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Picture of Practical Demonstration of Ohm's Law

When starting an introduction course on electronics one of the very first things you need to do is to teach the very important concepts of Voltage, Current and Resistance. They need to become intuitive, and an analogy that is often used is that with a flow of water, coming down from a reservoire.

Well, I've found that it doesn't always work! Many young students lack that feeling for physical fenomena, so I decided to show it experimentally.

I made this experiment in an after school course about basic electronics, for curious groups of students ranging from 15 to 18 years old. Even if some of the older ones already had this knowledge, doing this personally improved their understanding, which is what's really important when studying scince, otherwise it becomes just a bunch of formulas.

The experiment itself is easy, with a lot of room for error and the results are pretty nice. Also, I've found that seeing the fenomenon helps the students visualize later on.

PS: There's the Python script used to plot the data! Download from the step Analyzing your Data

Step 1: Build the Apparatus

Picture of Build the Apparatus
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The setup is very simple, you just need to hang one of the tanks somewhere and place the other tank below. In order to seal the point in which the tube enters the tank I used some epoxy. Also, making a couple of lines on the top tank (as visible in the photos) ensures that the amount of water trasferred is alway the same.

A possible variation is to constantly measure the mass, either by hanging the top tank from a scale or placing the second tank on a scale, and correlate the time to the mass transferred in that specific iteration. This should work fine, but notice that it introduces a non-constant error, since the average height of the water might not always be at the same distance from the point from which you measure the height.

Once everything is set up, you can add some coloring to the water, just to make it clearly visible. Food coloring works fine.

Remember to close the valve before filling!

ScottR2093 months ago
I applaud your effort on this one! As a tutor, teaching Level 2 Apprenticeships in England I know how difficult some learners can find this "apparently" simple concept.

I personally believe (and have believed since I was a student learning Ohm's law for the first time) that that the analogy (used since time immemorial) of a water tank (the voltage) pushing water (the electrical current) through a resistance (the pipe/tube) virtually useless! For one simple reason! Once the water reaches it's final container the pressure (Voltage) dissipates.

The true key to getting students/learners to understand Ohm's law (first time, every time) is to show them how a "closed/pressurised" hydraulic system works first!
I, as yet, have not found a single student who didn't understand the principles of pressure (Voltage), Flow rate (Current) and the relationship of two with respect to the diameter of the hydraulic pipes (Resistance).

Better still, using small hobbyist/experimental kits, small and easily alteraterable hydraulic circuits can be built and tested. Thereby making it easy for the student to investigate the difference between series/parallel hydraulic systems and to then be able to fully understand exactly how Ohm's law works!
Indeed, whenever I have used this method of teaching Ohm's law to teach a group of students I have never then heard a student talk about "Voltage through a component". The sure fire giveaway that the learner hasn't fully grasped the concept.


Using hydraulics is brilliant! Do you happen to have directions for how to make that demo?
I wish I did! I've got a brand new class full of apprentices arriving in September and I'm desperately trying to find something like a small 12 or 24 volt hydraulic pump that I can then use to begin designing something. So far, unfortunately I've come up empty handed.

I would have thought that there were miniature style hydraulic kits for children just as there are electronics and chemistry lab/learning kits for kids.

Maybe a gap in the market there!
3PO2 months ago
Thank you for the nice Ohm model. I always had problems with the basics of electricity because different teachers explain it differently. My question is this: You say the voltage is analog to the height of the canister but my inside feeling is that the voltage should be analog to pressure inside the small section tube rather than the height of the canister. The pressure in the tube depends mainly on the height of the water in the canister not on the height of the canister. Does this make any sense to you ?
DanM173 months ago
Unfortunately, in contrast to voltage drop in case of Ohm's law, pressure drop generally DOES NOT increase proportionally with flow rate ("current") in a tube - and your data prove it, despite the attempt for linearization. Otherwise, another simple law would exist for liquid flow in conduits in school textbooks.
But do not give up, there i a chance... Proportional relation between pressure drop (height) and flow rate works at LAMINAR flow conditions at low velocities and small tube diameter (so it is contraproductive to try bigger one!). Briefly: if simple shear friction is the source of pressure drop, linear relation prevails, if the energy is consumed to create eddies/turbulences, the relation is quadratic.
There is much more to study, please use Google to search laminar vs. turbulent flow condition, critical Reynolds number, and pressure drop calculation. In fact, there IS a "law" describing linear pressure drop/flow rate relation - but it has very limited use for conditions I mentined and you should work at - see Hagen-Poiseuille equation.
DanM17 DanM173 months ago
I have to correct or amend my previous answer because of additional dissimilarity, which could show at certain experimental conditions.
According to Ohm's law, current must increase to infinity if resistance approaches zero. That is not the case if you pluck off the hose from the bottle (and still there is "voltage" - water level in the bottle), the flow rate will stay quite low compared to "infinity". This is due to the fact that for fluid flow, there is not only energy dissipation, but also energy conversion - pressure at the bottom of the bottle is converted into velocity according to Torricelli equation, which is a special case of Bernoulli equation.
In my opinion, it is important to make experiments attractive, but without sacrificing scientific correctness. Using similarity is OK, but without ironing unwanted data - this is the difference between "just-a-show" and "true science" (no matter how simple the experiment is), or mnemotechnics and understanding. So it is important to comment/admit the limitations of the results - keep in mind that in physics, "electricity" will be followed by "hydrodynamics" after couple of semesters :-)
Kate Russell3 months ago
Thank you so much for creating a simple, memorable explanation of something has has evaded me for a while! Now, how do watts figure in?
Ohm's Law describes the relationship between Voltage, Current and Resistance.
Watt's Law describes the relationship, between Power and any two of the variables above. "Power is as easy as Pie." P= I * E, I squared * R, or E squared / R.

