Is this a proper analogy to understanding how current, voltage, and resistance work?

Discussion in 'General Electronics Chat' started by Cbcbrown, Aug 21, 2016.

  1. Cbcbrown

    Thread Starter New Member

    Aug 17, 2016
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    HI, I´m new to the AAC and this is my first forum post, sorry if this has been redundantly covered before. However, I´ve been trying to understand and grasp current, voltage, and resistance by finding an analogy that works for me. The water in the pipe one doesnt do it for me as it still gives me questions. So, I thought of this: Lets say that voltage is the height of an object being dropped, the current would be the mass of the object, and resistance would be pillows that deflect the objects energy. Example, the reason that high voltage low amperage current doesnt kill you is because its like a pingpong ball dropped from ten feet. Sure you´ll feel it, but it wont bother you, even though its moving fast, the mass thats moving is so small your body without any protection can resist it. Whereas a bowling ball dropped from a foot would be extremely painful. Thats because even though the voltage is low, the amps (bowling ball) is extremely high. Thats why you have to be careful with low voltage high amp circuts like car batteries. Same goes for the pingpong ball. Sure its small but if it were traveling fast enough, even though it has little mass, it can hurt you, so if you put a pillow in the way to resist its energy to you, you wont feel it near as much and it becomes safe once more. Does this make sense to everyone else?
     
  2. OBW0549

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    No. In fact, I would urge you to abandon this analogy as quickly as you possibly can; persist with it, and you'll only get yourself thoroughly-- maybe even permanently-- confused.

    There are times when it is appropriate to deploy analogies as an aid to understanding, and there are times when one must simply understand things in their own terms. This is one of the latter, especially when the analogy you're trying to use is as convoluted and off-the-wall as this one.

    One volt (a unit of electromotive force) will cause 1 ampere (a unit of charge rate of flow, meaning 1 Coulomb per second) to flow through a resistance of 1 ohm (a constant of proportionality having the dimensions of volts per amp). Looked at the other way around, 1 ampere of current flowing through a resistance of 1 ohm will cause a voltage difference of 1 volt to appear across that resistance. Either way you look at it, Ohm's Law prevails: current equals the voltage divided by the resistance.

    And that's all there is to it. I would urge you to stop playing around with analogies; they'll just slow you down.
     
  3. DGElder

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    Apr 3, 2016
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    I concur with OBW. I think the water pipe analogy might be the best analogy and is relatively easy to grasp for most people. And I think it is useful to give a newbie or someone with no interest in learning about electricity a quick and intuitive sense of how ohms law behaves - qualitatively anyway. But after a few days in the field of Electronics you should have moved past the analogy and have started to get comfortable with the physics and the terms and units that describe the physics of electricity. Going back and forth between analogies involves unnecessary mental gymnastics and as you move past the very basics of electricity the analogy will become more and more strained, inject unnecessary confusion and become all together useless and miss-leading.

    In fact if you really kept with the pipe analogy as you got beyond simple ohms law in DC circuits you would find the analogous physics of fluid flow in a pipe just as complicated as electricity and not anymore intuitive.
     
    Last edited: Aug 21, 2016
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  4. Papabravo

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    I agree as well. I think I understand electronics way better than Fluid Dynamics. Although I took the course and can wrestle with simplifying the complete Navier-Stokes equations to solve specific problems I still treat water and electrons a separate problems.
     
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  5. OBW0549

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    More complicated, even; I'm no fluid dynamicist, but doesn't flow rate in a pipe vary according to the square root (or approximately to the square root) of the pressure differential along the pipe?
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    If you want to use a height analogy, then the mass of the object is equivalent to the charge, not the current.

    But even then trying to understand the effects of current on the human body is well beyond the scope of this (or just about any) analogy because the effects of electricity on the human body are not related to energy or anything else in most analogies, but rather to electrical physiological processes.
     
  7. DGElder

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    Apr 3, 2016
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    Understanding the world is about building models of reality or analogs of reality if you will. We start with very simple models that are easy to understand and seem to work quite nicely, until they don't. We eventually find the simple model fails to predict some outcome and so we modify, learn or create a new more sophisticated model and are quite happy until again we have explored the limits of time, space or energy in which the model was conceived and we find it is no longer accurate. Each time we improve the model it gets more abstract. At some point the ultimate model will be pure mathematics without any "physical analogy" or at least not one that the human mind can conceive. One philosophical interpretation is that physical reality is math and what we envision as reality is just a conjuring of our minds, i.e a useful mental model that promotes survival.

