Take the 2-minute tour ×
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free, no registration required.

A similar question has been asked here, but mine is more general.

Here is the background: it seems to me that electricity is not so intuitive as it may sound. For example, many (unskilled, as I am) people think an electric plug is like a (water) tap: it delivers what it can, and stop at a maximum flow. But it is not possible to stop the water from flowing, by putting too large a bucket under a tap, whereas if you plug a washing machine you could break the current!

Improving the understanding with analogy seems to be an efficient way to learn (not talking about scholar stuff) about electricity and "feel" it (not too intensely, though).

Water seems to be a good candidate for these analogies: current/water flow, electron/molecule, tension/height difference, resistance/decrease of the pipe section, capacitor/volume with membrane. I also heard about an analogy between electricity/water for transistor.

My question is: is it possible to define a global analogy, which would: (i) allow to better understand any electrical circuit [ideally, a complete transcription would be possible], (ii) be exhaustive (i.e. work for any electronic/electrical component). Other examples include allowing to understand what would happen if the frequency of the electricity changed, or if a power plant produced energy in excess, etc.

I insist: I'm not an electrical engineering. I'm just curious :)

share|improve this question
    
Water is actually a pretty good analogy for DC electricity, but even a perfect analogy is only as good as your understanding of it. Putting a large bucket under a tap is the equivalent of a short circuit, while unplugging an appliance is equivalent to turning a tap off. –  Bruce Abbott Jul 31 at 23:30
5  
"All models are wrong. Some models are useful." - George Box. - Obviously, the only perfect model is the actual thing being modelled. There are many factors of EM fields that water cannot model. For a basic understanding it's superb. –  Russell McMahon Aug 1 at 2:00
    
So a battery would be a pipe full of anti-water connected to a loop-pipe of normal water. :) At one connection the water and anti-water come together and vanish, while at the other connection the water and anti-water are springing into existence and flowing off in separate directions. And finally, the two connections are intentionally made different, so a pressure-difference naturally appears and starts powering the flow. –  wbeaty Aug 1 at 6:35
    
A battery is a pump, rather than a water supply; further, all pipes are constructed with non-removable safety valves similar to those found on many pressurized gas canisters, such that fluid can only flow into another pipe or hose, and not into the outside environment. –  supercat Aug 1 at 18:59

5 Answers 5

up vote 10 down vote accepted

Using analogies to fluid dynamics (air/water) is useful for some purposes for the complete novice, but it also can lead to confusion. If someone really wants to understand electricity, they need to dispense with the fluid analogies and just learn how electrical fields and charges work. There really isn't a global analogy.

I find it useful to refer to William J. Beaty's articles about electrical misconceptions.

Also to the web site All About Circuits.

And finally, the more in-depth reference book, The Art of Electronics by Paul Horowitz and Winfield Hill.

share|improve this answer
    
That's sad (or not) but I'll have to do with it! Thanks for your answer (and Kamil's which is also interesting) and the references. –  anderstood Aug 1 at 1:02
1  
The good news is that intuition is developed from experience. On the day you were born nothing was intuitive to you. Least of all gravity which must have felt very strange compared to the world you knew in the womb. Over time we learned to infer "rules" from experience. The same will be for you as you get to know electricity. –  slebetman Aug 1 at 3:28

JYelton answer is very good in my opinion, but I will add alternative answer.

Electricity is unique and there is no complete analogy between electricity and something else. Is not just about moving electrons.

Electricity is complex and "contains" many diffrent phenomena

  • magnetic fields
  • electric fields
  • ionic reactions
  • chemical reactions

There is no complete analogy to electricity in "macro" world.

For better understanding how electricity electricity works you have to learn some chemistry, physics and try to build some picture of electric and magnetic fields in your mind.

You can read some very old books (1900-1950) about physics, chemistry and electricity. These old books are suprisingly good for beginners and most of them don't require modern engineer knowledge.

share|improve this answer
    
"...surprisingly good". Why are you surprised about the quality of "old" books?. I am not. According to my experience - and as far as basics are concerned - those old books are very often much better than new books. Certainly, there are reasons for that. –  LvW Aug 1 at 9:23
    
I meant that these books are good for begginers. –  Kamil Aug 1 at 15:16

Falstad physics applets: Circuits with visible electricity

If we use the water analogy, we find many interesting things that are hard to understand ...about water! For example, water flows slowly through a hose, yet the hydraulic energy flows incredibly fast. Also the hydraulic energy is counter-intuitive: it can easily flow backwards relative to the water flow. Or, water can be moving back and forth in a pipe, while the energy is racing along in a single direction. The water company doesn't sell us any water, they just offer an expensive pumping service, and all the hoses were pre-filled with water before we started. (After all, water isn't a form of energy ...and neither is electricity of course: electric currents aren't flows of energy, amps aren't watts.)