Thank you!
Pothuset3 months ago
This is how I explain the ratio between U, R and I in Ohm's law:
Forholdet mellem Volt, Ampere og Ohm.jpg
wclapie Pothuset3 months ago
Fantastic! "Voltage is the pressure that makes it go, electricity, electricity...."
SeanB103 months ago
I'll just leave this here...
http://lcamtuf.coredump.cx/electronics/
wclapie SeanB103 months ago
I studied a bit of physics and electron theory. Apparently I missed something. This is far beyond anything I studied.
necchiom3 months ago
Bravo Matias!!
jmacuk3 months ago
I like your method of showing flow rate as mass/time: too often my students confuse that with flow speed. DanM17 is correct that there are limits to modeling electric current with flowing water in tubes. Did you test with minimal length tubing, that is, straight drop with no loops? Try the same length of tube stretched straight, at various angles, to create different heights. Also, if practical, rig multiple reservoirs with different diameter tubes: if you can achieve laminar flow, it will model different resistances. If not, you'll have a great counterintuitive model to puzzle your students with : )
SteveDay723 months ago
My theory for the non-linearity is this:

The bottom bottle is filling as the top bottom empties - this is the equivalent of raising the ground voltage, thus reducing the potential difference of V as time progresses.

To confirm this simply remove the bottom bottle and let the water flow straight onto the ground, do this while re-timing and you should see a difference.
zanod3 months ago
Did you notice at 5:01 in the video, the HUGE current when the resistance was reduced to zero? ;-)
papa-ralph3 months ago
builderat66 --- You know, of course, that there are two types of current flow. One is ELECTRON CURRENT FLOW ( - to +) and the second is CONVENTIONAL CURRENT FLOW. (+ to -) It makes NO difference which one you use --- the results will come out the same.
Very EXCELLENT Instructable!!!!
builderat663 months ago
Unfortunately he's teaching incorrectly. East shows electron flow which is the water but current flow is in the opposite direction. It's been my experience that students have a problem adapting and understanding current flow when it is applied to semiconductors.


Meh. Most entry level physics are taught "incorrectly".
F=ma. Except when it is not.
E=mc^2. Except when it is not.

But the concepts are close enough for most cases. And more importantly, give most people a solid understanding of the universe that, otherwise, seems like magic.

"Except when it's not" is what makes more advanced topics interesting to those that want to learn more.
pixel tamer3 months ago
Non linearity can come from a lot of places.
1) Altitude is a factor. Atmospheric pressure is non linear for height. It isn't much. You could have been measuring an approaching weather front.
2) Another consideration is that in order to maintain a constant potential difference between the two liquid levels, both containers must be the same diameter and be a constant diameter from top to bottom. If the containers are of a different diameters, the difference in height will vary with time and will affect the flow rate, which will introduce errors.
3) There is also the turbulence of the liquid in the tube due to bends, the straighter or more constant the better. Using a long very small diameter tube was smart.
Your results were very good. Great Job! Better than the average college science lab.
mf703 months ago
How wonderful. There are some professional designers of USB charging cables that need to watch this. (They "discovered" that a larger gauge conductor in the cable or raising the base voltage charged a phone at a faster rate. https://www.androidauthority.com/fastest-charging-cables-793649/ )
Strato_caster3 months ago
I love the analogy to ohms law. Great explaination. I voted as well.
ViviY3 months ago
Nice and simple. Change for correct spelling for words phenomenon and chronometer in the introduction.
NicolasL623 months ago
Great idea ! Voted ;)
MatiasMamo (author)  NicolasL623 months ago
Thank you! Really appreciate the support