# Water analogy illustrations

I just love these illustrations from Dustyn Roberts’ Making Things Move book. Drawn by Sean Comeaux, they elegantly demonstrate the famous ‘water analogy’ of electrical theory.

Excerpted from Making Things Move (McGraw-Hill; 2010) by Dustyn Roberts, with permission by McGraw-Hill.

### 13 Responses to Water analogy illustrations

1. Just an observation that the circuit on the left will run down quickly, whereas its water analogy looks like it can run as long as the pump keeps pumping.

• Instead, consider a large chamber with an elastic membrane stretching across the center. As pressure increases on the left, the membrane stretches until its tension counterbalances difference in pressure.

______________
__/                \                 \___
high  pressure  __    –>        |                ___  low  pressure
\_______/______/

• dustynrobots on said:

I totally agree that I like this analogy better. However, I think that a semi-obscure pressure chamber with elastic membrane is a difficult concept for newbies. Just about everyone’s seen a water tower so it might be easier to grasp, although arguably less analogous. Plus, it was easier to explain to my illustrator!

• nobrainer440 on said:

To make the water tower analogy more accurate, the capacitor should be connected in parallel with the circuit, rather than in series, i.e. the capacitor should have one end connected to ground, and the other end connected to a wire running between the ground side of the fan and the ground side of the battery.

However, even though this setup would correct the accuracy of the analogy, it would still not use the capacitor properly, since the water tower (or capacitor) would continuously empty itself into the well (ground).

To make it more useful, the water tower could be inserted between the pump and the narrow pipe. (The capacitor would be connected between ground and the positive end of the voltage source). If the pump did not have a steady flow, putting the water tower here would help smooth it out.

• dustynrobots on said:

I totally agree this isn’t a perfect analogy, and thanks for your suggestions! I scoured books and the internet for water analogy pictures that covered ALL these elements, and most either do capacitors OR everything else, not try to combine them. And now I know why! It’s tricky to get them all right in the same diagram. So this circuit isn’t meant to be practical, just to stick in people’s brains of what water analog components are to the electrical components.

2. theophrastus on said:

This is delightful and very instructive! It recalls some of the wonders of Forrest Mims plus Larry Gonick.

A humble suggestion: add a couple of cartoons depicting how/when the hydraulic analogy fails. (there are some examples near the bottom of the wikipedia entry: http://en.wikipedia.org/wiki/Hydraulic_analogy )

• dustynrobots on said:

Thanks! I don’t think the hydraulic analogy ever totally fails, I just think it has to be applied carefully. Electricity does not equal water in all cases, but some components used to control the flow of water have electric analogs. For example, I’ve heard a capacitor described as a bucket that hold water, then the charge/water leaks out through a hole in the bottom of the bucket. This is wrong for two reason: DC current doesn’t flow through a capacitor like water flows through a bucket (in the top and out the bottom). It also doesn’t discharge slowly – in general, capacitor discharge very quickly.

If you have examples that can prove me wrong – please do! I love hearing how other people think of this analogy and if it works for them.

• anachrocomputer on said:

A couple of cases where the water analogy breaks down, for me at least: if you cut a pipe, water flows and pours out on the floor; if you cut a wire, the flow of electricity stops.

And with water flow, the water will flow faster or slower according to pressure and pipe size. Electricity, however, flows at nearly the speed of light regardless of voltage, current or wire size. You can’t “slow down” the flow of electricity, but you can slow down the flow of water.

3. The water analogy does help to explain how a circuit will work.

4. I’m doing (mechanical) engineering and I found it amusing that when we did pipes and stuff in a fluids course the lecturer used an electrical analogy! In the Scottish schools and education system there is far more stuff on electrical systems than fluids/pipes etc.

PS. Make – the comment system is still telling me that I’m signed in when I am apparently not. Not sure what’s wrong (and if the problem is at my end) but I wonder how many long detail comments have been lost because someone can’t be bothered re-typing!

• A lot probably It’s been broken since this site was created basically, and despite several reports they don’t feel like fixing it Just don’t check “remember me”.

5. Ahhh, but can you pump water in a circle and generate a magnetic field?

6. The capacitor/pump drawing is a little weird, it looks like gravity is inverted in the ground reservoir, and it’s not clear how the pump can pump water from it. I don’t think the ground reservoir should exist at all, actually. The ground symbol is just a convention, it could be anywhere.

Here are a few other analogies that helped me as a kid (found in the booklet of an electronics toy called Kosmos X1000) :
A resistor : a section of pipe with rocks slowing down the flow, or a narrower section of pipe
A battery : a water tower
A transistor : a floodgate attached to a lever pushed by the water coming from another smaller canal
A diode : a non return valve

They also used the elastic membrane model for the capacitor, and I had no trouble understanding it. An advantage over the water tower is that it shows clearly that the energy is stored inside the capacitor, and not as potential energy.