water.



(This analogy is limiting as you get to higher levels of electronics skill. For instance, I'm not sure how I would explain an inductor using this analogy. But for a beginner it's a pretty reasonable way to think.)



Chris mentioned in passing that a capacitor is considered to be like a water tank. The theory is that it takes a certain amount of water to fill the tank, but after it's full, water can flow by without trouble. This part of the analogy has always bugged me. And after a couple of days, I figured out why. A tank is usually hard-walled, and thus it doesn't really behave like a capacitor when it comes to the voltage/water pressure aspect of things. The pressure in a water tank would change from 0 psi (as it's filling up) to high psi (when it's full) in almost no time, due the rigidity of the tank walls. But that's only correct for a very small cap. With a bigger cap, the voltage rises much more slowly, smoothly, and gradually.







Instead, I think a better analogy for a capacitor is a water balloon. Imagine a straight section of pipe with a water balloon attached. This is like a wire with a cap to ground, something you see everywhere in electronic circuits. As the water flows through the pipe, some of it gets diverted into the balloon, filling it up. Just like with the tank analogy. But the pressure situation here is different. The psi in the pipe remains low (but not zero) as the balloon is filling up, due to the balloon walls stretching out more easily when the balloon is nearly empty. But as the balloon gets fuller, the voltage/psi in the pipe slowly rises as the tension in the balloon walls increases.



Eventually the pressure in the balloon and pipe become equal. At that point, there's no further water flow into the balloon. And so the water can freely flow from one end of the pipe to the other without disruption. Furthermore, if the pressure/voltage in the line should fall for some reason, the water balloon will push some water/current back out into the pipe, in an attempt to keep the pressures/voltages equal.



The capacitance of the cap can be thought of analogous to the size of the balloon. Big balloons take longer to fill up, and consequently take more water/current to change their back-pressure/voltage.



Finally, the analogy can be extended to a cap in series by considering the cap as a two-chamber water balloon. Water into one chamber pushes water out of the other chamber. Water flow cannot go through, but pressure waves can, and do. (If the concept of a balloon with two chambers weirds you out, try and imagine a two-chamber water tank with a flexible membrane between the chambers. However, that isn't a great analogy, for all the same reasons that a tank isn't a good analogy either.)



I think this is nicely intuitive and gives a good "feel" for how a cap works in a circuit. Much better than the tank analogy, anyway. It gives you a much better sense of how pressure/voltage and current/water flow are related in a circuit that uses a capacitor. More voltage? That means more water pressure. Which means the balloon will fill up faster. And by analogy, you can also say that the cap will suck down more current (water) while it's charging (filling). Bigger cap? That means bigger balloon, so it will take more current to fill it up.



I think this also shows how a cap acts as a "flow smoother" device on a wire. Once the cap/balloon fills up, any small variations in flow will be compensated for by the balloon shooting some water back into the pipe. Similarly, any spikes or dips in water pressure will be moderated by the elasticity of the balloon. And lastly, of course, the pressure in the balloon will never be much different than the pressure in the pipe. Only as the balloon is inflating or deflating can there be a difference in pressure (= voltage) between the balloon and the pipe. And even then, only so long as the balloon can accept or provide water (= current). Of which the balloon has only a finite amount.



Parasitic capacitance? Your pipes expand or contract slightly when the pressure changes, thus leading to a tiny (but sometimes important) waste of water/current during pressure changes. In effect, it's like there's a tiny water balloon attached to the pipe, stealing a tiny bit of water when the pressure rises. And giving it back when the pressure falls.



