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#1
12-20-2007, 09:01 PM
 Mr. Sharkey Senior Member Join Date: Jul 2007 Posts: 207
Disagreement on Wiki Entry

I have to take issue with the latest Wiki entry "Amps, Volts, etc." Using the "water analogy" should not use "speed of water" to explain volts, nor should current be associated with the "width" of the water flow.

Here's why I disagree:

1) The speed of electricity is for all purposes, constant. Comparing it with the velocity of water is misleading, and infers that electrons can and do travel slower than others (in practical terms).

2) When determining water delivery, the "speed of the water" is totally irrelevant to the equation. Municipal utilities don't figure system losses in terms of speed, they figure it in terms of pressure loss. The pressure of the water is what is used in the figuring of water delivery. If you're a fireman, you don't care how fast the water is travelling in the fire hose, you care about how much pressure it has at the nozzle. Also, the pressure drops in a closed water system when the flow (current) is increased. This has a direct relationship with voltage drop in an electrical circuit. Saying that the speed of the water is like voltage creates a situation where if the valve at the end of the pipe is closed, there is no velocity, so there is no pressure? Does the source voltage go away when the switch in a circuit is opened? Both situations are answered by comparing to pressure. You can have pressure in a shut-off garden hose, just as you have it in an open circuit.

3) It doesn't matter how "wide" the conductor is in either liquid or electrical circuit, at least in terms of calculating the amount of work accomplished, as long as the conductors/pipes are large enough to not introduce losses. A 4 foot diameter pipe can deliver as little as a gallon per minute, and a 1" pipe can deliver 100 gallons per minute (assuming enough pressure). Current would best be described as the flow, or amount of water passing though the pipe any given interval, ignoring the size of the vehicle that carried it there. Electrical current is defined as a measured number of electrons passing a fixed point in an interval of time, rather like saying that a well produces 5 gallons per minute.

Water: Pressure x flow = quantity (delivery)

Electricity: Amps x volts = watts (delivery)

At any rate, this is how I've heard the water/electricity analogy explained for the last 45 years or so. I'd suggest some research to dig up a reference source if you want to use the speed/width explanation, I think it's faulty.
#2
12-21-2007, 05:48 AM
 mattW Super Moderator Join Date: Sep 2007 Location: Sydney, Australia Posts: 882 Blog Entries: 18
Re: Disagreement on Wiki Entry

I'm inclined to agree, sorry Robert. Using Pressure and rate of flow is a little bit more useful as an analogy but Mr Sharkey I suggest you go ahead and edit the article yourself (though it was a good policy to ask first). That is what the wiki is for; so we can hone and polish each other's ideas. Just Click 'Advanced Edit' at the bottom of the article and its just like editing a forum post. Cheers
#3
12-23-2007, 12:14 AM
 rbgrn Administrator Join Date: Jul 2007 Posts: 691 Blog Entries: 13
Re: Disagreement on Wiki Entry

That is the whole point of the wiki! Thanks for being democratic. I take no offense to any changes and I highly encourage a correction if it's needed!
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#4
05-16-2008, 11:57 AM
 Wirecutter Senior Member Join Date: Jul 2007 Location: DC metro, East coat USA Posts: 232
Re: Disagreement on Wiki Entry

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
#5
05-16-2008, 11:59 AM
 Wirecutter Senior Member Join Date: Jul 2007 Location: DC metro, East coat USA Posts: 232
Re: Disagreement on Wiki Entry

Pardon my ignorance and late entry to this thread, but can just anyone edit the wiki? I'll step up and do it if nobody else wants to..

-Mark

-edit - never mind, I found my answer. I'll edit it, and check back to see if we all agree.

Last edited by Wirecutter; 05-16-2008 at 12:02 PM.
#6
05-16-2008, 12:23 PM
 rbgrn Administrator Join Date: Jul 2007 Posts: 691 Blog Entries: 13
Re: Disagreement on Wiki Entry

Yes, it's a wiki. It seems everyone is in agreement that the water analogy needs to be reworded a bit. Feel free to fix it!
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#7
07-02-2008, 01:24 PM
 Bottomfeeder Senior Member Join Date: Jun 2008 Location: Pasadena, CA Posts: 509
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.
#8
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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#9
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

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