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From the linked notes: I never really understood capacitors until I started trying to construct proper water-analogies for them. Then I discovered that my elec

• seiferteric 10y ago
  Not really sure I like that link. He seems to suggest that capacitors store "energy" instead of charge, which is just as ambiguous really. It's not like there is some sort of energy particle either. Of course what's really happening is that you are creating an electric potential between two plates. It's true that the net charge is the same, but you are moving electrons from one plate and forcing them (doing work) into the other. Then when you remove the battery and have an open circuit, the potential remains because they have no way to move back to the other plate until you close the circuit again. Also, I don't think the water analogies work very well because water does not attract other water in any way, whereas in a capacitor, the electrons attraction to the electron holes in the opposing plate is an essential part of how a capacitor works and explains why the distance between the plates and the surface area are important.
  • robbrown451 10y ago
    I think you are missing the point of the water analogies. It is what made electricity make sense to me as well. But don't take it too literally, it's an analogy, not an identity. If you start with things like "a motor is like a turbine, a generator is like a pump, a battery is like an elevated tank", a lot of things can fall into place. The point is you can visualize it.
    • gohrt 10y ago
      It's hardly fair to say that water analogies are good but "capacitors store charge" is bad, they are both weak analogies
      • CamperBob2 10y ago
        There's nothing weak about saying capacitors store charge. That's exactly what they do. You measure charge in coulombs, which is how you count electrons. Capacitance is just the value that relates stored coulombs of charge to available voltage.
        • gugagore 10y ago
          I didn't agree with what you wrote and maybe I can explain why. Springs store inches. You measure displacement in inches. Spring constant is just the value that relates stored inches to available force. (You can swap roles of force and displacement if you wish, the point is the same. It sounds bad to say springs store force or displacement, to me.)
          • CamperBob2 10y ago
            But inches are an abstract measurement of distance. Electrons are a thing. (Well, depending on who you ask... no one has ever seen one, and some people have claimed half-jokingly there's only one electron in the entire universe: https://en.wikipedia.org/wiki/One-electron_universe https://en.wikipedia.org/wiki/One-electron_universe .)
            • gugagore 10y ago
              What?? Okay then count by lengths of the spacing of carbon molecules in graphite, or Plank lengths, or whatever. I don't see why your objection is on inches or otherwise any unit I chose for dimension of length.
              • 10y ago
  • justifier 10y ago
    i agree, I linked more for the similarity in experience between the gp and the author in regard their unlearning I prefer your clarifications when considering the function of capacitors That said, water does attract water.. the term used to describe the phenomena is cohesion https://en.m.wikipedia.org/wiki/Cohesion_(chemistry) https://en.m.wikipedia.org/wiki/Cohesion_(chemistry)
    • gohrt 10y ago
      Cohesion is not an inverse-square field
      • justifier 10y ago
        gp> because water does not attract other water in any way though i was merely addressing this quote your comment confuses me both in fillip and content from cohesion wiki(o): Water, for example, is strongly cohesive as each molecule may make four hydrogen bonds to other water molecules in a tetrahedral configuration. This results in a relatively strong Coulomb force between molecules. from coulomb force wiki(i): Coulomb's law or Coulomb's inverse-square law, is a law of physics that describes force interacting between static electrically charged particles. (o) https://en.wikipedia.org/wiki/Cohesion_(chemistry) https://en.wikipedia.org/wiki/Cohesion_(chemistry) (i) https://en.wikipedia.org/wiki/Coulomb%27s_law https://en.wikipedia.org/wiki/Coulomb%27s_law
        • seiferteric 10y ago
          Yes I knew water attracts water, but it is not relevant to the capacitor analogy. You might as well have said water attracts water via gravity, it's just not relevant in this situation.
          • dotancohen 10y ago
            Gravity does have an inverse-square attraction. Though obviously it is several orders magnitude less than electromagnetism, in fact the analogy holds.
  • ambrop7 10y ago
    My water analogy for a capacitor is a piston with pipes attached to both ends, with a spring system that pushes the piston towards the center position. Is that not a mathematically correct equivalent? (in an idealized system with no water resistance/inertia and disregarding that the piston is of fixed length - not that real capacitors have zero resistence, inductance or can store infinite charge)
    • strictfp 10y ago
      Or just a membrane between the two sides?
      • gugagore 10y ago
        Yeah, but maybe it's trickier to relate the restoration force of a membrane with displacement of water.
    • javajosh 10y ago
      From an energetic standpoint, a capacitor is something that takes a trickle over a long time, and releases a flood over a short time. So it would be a water tower with a small input and a large gated output.
      • gugagore 10y ago
        It's just not clear to me how to think of the water tower as being an element of a hydraulic _circuit_.
      • wbeaty 10y ago
        But a water tower only has one terminal. A better "capacitor" would be a pair of water towers side by side. To "charge" this double-water-tower capacitor, pump some water from one to the other. And, when the water-tower capacitor is entirely "discharged," the towers both have the same water level inside. (As with a real capacitor, the total amount of water never changes.)
