Explaining current flow in series/parallel circuits to kids using the water analogy

[muesiocks]

This has stumped a few people at work, so I thought I'd come over here looking for a drop-dead simple way to present this. I often do electronics demos at a middle school and try to bring home the concept of current and resistance using the old water flowing in a hose analogy. If you connect a 10 foot long, narrow diameter hose to a faucet and turn it on, water comes gushing out the end of the hose - the amount of water per unit time being a proxy for resistance. If you put a Y adapter on the faucet and connect a 10 foot long, narrow diameter hose to each output you get twice as much water. Every kid gets that, and it's perfect for explaining parallel circuits and halving the resistance.

So now take those two 10 foot long, narrow diameter hoses and put them in series in an effort to explain series resistance as a mechanism to halve the flow and every kid's life experience tells them the same amount of water per unit time will come out the 20 foot end i.e. the resistance isn't doubled.

I think I have an explanation for this, but it's nothing that'll end up being simple for kids and it requires me to toss the analogy and look for something else. Anybody willing to take a ***** at using this analogy to explain series/parallel resistance...

 

[ads-ee]

It doesn't work because the analogy is flawed and your explanation (whatever it is) would be incorrect.

It's the cross section of the pipe that the water is flowing through that determines the flow rate for a given water velocity.

The cross section of the inlet to the Y must be 2x or greater than the outlets to ensure the flow doubles (compared to a single outlet). To reduce the flow (resistance) you would have to reduce the size of the hose so the cross section is half, i.e. use pi*r^2.

If what you assume was occurring the you would have to keep increasing the size of a pipe to keep the flow the same as you increase the length.

- - - Updated - - -

Some water velocity and pressure information here

 

[BradtheRad]

Water hydraulics provides many useful analogies for electricity. The site below presents pipe constriction as an analogy for a resistor. Is the effect doubled when we make two constrictions? Not sure.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir.html

 

[chuckey]

That link did not work for me. The analogy that I like is a tank of water with a series of holes drilled down the side. As the depth of water increases (V) the spurt of water coming out of the constant sized holes (R) increases (I).
The reason the hose analogy does not work is that the mains water pressure is very high with a high source impedance, so it is acting more like a constant current generator, compared to the pressure drop of the length of hose. Turning the tap down only increases the source resistance.
Frank

 

[BradtheRad]

That link did not work for me.

I first got an error 'server not responding' when I first visited the hyperphysics website. A minute later it was fixed, apparently.

 

[muesiocks]

It doesn't work because the analogy is flawed and your explanation (whatever it is) would be incorrect.

It's the cross section of the pipe that the water is flowing through that determines the flow rate for a given water velocity.

The cross section of the inlet to the Y must be 2x or greater than the outlets to ensure the flow doubles (compared to a single outlet). To reduce the flow (resistance) you would have to reduce the size of the hose so the cross section is half, i.e. use pi*r^2.

If what you assume was occurring the you would have to keep increasing the size of a pipe to keep the flow the same as you increase the length.

- - - Updated - - -

Some water velocity and pressure information here

Agreed - that's why I specified a narrow hose (although I could have been clearer about that). The intent was that the opening at the faucet was significantly bigger than the sum of hose diameters so that 2x the water would definitely flow when two hoses were attached...

It's always easy to explain parallel circuits and double the current with this analogy. The series circuit and half the current is the sticking point. I'll read the article in your link later today while I'm munching on my lunch.

I'll present my explanation a little later if nobody else chimes in. Again, it's not anything kids would intuitively relate to. I'd also be interested in any other kind of simple analogy if you have one.

 

[D.A.(Tony)Stewart]

This explains the Newtonian analogy flaws and differences why water flow is not the same as current flow.
https://www.slideshare.net/PDiCEOTh...explain-the-laws-of-electricity-and-magnetism

 

[FvM]

I think, criticism of the water analogy is overshooting a bit.

But you can simply consider that flow resistance for a particular fluid, e.g. water is generally not constant but a function of the flow rate. For very low flow rates, a laminar flow with linear pressure drop versus flow rate relation is achieved. For higher flow rates, the pressure drop relation becomes almost quadratic. Skipping the details (e.g. the Reynolds number and other parameters determining flow pressure drop), you'll end up stating that water flow in a tube is effectively never laminar for flow rates of interest.

