Analogies- are they correct?

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peter d

peter d

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I have come across two analogies that I would like to run by the forum to see if they are accurate. One was in a DIY book, the other in a student book.

Both explain the principles of current and voltage.

#1. The plumbing analogy

The supply lines are like the hot wires. They carry current and voltage (water under pressure). The neutral wires are like the drain pipes. They carry the waste (current), but at zero pressure ( 0 voltage).

#2. The dump truck analogy.

The dump trucks deliver their load to the site which is current times voltage = power. The load represents voltage, the truck is the current. After they have delivered their load, current is still there but the load is gone (voltage).


It's been a while since I read these so I hope I am explaining it right. No doubt someone else has heard these and can correct me if need be. :)

[ January 19, 2005, 04:22 PM: Message edited by: peter d ]
 
Re: Analogies- are they correct?

Originally posted by peter d:
#1. The plumbing analogy

The supply lines are like the hot wires. They carry current and voltage (water under pressure). The neutral wires are like the drain pipes. They carry the waste (current), but at zero pressure ( 0 voltage).
Click to expand...
Peter #1 is way off IMO, the neutral is under as much 'pressure' as the other.

Originally posted by peter d:
#2. The dump truck analogy.

The dump trucks deliver their load to the site which is current times voltage = power. The load represents voltage, the truck is the current. After they have delivered their load, current is still there but the load is gone (voltage).
Click to expand...
I don't get that one at all. :cool:
 
Re: Analogies- are they correct?

Peter #1 is way off IMO, the neutral is under as much 'pressure' as the other.
Click to expand...
This is one that always confuses me. It is under pressure but at 0 volts. How can current move without voltage. My only guess is because the "neutral" is connected to an earth reference.


I should add that #2 is my best verbal desciption of the illustration in the book of dump trucks dropping off their "loads."

I completely forgot the text but remembered the graphic quite well.

Yes, one of Ed's illustrations would be very nice. :D

[ January 19, 2005, 04:38 PM: Message edited by: peter d ]
 
Re: Analogies- are they correct?

Originally posted by peter d:
It is under pressure but at 0 volts. How can current move without voltage. My only guess is because the "neutral" is connected to an earth reference.
Click to expand...
It is at zero pressure in relation to earth.

However a neutral is as much a supply conductor as the ungrounded conductor(s).
 
Re: Analogies- are they correct?

I recommend forgetting both analogies. They are both too superficial; neither is any help in conveying what happens in a wire. For example, in neither case is a complete circuit involved. The drain pipe does not bring water back to the source, and the truck driver can go home after dropping the load. By contrast, the wires carry current from the source to the load, and back again to the source.
 
Re: Analogies- are they correct?

I always have a difficult time using those analogies because to me and others here, those analogies do not do electricity justice, and we know there is much more to it than just water flowing through a pipe. It is true that current does not flow without voltage on the neutral, but there is a potential difference between the hot and the neutral, which is all part of one circuit, and the current ideally will be the same in both (I'm probably going to get reamed for that last general statement). :eek:
 
Re: Analogies- are they correct?

No mechanical analogy will be 100% accurate, but I've used the plumbing system with some success.
What should be stressed is the pressure difference, as that is what voltage is, the difference in potential energy between the two circuit conductors.

As for current, the electrons repel each other and are attracted to positive charges, so they move from an area with a surplus of electrons, (negative terminal) through any path that is a conductor, to an area that has a deficiency of electrons (positive terminal).

:)
Ed

power.gif
 
Re: Analogies- are they correct?

I think that using the analogy of a pump forcing water through a closed piping system is a very good way to teach rudimentary electricity to beginners. Resistance is modeled by restrictions in the piping system.

Coincidentally, there are simple methods for modeling both capacitance and inductance in the closed system water analogy.

DC is the easiest to model of course... pump is always going the same way. But AC can be modeled by switching the pumping direction of the pump.

In my mind and in teaching, I have found the water analogy to be effective.
 
Re: Analogies- are they correct?

Ed
I like the hole flow diagram. Are they your creation? That is a good way to illustrate it!
I would be interested in using this if I ever go back to teaching Theory 1
 
Re: Analogies- are they correct?

I think the water in the hose analogy is a pretty good one. The thing is though, you have to consider only a section of the hose to represent only a section of a circuit. It seems like that's almost always neglected or handled poorly. I think most people can easily complete a circle from a section, in their head.
 
Re: Analogies- are they correct?

I like the bicycle-wheel analogy.

Turn the bicycle upside-down in your mind. Give the front tire a spin. When you spin the wheel, your hand injects energy into the entire wheel all at once. Now put your hand lightly against some part of the tire so the spinning wheel is slowed and stopped by friction. Your hand gets hot. Your hand extracts energy from the entire wheel, all at once, and the whole wheel slows down. Finally perform both tasks at once: rub one hand lightly against the tire while you use your other hand to keep the wheel spinning.

