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Electronics: using water pressure & flow analogy to explain voltage & current 
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#1
Aug1108, 07:45 PM

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Electronics / electrical engineering forums often get questions from novices who have trouble understanding the concepts of voltage and current. Invariably, a more knowledgeable person will explain it by giving the standard analogy between electricity and water:
voltage <> water pressure current <> water flow rate (eg. liters/sec of water) I'm interested in people's thoughts on whether this analogy actually gets the concept across to the novice? While the analogy works in principle, I have never seen a novice person respond with "Aha, thanks, I get it now. That water analogy makes total sense to me". After more than 3 years participating in the usenet group sci.electronics.basics, and several months here at PF, I have yet to see that sort of response from the many novices who have had this fundamental misunderstanding. So, I am highly suspicious that this electronics<>water analogy, while true, is not beneficial in terms of teaching beginners about voltage and current. What do others think? 


#3
Aug1108, 08:12 PM

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I was taught this analogy as a teen and it was a huge "AHA!" moment for me. It was useful.
Of course I had to relearn it all over again when I got to upper division E&M. 


#4
Aug1108, 08:18 PM

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Electronics: using water pressure & flow analogy to explain voltage & current
I think it is a very hard concept to understand and it can take years to fully understand current/voltage. There are too many "why?".
This analogy never helped. I just struggled with these concepts for some several months ... Now, after finishing first year of EE, I have some basic knowledge about current and voltage but still not sufficient. 


#5
Aug1208, 01:53 PM

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It is not useless. It is very useful. I have had many students who "get it" completely when I use the water analogy. I am careful, however, to add a caveat (repeatedly) that it is an analogy, which means that it is not "it" exactly, and understanding it fully is something you "get" after a long time of pondering and taking in more information from further studies. Sometimes I'll remind students that no one ever "fully" understands anything. All we can do is get a little closer to building a mental model that "works," which means something that allows us to correctly predict an outcome of any given circumstance ("look at the computer screen and tell me if someone predicted these electric circuits correctly").
The concept of electric potential is often the hardest to understand, so calling it "sort of like an electric pressure" is the closest to "not wrong" that any analogy can get. With current, I have to be careful to make sure that it is not equivalent to net speed (so electric current is not the same as watter current) but the net quantity f electrons going by. I use a "wide river vs. garden hose" analogy for this one. If it is carefully presented, with full discloser about the ultimate failure of all analogies, the water analogy is the best method for learning to distinguish between current, voltage, and resistance. 


#6
Aug1208, 02:14 PM

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Here is a PF thread where we debated the usefulness of the water analogy:
http://www.physicsforums.com/showthread.php?t=124517 Using it for a brief analogy is okay, IMO, but its usefulness is very limited past that, again IMO. After using the water analogy briefly, I prefer to then use more of a gravity and ping pong ball analogy, to get the student thinking more in terms closer to the real physical situation (discrete electrons moving in the conductor lattice). I like to picture something like a lifting conveyor belt for the balls to represent the battery, and a vertical pipe with stationary obstructions (similar to a pachinko machine) as the resistive wires and devices. The balls fall at a rate determined by the rate of collisions with the stationary lattice, analogous to I = V/R. 


#7
Aug1208, 08:28 PM

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Thanks to everyone for the responses, and to berkeman for posting the link to the past PF thread.
My thinking is that students really have to understand the concept of potential energy in order to understand voltage. Usually in physics books electricity is introduced with Coulomb's law, giving the force between two point charges. People seem to have no trouble with the concept of electric charge, and are quite familiar with forces at this point in their studies. Over the course of several chapters, force is used to explain electric field, which in turn is used to explain electric potential (i.e. voltage). This all takes some time, but perhaps in the end is the best way for many to grasp it. I appreciate that the water analogy (and others) have helped some people. It's been so long ago that I learned this stuff myself I'm not quite sure just how the understanding came about, but I know it wasn't the analogy route. 


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