Assuming turbulent flow and constant input pressure, the flow resistance will drop with flow rate in the series circuit, resulting in a flow somewhere between 1/2 and 1/√2 of the single hose, but not "the same amount of water per unit time".

 

[muesiocks]

My inability to properly use the water in a hose analogy to explain parallel/series resistance can be explained using another analogy - a scuba diving tank and a regulator. The pressure in the tank is obviously enormous and varies over a large range as the tank empties over time, but the air going into the diver's lungs is regulated at a more or less constant rate by regulator at the mouthpiece. That instrument is simply a tiny hole with a cover, and over a vast pressure range in the tank, that little hole provides a more or less constant air flow while breathing in i.e. rather than thinking of the tank as a voltage source, that small hole in the regulator effectively turns the flow into a current source (provided the pressure in the tank is big enough to keep the system saturated). So back to the water analogy. The water pressure is very high and the valve cross-sectional areas to the hoses sum up to less than the cross-sectional area of the water pipe area feeding the valves. So "making a hole" for the water to flow through yields a restricted flow rate (current source) and that explains why a series circuit of identical diameter holes doesn't change the flow rate, but two hoses in parallel does.

Not the sort of thing kids can easily relate to. And so I'm still on the look out for something else intuitive to explain series/parallel resistance and current flow to kids...

 

[FvM]

1. The comparison is flawed because diving regulators are using demand valves. No constant flow rate involved.
2. The flow resistance of compressible fluids (e.g. air) is pressure dependent and IMHO beyond the useful scope of electric analogies. Although I know how to calculate it, it actually doesn't help here.
3. "a series circuit of identical diameter holes doesn't change the flow rate" Are you referring to water or air with this? That statement is correct in neither case, where did you get it? The flow resistance of a nozzle can be roughly calculated from the specific gravity and squared flow speed.

 

[D.A.(Tony)Stewart]

It may be better to use the bucket brigade analogy using water where each person is a resistance and there is only 1 bucket per line. The number of buckets per minute reduces with the number of people in series. Each person also has a capacity limit C so the time delay also increases with the number of people in series for for series and parallel..

But if you have parallel lines flow of water increases with the number of lines where conductance is the inverse of resistance.

 

[muesiocks]

1. The comparison is flawed because diving regulators are using demand valves. No constant flow rate involved.
2. The flow resistance of compressible fluids (e.g. air) is pressure dependent and IMHO beyond the useful scope of electric analogies. Although I know how to calculate it, it actually doesn't help here.
3. "a series circuit of identical diameter holes doesn't change the flow rate" Are you referring to water or air with this? That statement is correct in neither case, where did you get it? The flow resistance of a nozzle can be roughly calculated from the specific gravity and squared flow speed.

1. I got the regulator explanation from a friend of mine who's a (casual) diver, and once it clicked that an analogy to a current source could be drawn then everything kind of fell into place for me. If you're saying his explanation is wrong then I guess I'm back to square one.
3. I was considering only water (incompressible). Regarding your quest for the origin of "a series circuit of identical diameter hoses doesn't change the flow rate", that came from me assuming a current source (of water) would continue doing its thing independent of the length of the hose. I guess you're saying I got that one wrong too :-/...

 

[muesiocks]

It may be better to use the bucket brigade analogy using water where each person is a resistance and there is only 1 bucket per line. The number of buckets per minute reduces with the number of people in series. Each person also has a capacity limit C so the time delay also increases with the number of people in series for for series and parallel..

But if you have parallel lines flow of water increases with the number of lines where conductance is the inverse of resistance.

That works! Thanks .

 

[D.A.(Tony)Stewart]

Re: Explaining current flow in series/parallel circuits to kids using water analogy

However, electrons don't flow like buckets, its more like a domino effect where the apparent speed of the wave
( depending on dielectric constant )...
is a fraction of speed of light but actual change in position of electrons over time in a conductor is very slow.