    • VLM 10y ago
  • thisrod 10y ago
    > water does not attract other water in any way Um ... once we've cleared up all the misconceptions about electricity, we might want to move on to this other thing called gravity :-) The usual reason that people are mislead by "water" analogies—more precisely, the analogy between height and electric potential—is because they misunderstood gravity to start with. If you start by writing Newton's law and Coulomb's law side by side, and develop the analogy in a precise way, smart students will be able to debug their own fallacies. For example, a capacitor does have a precise gravitational analog, where you have two tanks floating in outer space, and water gets sucked into them by gravitational attraction. Once you understand that, you can think about the effect of removing the minus sign from the gravitational energy law, and about the things you can do with two types of charge, but can't do with only one type of mass.
    • seiferteric 10y ago
      Yes as I said in a later comment of course cohesion and gravity exist, but they are not relevant in this analogy.
    • solipsism 10y ago
      where you have two tanks floating in outer space, and water gets sucked into them by gravitational attraction As a person who has always struggled to understand electricity (but understand Newtonian gravity well enough), please tell me more! Water is going from where to where?
      • thisrod 10y ago
        Picture a pair of tanks shaped like a parallel plate capacitor, a circuit shaped pipe connecting them, and a mixture of water and air filling the pipe. (The air lets the water move around without creating a vacuum.) All other things being equal, the water gets sucked into the plate shaped tanks to minimise the gravitational energy; the resulting gravitational field is identical to the electric field you would get if you forced positive charge onto both plates of a capacitor. Some of the differences with electricity are: Like charges repel, so you have to force the positive charge to go where the water would pull itself. Mass is always positive (dark energy aside), but charge can be negative, so you can have "negative mass pipes" that cancel out the mass of the water. You can squash positive charge into one tank, and negative into the other. Unlike the parallel water tanks, this takes work, because the charge on each plate repels itself-mass can't do that. But less work than when there is positive charge on both plates. The resulting dipole field is something that you can't get with gravity. This is how real capacitors work.
  • crador 10y ago
    Capacitors do store energy, in their electric field. Potential energy in general, as far as I'm aware, is stored in fields of some sort -- water turning a wheel (gravitational), tension in a spring (electromagnetic), capacitors (electromagnetic), chemical energy in gasoline (electromagnetic), nuclear energy (weak/strong fields I believe, not as well versed here) and so forth.
• nilkn 10y ago
  I still remember struggling with fractions when I was young. The funny thing is that I ended up majoring in math in college (at a highly ranked one at that), and I did very well, even publishing research in differential topology as an undergrad. How can someone who struggled to even grok fractions end up being successful in pure mathematics? I think it's because all my teachers operated off the assumption that kids can't possibly understand abstract definitions, so everything must be explained via analogies. So nobody could tell me that fractions are equivalence classes. Everything was instead explained in terms of pies, which certainly seems like it would be intuitive, and there's really nothing technically wrong with it, but it encouraged me to think of, say, 1/2 and 2/4 as different entities instead of two representations for the same entity (since, after all, cutting a pizza into two slices instead of four is just not the same thing). I didn't "get" fractions until on my own I came to the conception that these are actually kind of abstract concepts that go by many different names. For me, I was taught too much to identify numbers with concrete objects in the real world. This sort of works for integers (as long as you ignore negative integers and even zero to some extent), but it basically breaks down for all other types of numbers, including rational numbers. So it's a really bad expectation to set in my opinion. But you can hardly find an elementary school teacher who will dare approach this topic. I honestly think that if someone had just explained that "1/2" is a symbol we've devised to represent the solution to the equation 2x = 1 -- an equation which otherwise has no solutions at all in the integers -- I would've grokked it a lot faster. It would've been clear to me that we're introducing an entire new set of numbers distinct from the integers. These new numbers do not have a direct correspondence with physical objects in reality. And it would've been easier to see that the same solution must go by many other names as well. For instance, multiplying that equation by 2 on both sides gives us a new equation which must have the same solution: 4x = 2. So "2/4" must represent the same entity as "1/2". And so on. But because US educational culture has decreed that basic algebra is more advanced than fractions, the very idea of discussing solutions to an equation will never be allowed in an elementary school classroom. It is assumed as a basic fact of human life that kids cannot understand the concept of solving an equation before learning about fractions. We'll teach kids to perform mechanical manipulations of decimal expansions before we ever begin explaining what any of those digits actually mean.
  • 10y ago
• tomkat0789 10y ago
  +1 for drainpipe theory! As a ChemE grad student I took an EE control class after basically forgetting all the circuits knowledge I learned as an undergraduate (my high school didn't teach electronics and magnetism!). Water analogies were the only way I could do the problems with circuits. Drainpipe theory actually made me wonder what was ever so hard about that part of physics class...
• esmi 10y ago
  People say "capacitors store electric charge" because that's how the integral form of the capacitor equation works. [1] When tutoring EEs in university I spent a lot of time getting them to unlearn water analogies. They break down in some very fundamental ways. [2] I agree the analogies are useful when introducing the concept of capacitance and inductance but to me the analogies are the dead-end path. [1] http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capeng2.html#c3 http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capeng2.... [2] https://en.wikipedia.org/wiki/Hydraulic_analogy#Limits_to_the_analogy https://en.wikipedia.org/wiki/Hydraulic_analogy#Limits_to_th...
• somedangedname 10y ago
  I suspect that the top search result for "water analogies" was hugged to death because of this comment.