Also, in a closed water-filled loop (a hydraulic circuit,) the start-up transients are fairly hard to grasp. So are the energy reflections from joints and T-connections, resonance phenomena for "AC water," and energy loss from the hoses as radiated sound (which is analogous to antennas and energy lost as radio waves.)

Also, water is invisible. Well, actually it's much like electricity: if you have a water current in a transparent pipe, you see nothing, it just looks like a static rod of solid glass. It could be moving slowly, or flowing fast, or wiggling back and forth, but without dirt or bubbles we have no clue. In this it's just like electricity: electric currents are invisible, while electricity itself is easily seen, the mobile electrons within metals are silvery in color. Metals are reflective like mirrors because any dense "fluid" of electrons is reflective like a mirror.

Heh, so we should start using glass pipes full of liquid mercury as an improved fluid analogy for electric circuits: generators are liquid-mercury pumps, resistors are pipes full of mercury and compressed sand, frictional pipes which get hot whenever there's an Hg current. The silvery Hg stuff always goes slowly in a circle, while the hydraulic energy races from place to place in the pipes at extreme speed, yet when we're looking at transparent Hg-filled tubes, we see nothing happening, just some thin silvery rods with a transparent plastic coating.

share|improve this answer
    
Thanks for this very instructive comment. The app seems highly informative too and very educational! –  anderstood Aug 1 at 7:04
    
@anderstood - while the falstad circuit simulator is a very nice tool, it should be understood that it too is only approximate. In particular, the falstad simulator uses ideal models for the great majority of it's components, which can lead to behaviour that does not match reality in a number of cases. –  Connor Wolf Aug 1 at 11:46

One analogy I liked to use for understanding current is a train. Think of the electron as connected carts on a train. If I push one cart (even just a little) this first cart will move since they are connected.

Then parallel tracks or larger carts may describe other properties.

share|improve this answer
    
With real trains we can hear the energy-flow: when the locomotive applies force, the couplings between cars tighten up and go BAM BAM BAM in succession, with a wave running rapidly backwards as the cars themselves move slowly forwards. Here's a coil, a train-inductor youtube.com/watch?v=wevbi_zEmxc –  wbeaty Aug 1 at 6:29
    
I think this explains why light switches on "immediately" after touching the switch, while electrons move around 1m/s! I'll think about trains, thanks. –  anderstood Aug 1 at 7:06

The fluid-analogy is flawed in some ways, e.g. due to allowing open circuits and the fact that the fluid molecules actively move through the tube while the electrons in the conductor actually merely nudge their neighbours, who in turn nudge their neighbours and so on, which propagates much faster than the actual electron movement speed (which is slow).

So in my opinion a better, though not trivial, analogy would be a (closed) track of magnetic wheels:

<===============================> connected to start
   S     S     S    S   .......
   |     |     |    |
   N     N     N    N   .......
<===============================>

If you push the one on the left side, it will repel the next one and slow down (and maybe move slightly back, but not entirely to its origin), while the next one accelerates until it repels the second-to-next one and so on.

  • The fact that the wheels will slightly move towards the direction of "conductance" explains why the "circuit" must be closed, otherwise you'll run out of wheels
  • magnets in the track itself can simulate resistance
  • a junction plate, the switch of which is equipped with a magnet as well, can act as a transistor

(I haven't yet come up with an analogy for a capacitor though)

share|improve this answer
    
Um ...electrons actively move through the wire. And with water, the molecules nudge their neighbor, which nudges the next neighbor, etc., so the underwater sound wave propagates much faster than the H2O movement speed, which is slow. (Charge-flow and water-flow match: electrons really do flow along through metals, that's why we use fluid analogies.) –  wbeaty Aug 5 at 5:14
    
@wbeaty I didn't want to go into too many details. yes, the water molecules also nudge one another, but the common understanding of the water analogy is about the actual flowing and due to the low compressibility of water the major means of energy transportation in water is flow. electrons move indeed, but with a velocity of a few centimetres per second IIRC, while the signal propagates with almost the speed of light due to said nudging. though for a detailed discussion on this I propose a new question at physics.stackexchange.com –  Tobias Kienzler Aug 5 at 8:04

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.