    Anyway, at this point you would be best served by thinking of an amount of charged particles (Coulombs) being pushed by an E-field force (Newtons/Coulomb), at a rate measured in (Coulombs/sec or Amps), through a wire from a point of high potential energy (volts) to low potential energy (fewer volts); and those charged particles bump their way through a wire that is providing a resistance (ohms) to the flow of charge. You can build on that model. Not so much with bowling balls falling on peoples heads.
     
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  8. DGElder

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    I don't know, not in my wheelhouse, but that seems reasonable. I would think it is linear - depending on pipe cross sectional area - at low flow rates. But as rates increase friction, turbulence, acoustics and stuff I don't know anything about would become a factor.
     
  9. OBW0549

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    From working at a firm that manufactures flow meters, I recall (I think I recall) that the pressure differential across an orifice plate placed in line with a fluid flow increases according to the square of the fluid flow rate; I'm not 100% sure, but I suspect the same applies to a liquid flowing through a pipe.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    As noted, if you want an electrical analogy, then the water analogy is likely best.
    Your analogy is not correct.
    Here's my version:

    Voltage is the pressure applied to the (water) charges at one end of the wire (pipe) relative to the other end.
    Note that voltage is always relative between two points just as water pressure is relative between two points.
    For example if the pipe outlet goes to the ambient air, than the pressure applied to the input is relative to ambient air pressure at the other end of the pipe.
    Similarly the voltage is that measured from one end of the wire to some other point, such as across a resistance or to common/ground.

    Current is the rate the charge in coulombs/sec (amperes) flow in the wire ( water in liters/sec in pipe).
    Note that, in order for the flow to continue indefinitely, the water from the outlet of the pipe must return to the source.
    Similarly all electrical circuits must always have a complete return path (sometimes common or ground) back to the voltage source.
    You cannot have a single wire conducting current.

    A resistance is like a small orifice in the pipe. The smaller the orifice, the higher the resistance to the flow and the smaller the flow for a given voltage drop (pressure drop) across the resistance (orifice).

    A capacitor is like a tank (size is capacitance) with a flexible membrane sealing the tank across the middle and outlets on both sides of the membrane. The more voltage (pressure) you apply and the larger the tank, the more charge (water) will be transferred from one side of capacitor (tank) to the other.
    But note that you cannot have a steady flow (DC) going through a capacitor.
    With a steady applied voltage (pressure) it will charge to a certain level and then stop.

    An inductor's inductance can be viewed as the inertia of a heavy slug that fits snugly in the pipe with no leakage past it but it can readily move without resistance. The inductance (slug inertia) takes energy to get the flow moving, and once it's moving it tends to keep the flow moving. The more inductance (the heavier the slug) the more inertia to a change in flow.

    You can't use this analogy as a exact comparison, but in general it should help you understand the basics of electricity.
    Any questions about this, just ask.
     
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  11. DGElder

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    Hmmm.... (I^2)*R
     
  12. AnalogKid

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    Aug 1, 2013
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    In the water analogy universe, that is the pseudo equivalent or skin effect. But, like all analogies, the more complex the concept you are analogizing, the shakier the analogy is.

    ak
     
  13. AnalogKid

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    There is nothing wrong with the traditional water analogy as long as you remember that it a very simple analogy for a very complex(!) physical structure. And by complex, I mean the complex operator, the square root of -1, a cornerstone of all of the math of things electric.

