# Practical Demonstration of Ohm's Law

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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

### Supplies:

• Two containers, at least one transparent
• Some sort of tubing of constant legth
• A cronometer
• Measuring tape
• Water, preferably colored

## Step 1: Build the Apparatus

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!

## Step 2: Measure

I suggest to measure the height after filling, as the system might have to adjust a bit to the weight. To measure the mass I used a hanging scale, filled the tank to the empty line, weighted, and repeated for the full line.

Tip:fill the tank over the full line, so you can open the valve and then start taking the time once the level reaches the line.

Once everything is ready, open the valve, take the time, and wait.

Take notes and repeat

## Step 3: Analyzing Your Data

Clearly you can analyze the data in several different ways, depending on your/your students' knowledge. The easiest is by hand, which in this case is still possible, and can also be used to teach how to graph and make predictions starting from a graph.

I used a python script which you can easily modify to work on your data, just replace the values for heights, times and mass (lines 10, 11, 12).

This script makes a linear fit, a quadratic fit and plots the data. The last five lines make a height prediction based on the time (in seconds) set in line 39 (variable time_set).

## Step 4: Interpretation

The data points that I collected show a kind of linear behavior. Some of the variance is for sure due to my imprecise measures, but I think that the tube also had a great influence on non linearity. If I had to do it again, I would use a tube with a larger diameter, like 10mm ID or more. That should ensure that the interaction between the flow and the walls of the tube becomes less influent.

The python script gives you the slope of the curve, which is the "resistance" of the system.

## Step 5: Testing Its Predictive Power

A nice thing you can do is show that your model can predict the behaviour of the system. In this case I desided that I wanted the tank to be empty in 5 minutes, and used that and the slope found before to calculate at what height the tank needed to be.

The python script does that too, you just need to replace the time (300 seconds) with the time you want. Last thing, I don't expect the predicion to work with times that are too small.

When testing the prediction, I got a time of 5:04 minutes, way better than I expected, to be honest!

## Step 6: Send Me Something!

Send me photos, videos, plots, data, questions, whatever! I'd really love to know that someone was inspired and made the experiment!

If you send me something and want to be featured in one of my videos on YouTube, please let me know.

Also, are there any mistakes?

This is an entry in the
Classroom Science Contest

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## 24 Discussions

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 ?

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.

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 :-)

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?

2 replies

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.

This is how I explain the ratio between U, R and I in Ohm's law:

Fantastic! "Voltage is the pressure that makes it go, electricity, electricity...."

I studied a bit of physics and electron theory. Apparently I missed something. This is far beyond anything I studied.

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.

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 : )

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.

Did you notice at 5:01 in the video, the HUGE current when the resistance was reduced to zero? ;-)

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!!!!

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.

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.