One hand spins the wheel and fills the whole thing with kinetic energy... and the rubbing hand sucks the energy back out at the same time. The wheel rotates, and energy flows almost instantly across the wheel, going from the "spin" hand to the "friction" hand.

The bicycle-wheel analogy has no problem explaining AC. Just wiggle the bicycle wheel back and forth instead of spinning it continuously. The wiggling wheel will rub upon the distant unmoving "friction" hand, and will heat up that hand. Energy can essentially travel instantly across the bicycle wheel, even though the wheel itself rotates slowly. Energy can travel instantly between the two hands even if the wheel moves back and forth instead of spinning. What determines the direction of this energy travel? It's simple. If one hand spins the wheel, it throws energy out into the wheel, and if another hand rubs on the tire, it extracts energy from the wheel. Notice that the energy doesn't care about the wheel's rotation. The energy flows one-way, from one hand to the other, even if the wheel reverses direction, and even if the wheel vibrates back and forth rather than continuously turning.
 
Re: Analogies- are they correct?

peter d posted January 19, 2005 04:34 PM It is under pressure but at 0 volts. How can current move without voltage.
Click to expand...
While we think in "big picture" that the grounded conductor is at zero volts, it actually isn't exactly zero until the current reaches the low side terminal of the source, that is, essentially, the terminal on the side of the PoCo transformer.

Out on the branch circuit, where the load is, we can measure the voltage across the load. That voltage will be smaller than the voltage at the terminals of the transformer(xfmr). There is a voltage drop across the circuit conductor between the xfmr and the load, and another voltage drop across the "grounded conductor". These two voltage drops and the voltage across the load added together will equal the voltage at the xfmr.

Here's the key piece, for me, at least. . .The resistance of the two conductors (ungrounded and grounded) and the load resistance all get their fair share of the source voltage, as determined by Ohm's Law. So,

As Iwire says, the pressure on the return current is the same as the pressure on the feed current.

Bryan, that bicycle wheel analogy is quite interesting. I don't recall hearing it before. Push in one hand, heat in the other hand. Actually demonstrates AC, as well as DC. And the paradox of whether my motivating hand is "pushing" or "pulling" the tire around the "circuit" is satisfyingly clear.

[ January 20, 2005, 11:39 AM: Message edited by: al hildenbrand ]
 
Re: Analogies- are they correct?

I have never seen or thought of the bicycle wheel example. That is EXCELLENT, I like it, and will pass the idea on to my fellow instructors.

I don't see any way to model indcutance and capacitance and current out of phase with voltage like you can do with a closed water pipe system, but it is definitely usable for basic electricity classes.
 
Re: Analogies- are they correct?

I've never heard that one Bryan. It's ingenious.

The only problem is that every body insists the electrons be represented by little balls or something. Which is probably why even people educated in electricity think that electrons are coming out the wire. It takes years to unteach that to people.
 
Re: Analogies- are they correct?

The little ball theory is what I was taught back in my apprenticeship and the elementary texts still teach that. Now of course, I know that electrons are something more than little balls.

But, physis, I would like to you expand on your idea here. I have always thought of the electrons actually traveling through the conductor, jumping from atom to atom. As electrons are being pulled into the positive side of the source, an equal ampount is being pushed out of the negative side, and the valence electrons in the atoms of the conductors are all jumping along from atom to atom in between.

I think this is how most laypersons will view it. I would love to know more about the theory.

Can you give me a link, a source, or give me your thoughts here about what you mean?
 
Re: Analogies- are they correct?

As already has been mentioned, analogies aren't an exact replacement for the real thing. Consider the hydraulic scenario. This analogy is only truly affective if the student already has a rudimentary understanding of line loss, pressure drop, and flow. By the time one explains these concepts for hydraulics, it would seem more prudent just to explain the electrics.

Bob
 
Re: Analogies- are they correct?

1st year electrical apprentice: "Now tell me again, how does that voltage and current stuff work?"

Electrical instructor: "It's simple. Just think of it like water."

Meanwhile, over at the plumbers apprentice school:

1st year plumbing apprentice: "Now tell me again, how does that pressure and flow stuff work?"

Plumbing instructor: "It's simple. Just think of it like electricity."
 
Re: Analogies- are they correct?

Originally posted by physis:
I've never heard that one Bryan. It's ingenious.

The only problem is that every body insists the electrons be represented by little balls or something. Which is probably why even people educated in electricity think that electrons are coming out the wire. It takes years to unteach that to people.
Click to expand...
Okay, I tried to figure out what you meant by this. I am assuming you are comparing water flowing out of the end of a hose to electrons flowing out the end of a wire. If I am correct, then I understand your point, and no other elaboration is necessary.
 
Re: Analogies- are they correct?

What I meant was that the initial analogies used to explain electricity often end up not going away after someone has supposedly learned the real theory.

Edit: Yes, that's what I meant. I left out the "of" in "out of the wire". Oops.

[ January 20, 2005, 02:55 PM: Message edited by: physis ]
 
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