So that's it. If electricity is water, than capacitors are not tanks - they are water balloons. I was listening to Chris Gammel's first podcast the other day, and it got me thinking about the commonly used water & pipes analogy for electronics. Particularly as it relates to capacitors.The standard water-flow analogy for electronics is that electrons are water, and wires are pipes. Voltage is equivalent to water pressure (psi), and current is analogous to water flow (gallons per minute). Batteries are like pumps that supply an endless amount of water/current at a certain psi/voltage. Resistors are considered to be like very narrow sections of pipe, which restrict the flow rate of the water.(This analogy is limiting as you get to higher levels of electronics skill. For instance, I'm not sure how I would explain an inductor using this analogy. But for a beginner it's a pretty reasonable way to think.)Chris mentioned in passing that a capacitor is considered to be like a water tank. The theory is that it takes a certain amount of water to fill the tank, but after it's full, water can flow by without trouble. This part of the analogy has always bugged me. And after a couple of days, I figured out why. A tank is usually hard-walled, and thus it doesn't really behave like a capacitor when it comes to the voltage/water pressure aspect of things. The pressure in a water tank would change from 0 psi (as it's filling up) to high psi (when it's full) in almost no time, due the rigidity of the tank walls. But that's only correct for a very small cap. With a bigger cap, the voltage rises much more slowly, smoothly, and gradually.Instead, I think a better analogy for a capacitor is a water balloon. Imagine a straight section of pipe with a water balloon attached. This is like a wire with a cap to ground, something you see everywhere in electronic circuits. As the water flows through the pipe, some of it gets diverted into the balloon, filling it up. Just like with the tank analogy. But the pressure situation here is different. The psi in the pipe remains low (but not zero) as the balloon is filling up, due to the balloon walls stretching out more easily when the balloon is nearly empty. But as the balloon gets fuller, the voltage/psi in the pipe slowly rises as the tension in the balloon walls increases. Eventually the pressure in the balloon and pipe become equal. At that point, there's no further water flow into the balloon. And so the water can freely flow from one end of the pipe to the other without disruption. Furthermore, if the pressure/voltage in the line should fall for some reason, the water balloon will push some water/current back out into the pipe, in an attempt to keep the pressures/voltages equal.The capacitance of the cap can be thought of analogous to the size of the balloon. Big balloons take longer to fill up, and consequently take more water/current to change their back-pressure/voltage.Finally, the analogy can be extended to a cap in series by considering the cap as a two-chamber water balloon. Water into one chamber pushes water out of the other chamber. Water flow cannot go through, but pressure waves can, and do. (If the concept of a balloon with two chambers weirds you out, try and imagine a two-chamber water tank with a flexible membrane between the chambers. However, that isn't a great analogy, for all the same reasons that a tank isn't a good analogy either.)I think this is nicely intuitive and gives a good "feel" for how a cap works in a circuit. Much better than the tank analogy, anyway. It gives you a much better sense of how pressure/voltage and current/water flow are related in a circuit that uses a capacitor. More voltage? That means more water pressure. Which means the balloon will fill up faster. And by analogy, you can also say that the cap will suck down more current (water) while it's charging (filling). Bigger cap? That means bigger balloon, so it will take more current to fill it up.I think this also shows how a cap acts as a "flow smoother" device on a wire. Once the cap/balloon fills up, any small variations in flow will be compensated for by the balloon shooting some water back into the pipe. Similarly, any spikes or dips in water pressure will be moderated by the elasticity of the balloon. And lastly, of course, the pressure in the balloon will never be much different than the pressure in the pipe. Only as the balloon is inflating or deflating can there be a difference in pressure (= voltage) between the balloon and the pipe. And even then, only so long as the balloon can accept or provide water (= current). Of which the balloon has only a finite amount.Parasitic capacitance? Your pipes expand or contract slightly when the pressure changes, thus leading to a tiny (but sometimes important) waste of water/current during pressure changes. In effect, it's like there's a tiny water balloon attached to the pipe, stealing a tiny bit of water when the pressure rises. And giving it back when the pressure falls.So that's it. If electricity is water, than capacitors are not tanks - they are water balloons. Post A Comment | 4 Comments | Share | Flag | Link









http://www.robotshop.ca/PDF/eck-10-manual.pdf



This is a completely awesome explanation of basic electronics, including diodes and transistors, in the style of the water analogy.



Thanks for redditor This is a completely awesome explanation of basic electronics, including diodes and transistors, in the style of the water analogy.Thanks for redditor yumz for showing me this! Reply | Thread | Link





http://en.wikipedia.org/wiki/Hydraulic_analogy



You'd think that me, of all people, would go looking for something on Wikipedia before reinventing the wheel...



Thanks to redditor You'd think that me, of all people, would go looking for something on Wikipedia before reinventing the wheel...Thanks to redditor 80hd Reply | Parent | Thread | Link





Nitpick : Except that's not the way balloons work.



It's more difficult to start filling a balloon than it is to continue filling it. The material of the balloon is thicker when it is not inflated; leaving more material to resist the initial introduction of pressure. When inflated the material of the balloon necissarily becomes thinner to enclose a larger volume. Consequently the material becomes weaker. Any pressure that can initially inflate a balloon is enough to continue to inflate it and eventually cause catastrophic structural failure (bursting). Pressure will not equalize. Reply | Thread | Link





Yeah, it's really more like, "the way people expect a balloon to work" than "the way a balloon really works." Reply | Parent | Thread | Link





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