# "Water circuit" equivalents of basic electric components

1. Apr 21, 2015

### rumborak

There were two threads in short succession in this forum about basic electric circuitry (RC circuit, RL circuit), and it struck me that over the years I've been surprised that the "water circuit" analogy isn't used more than it is.
That is, in a system with a circular pipe that contains water:

Electrons = Water molecules
Battery = Pump
Voltage = Water pressure
Current = Water flow
Resistor = Narrow pipe

Now, what would the equivalents of the capacitor and the coil be? That's what I've been wondering.

EDIT: Ooh, a coil could be imagined as a heavy flywheel that is moved by the water current. It will oppose any change in flow through it.

2. Apr 21, 2015

### Quantum Defect

A Battery is more like a water tower, I think. It just sits there with potential energy until you tap it. Water also leaks out of these over time, running down -- just like a battery.

3. Apr 21, 2015

### Hesch

A capacitor is a watertank with the inlet and outlet placed in same height (at the bottom). If more water flows in than out, the tank will be filled with water, increasing the water pressure at the bottom. (The capacitor will be charged, increasing the voltage over the capacitor).

A coil is a turbine loaded by some inertia. Water pressure will accelerate the turbine, so that water flow through the turbine gradually increases. Lowering the water pressure (or changing direction of the pressure), the inertia will drive the tubine, so that the turbine will act as a pump while decellerating.. (You cannot immediately alter the current through a coil).

Last edited: Apr 21, 2015
4. Apr 21, 2015

### rumborak

Hmm, but no matter how high you charge the watertank, you are not impeding the flow, right? That's what a capacitor does, on direct voltage it eventually blocks the current completely.

5. Apr 21, 2015

### rumborak

Ah, a capacitor could be a simple tube with a divider plate in it, where the divider plate would be moved by the water pressure on both sides. However, a spring would be holding it in place, where there more it gets displaced from its rest position, the harder it becomes to move it further.

6. Apr 21, 2015

### jbriggs444

Agreed. My picture of a "hydraulic capacitor" is a horizontal tank with a narrow inlet on one end, a narrow outlet on the other end and a rubber diaphragm in the middle.

Edit: Rumborak beat me to it.

7. Apr 21, 2015

### rumborak

Lol, that's exactly what I was suggesting in my previous post. I like the rubber diaphragm idea, makes it conceptually simpler.

8. Apr 21, 2015

### Hesch

I see, but give me another try:

It's a piston with an attached spring in a cylinder with inlet/outlet at the top/bottom of the cylinder. Water flow through the cylinder will compress/stretch the spring, resulting in a water pressure.

9. Apr 21, 2015

### rumborak

That still doesn't cut off the flow on steady pressure.

jbrigg's and my equivalent does exactly that. Horizontal tube, with a rubber diaphragm in the middle. The equivalent is even good enough that you could show an isolated, charged capacitor that way. Charge the device in a water circuit so that the diaphragm is "loaded", then cap off the two sides, and remove the device. Now you got a "charged" capacitor that, when reconnected to another circuit, will unload its energy.

EDIT: Sorry, misread your post, you had changed one of the outlets to the top. Yeah, then it becomes equivalent to the rubber diaphragm device.

10. Apr 21, 2015

### sophiecentaur

There are a number of Mathematical Equations that describe aspects of water flow are of the same form as some of the Equations used for electrical circuit analysis. That much is true. However, the 'Water Analogy' is actually pretty hopeless as a model for Electricity. The main problem is that there is no appreciable kinetic energy involved in the energy transfer in electric circuits. There is also the problem of open circuits and water leaking out of the open ends of pipes.

If you really really want to use an analogy, rather than dealing directly with a phenomenon then you are always welcome to try but the caveats are enormous and you will never be sure that any conclusions that are drawn from the analogue model are valid for the real thing.

Just what is the point, when the sums involved in basic Electrical theory are extremely simple and you can rely on the results?

11. Apr 21, 2015

### rumborak

I don't know, what is ever the point of using analogies? I think the water analogy holds actually pretty well, and particularly, it makes for example people realize that just like water, the electron gas "bounces" off a resistor, with waves sloshing to and fro until the circuit is settled. I remember only learning about these dynamic effects pretty far into my EE, and I was annoyed because I always *thought* that's how it must happens, but the educational material up to that point had made it look as if electricity just magically knows that five elements down the circuit there's a resistor and thus it can only have this much current.

12. Apr 21, 2015

### OmCheeto

I thought I did a great job of explaining the simplicity of this a few years back:

And my post from last week was almost flawless:

Though, I don't think I finished my thoughts on the subject. hmmm......

So resistance would be (fear of death)/(fans/second)
which would be (fear of death seconds)/fan
fear of death seconds being equivalent to a weber(volt second)

Which is not quite right, as it is a change of a volt in a second that generates a "Weber".

I'm glad I could clear this up.

Rumborak, you are welcome.

13. Apr 21, 2015

### OmCheeto

ps. Whenever I do these things, I have a weird feeling, that I'm simultaneously channeling the spirits of Dr. Seuss & Rube Goldberg, as the imagery, and complexity, get very strange.

14. Apr 21, 2015

### Quantum Defect

But a "Weber" is a barbecue grill? I guess if you used too much lighter fluid, you could have some reasonable fear of death ...
... and is it a "fear of (death seconds)" or a "(fear of death) seconds" ... ?

My favorite "analogy" of all time was in a quantum mechanics class taught by an eccentric professor. He said that time reversal invariance was like running the film backwards through a movie projector and having nobody laugh.

15. Apr 21, 2015

### rootone

Water equivalent of an LED would be a hard one.

16. Apr 21, 2015

### OmCheeto

Don't even get me started, on diodes.......