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#11
05-23-2008, 07:31 PM
 mattW Super Moderator Join Date: Sep 2007 Location: Sydney, Australia Posts: 882 Blog Entries: 18
Re: Disagreement on Wiki Entry

An optimist would say the container is half full, a pessimist that it is half empty and an engineer would say that the container is too big...
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#12
07-02-2008, 01:24 PM
 Bottomfeeder Senior Member Join Date: Jun 2008 Location: Pasadena, CA Posts: 508
Re: Disagreement on Wiki Entry

Quote:
 Originally Posted by Wirecutter At one point, I thought I'd actually invented the water-electricity analogy. I know, I didn't - but I did come up with it independently. Others simply did it first. I also disagree with the wiki entry, once I found it. I think I mostly agree with Sharkey, but maybe say it a little differently. Volts are not analogous with water speed, they're analogous with the pressure. Amps are defined as the number of electrons moving past a given point. That's your speed (or flow) analogy. So more voltage (pressure) can facilitate more amperage (speed). More voltage (pressure) also means you can move more watts (gallons per minute) through the wire (pipe). Watts are analogous to "gallons per minute" or some other quantity of water per unit time. Kilowatt hours similarly relate to the amount of water delivered. The analogy has to get creative to explain magnetism, but you can still use it to explain resistance, inductive reactance, and capacitive reactance. But all that's for the more advanced course. I always though of the electricity-water analogy as a way to explain it to non-electrical types. My \$0.02 -Mark
Hmmm, this just kind of bothers me a bit. Voltage is definitely analogous to the pressure, no debate there, but amperage is analogous to the flow (in gallons per minute, say) not the "speed". The speed would be analogous to the current density. Consider this, the Mississippi has a much higher current than a firehose, yet the water speed is higher coming out of the nozzle. If we were looking at just the speed it'd appear that a firehose has more current than a slow moving river. And that just ain't so.

Wattage is not analogous to the flow the water. It's more analogous to how much work the water can do. I can cut any metal with a high pressure stream of water, even though the gpm of the water is very low. There isn't a clean analogy to Wattage unfortunately. It's really flow*pressure.

Voltage => Pressure
Amperage => Flow
Flow /= Speed
Watts => Pressure * Flow (whatever that means)

Another 2 cents for the pile.
#13
07-02-2008, 03:30 PM
 rbgrn Administrator Join Date: Jul 2007 Posts: 691 Blog Entries: 13
Re: Disagreement on Wiki Entry

It seems that everyone is in agreement that the article needs to be changed to reflect a better water analogy. Could someone please just go ahead and make the changes?? Remember - everyone can edit!
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#14
07-04-2008, 09:28 AM
 Wirecutter Senior Member Join Date: Jul 2007 Location: DC metro, East coat USA Posts: 232
Re: Disagreement on Wiki Entry

Quote:
 Originally Posted by Bottomfeeder Hmmm, this just kind of bothers me a bit. Voltage is definitely analogous to the pressure, no debate there, but amperage is analogous to the flow (in gallons per minute, say) not the "speed". The speed would be analogous to the current density. Consider this, the Mississippi has a much higher current than a firehose, yet the water speed is higher coming out of the nozzle. If we were looking at just the speed it'd appear that a firehose has more current than a slow moving river. And that just ain't so. Wattage is not analogous to the flow the water. It's more analogous to how much work the water can do. I can cut any metal with a high pressure stream of water, even though the gpm of the water is very low. There isn't a clean analogy to Wattage unfortunately. It's really flow*pressure. Voltage => Pressure Amperage => Flow Flow /= Speed Watts => Pressure * Flow (whatever that means) Another 2 cents for the pile.
Not to put too fine a point on it, but... The current of the Mississippi is much lower than the firehose, but the "watts" (gallons per minute) delivered is still high. Why? because with such a huge (mile wide in many places) pipe, the resistance is incredibly low. Think of the Mississippi as a kind of "superconducting" water pipe. Aw, hell. This doesn't work, either.

I can't claim the water analogy is perfect by any means. It's a handy way to describe electrical terms to non-electrical types. You can kind of create other analogies for things like capacitance and resistance using the water model, but when you look closely, the analogy breaks down. Consider inductance, for example.

Wouldn't it be cool if you could make water flow in a pipe simply by placing it next to another pipe with water flowing in it? (Ok, with water flowing back and forth in the pipe. You get the idea.)

Anyway, it's an interesting discussion. One of my former employers once talked of a college course he took while getting his BSEE. For a good portion of the class, the instructor strenuously avoided electrical terms, and taught the concepts using analogies from the physical world. My friend said it was a tough course, and really made everyone think, but it was interesting. I think I'd better stick to electricity, since I can understand it (in as much as I do understand it) just as it is.

-Mark
#15
07-04-2008, 10:04 AM
 Bottomfeeder Senior Member Join Date: Jun 2008 Location: Pasadena, CA Posts: 508
Re: Disagreement on Wiki Entry

Quote:
 Originally Posted by Wirecutter Not to put too fine a point on it, but... The current of the Mississippi is much lower than the firehose, but the "watts" (gallons per minute) delivered is still high. Why? because with such a huge (mile wide in many places) pipe, the resistance is incredibly low. Think of the Mississippi as a kind of "superconducting" water pipe. Aw, hell. This doesn't work, either.
Actually, you've got this exactly wrong. Electrical current is a measurement of the flux of electrons through an area. That area is usually a plane normal to the wire. So by counting how many electrons go through a specific slice of the wire every second you can get the electrical current. Of course, this is really hard to do in the real world, but it's fundamentally true. In the water analogy, you can replace discrete units of charge (electrons) with discrete units of water (gallons). Replace the wire with a fire hose or river and you can see what happens. The analogous current for water is the amount of gallons that pass through a certain slice of that hose or river per second. The Mississippi has a HUGE current. But, it's Voltage is very low (not a very steep river near the end, where I grew up in Louisiana). You are correct though the resistance is definitely very small. That's what allows the current to be so high for such a low voltage. Low V = Large I * Minuscule R.

Is that more clear?
#16
08-25-2008, 12:06 PM
 rfengineers Senior Member Join Date: Jun 2008 Location: Gainesville, Florida Posts: 401
Re: Disagreement on Wiki Entry

Oh what-the-heck, just to be argumentative, let me raise some ridiculously picky and utterly meaningless points.

1) Charge carriers are responsible for current flow. They can be electrons or ions (as in the fluid between battery plates) or any other charged particle you can think of.
2) Electrons, and other charge carriers, DO travel at very different speeds under different conditions, although the water pressure analogy is more correct than the speed analogy.

I've found that if you abandon the idea that electricity is exclusively electrons-in-motion and grasp the concept of charge carriers then batteries of wet-cells suddenly make a whole lot more sense.

All that said, I really do like the Wiki water analogy as it is now written.
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