    The water analogy is a grade-school-level description. If you have mastered 6th grade arithmetic, its time to move on.

    ak
     
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  14. Cbcbrown

    Thread Starter New Member

    Aug 17, 2016
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    Wow, I wasn´t expecting so many answers from everyone, I guess if the analogy confuses me because it gives me more questions than answers it is time to abandon it. I just get confused with Volts and Amps because Ive always struggled with the concept that something can have thousands of volts in the first place with such little current. Like how tazers work. I can do the math for resistors in each circuit and combined. along with most basic DC electronics calculations. I struggle with visualizing how something can have so many volts going though it yet have no current. Doesnt having a tens of thousands of volts mean that there is strong current flow? For example if a tazer were to instead continuously shock you, instead of it being pulsed, would it electrocute you? Is it the time spent shocking you thats also a factor? How can something have many volts but almost no current, and how can something have a ton of current, but low voltage? Could you step down a nine volt battery to make it have a high amperage to actually harm you? Questions like this continously pop into my head and though I can do many of the calculations with Ohms law, these questions pop in my head. What am I not grasping? Im truly not being purposely dense, Also with the water analogy I get confused because if you have alot of pressure, but a small opening water can end up cutting metal. Can the same be said for electricity? Or am I taking this analogy way too far? Im feeling lost in the sause so to speak.
     
  15. AnalogKid

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    Not automatically. First, volts do not go through anything. Amps do. Volts appear across things, like resistors. Assuming a 100 megawatt power supply, then 10,000 V across 1,000,000 ohms is only 10 mA, and dissipates 100 W, while 10,000 V across 1 ohm is 10,000 A, and dissipates 100 MW. When a voltage is impressed across a circuit or device, the current drawn is set by the characteristics of the circuit or device, not directly by the voltage itself. Your view of cause and effect isn't quite there yet.

    ak
     
  16. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    I agree with the water analogy as the best that a normal person can embrace to try to grasp the fundamentals of voltage and current. But as OB has implied in previous posts, it's not a perfect one, and eventually you're going to have to abandon it (like removing the third wheel from your bike) and accept electricity as a completely separate entity if you want to move forward on the subject.
     
  17. OBW0549

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    Mar 2, 2015
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    No, it doesn't.

    Volts don't "go through" things. Current does. They are two completely different things. If you need an analogy, consider that voltage is a sort of pressure, and current is a sort of flow. And resistance is a bit like a restriction in a pipe. Use the analogy loosely, and don't let yourself draw any elaborate conclusions from it.

    But just as you can have a high pressure in an air tank but no flow (e.g., if the valve is shut off), so can you have high voltage but no current.

    An air tank filled to 10,000 PSI with a teeny tiny pinhole in it has only a tiny outflow of air; and in contrast, the Amazon river has an enormous rate of flow, but very little pressure drop (i.e., elevation change) along it. This is analogous to high voltage/low current versus low voltage/high current. The pinhole in the air tank has a huge resistance to the flow of air; the channel of the Amazon river has almost no resistance to the flow of water.

    It looks to me like you're struggling with the concept of resistance, as well as an over-reliance on analogies.

    No. What creates the metal-cutting ability is not the fact that the opening is small, but the volume of water flow and the huge pressure behind that flow. If you had a big opening and could somehow maintain that high pressure, believe me, it would cut the metal REALLY fast.

    Yes, way, way, WAYYYYYY too far.

    Time to abandon analogies, and understand electricity in its own terms. Voltage is electromotive force. Current is the flow of electrical charges. Resistance is a physical property of matter that impedes the movement of charges. Ohm's law: current (in Amperes) is equal to voltage (in Volts) divided by resistance (in Ohms). No exceptions.
     
  18. Cbcbrown

    Thread Starter New Member

    Aug 17, 2016
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    Thanks for all the help from everyone, its really helped me understand how electricity does and doesnt relate to the water analogy. Moreover I´m going to abandon analogies and like you said, understand things in their own terms. Though due to everyones help I can rest my mind a bit now and move forward.
     
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  19. OBW0549

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    Excellent choice. May you enjoy learning electronics and, even more so, have plenty of fun with it!
     
  20. hp1729

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    Nov 23, 2015
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    Take up the challenge and learn it the right way. It will serve you better in the long run. Personally I think your analogy is good if it works for you. OBW's stuff is great if you are going into engineering. But to a first year person it just introduces more unknown terms. Yes, it is most correct in the long run, but I think it has been too long since he was learning from the start.
    Voltage is the pressure push or pulling on the electrons. Resistance is the opposition to the electrons. Current is the result of the pressure and resistance. A matter of how many electrons are in motion.
    The older terms were more descriptive. Voltage used to be Electromotive Force (EMF), the force moving the electrons. Current was Intensity (I), the magnitude of the electrons in motion.
     
    Last edited: Aug 23, 2016
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