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T³: Adventures in Science – Ohm's Law

Let's look at the resistor and how it affects current and voltage in a circuit. Here's a hint: if you know two of either voltage, current or resistance, you can solve for the third.

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Analyzing voltage and current in a circuit is a great place to start to understand what that circuit is doing. In this episode of "Adventures in Science," we introduce the resistor and use it to help demonstrate Ohm's Law.

This interesting law of physics was named after Georg Ohm, and states that the current between two points is directly proportional to the voltage across those two points:

Ohm's Law

With a little bit of algebra, we can move the variables around and arrive at the more memorable:

Redefined Ohm's Law

In the video, we demonstrate voltage and current in a fluid-based circuit, and show how a resistor acts like a piece of steel wool used to restrict the flow of water. We also construct a real circuit using a resistor, measure the voltage and current, and then calculate the resistance using Ohm's Law.

The information in the video was inspired by the following tutorials:

Voltage, Current, Resistance, and Ohm's Law

Learn about Ohm's Law, one of the most fundamental equations in all electrical engineering.

Resistors

A tutorial on all things resistors. What is a resistor, how do they behave in parallel/series, decoding the resistor color codes, and resistor applications.

How to Use a Multimeter

Learn the basics of using a multimeter to measure continuity, voltage, resistance and current.

I've gotten some good feedback on the previous tutorials, so thank you to everyone who commented! For this video, I put the "intuitive" demo first before touching any math, so hopefully that helps to make things clearer for students. What can be improved for episode 4?


Comments 7 comments

  • First, thanks for showing the number of electrons needed for an Ampere of current, but you really don't need to know "scientific notation" to get started in electronics, so you might write out the number so that folks get the idea that it's "really big". (You might also say something like "Don't be scared by this number, we are not going to put it on an exam question!"

    Next, the "zig-zag" line is the standard in the U.S. Some parts of the world use a rectangle. (That's the "default" for Buzzard, er, Eagle, and one of the more minor reasons I strongly dislike that program!)

    As to the measured value being slightly different than the calculated value, there's the accuracy of the meter. If you look up the value for your particular meter, I would guess that it's around 1%. (There are meters that are much more accurate, but they have much bigger prices, and need to be calibrated regularly. The calibration alone likely costs more than your meter.) And also, although they're gotten a lot better in recent decades, power supply built-in meters are notoriously inaccurate. (Try this experiment: Set your DMM to "current", and put it in series with the resistor. I bet you'll get a number that's at least a couple of "counts" different from the built-in meter.) There's also a saying in laboratory work: if the measured value exactly matches the predicted value, look for what you did wrong.

    At some future point you're going to need to introduce the concept of tolerance. I can't see the resistor in the video well enough to be sure, but I think it's 5%, so 101 ohms measured (or calculated) for a 100 ohm marked is well within the allowable range of 95 to 105 ohms. (BTW, when I started in electronics, 20% resistors were the norm.) Off on a tangent, the ones you've scraped off the covering for show the spiral cut where the manufacturer actually "trimmed" the resistor to the desired value.

    • Great feedback, as always :) Good point on the scientific notion. I assumed it would be understood, but I don't think I learned about it until high-school, so it's probably not great for younger crowds.

      I always took the "rectangle" symbol to mean some kind of load, and that could be resistive, inductive, or capacitive, whereas the "zig-zag" specifically meant resistor. I've seen the rectangle used as resistor in programs, books, etc., though, so I can see how it could mean both. If I take the series beyond these few episodes, I'll likely do one on just the resistor: how it's made, how to read the colors, tolerance, etc.

      The readings on the supply are definitely inaccurate, and I was quite surprised to be within 1% of the measured resistor value. It was just a ballpark figure that worked well enough for the demonstration. Once again, I really would like to re-do the "how to use a multimeter" video to be inline with these episodes (and talk about things like accuracy).

      • You know, I'm also surprised that your 5% resistor was within 1%. I always thought that resistor manufacturers binned their production, marking the ones that fall within the tighter tolerances with the tighter value. Based off that assumption (and, yes, I know what assuming does...) I would have expected the 5% 100ohm resistor to actually be either 95-98 or 102-105 ohms. Either that is how it used to happen or has always been an incorrect assumption.

      • The zig-zag symbol is the ANSI (US) style, and the open rectangle symbol is the IEC (International) style. See http://www.resistorguide.com/resistor-symbols/

        The ANSI style should be depreciated, but I prefer the look of it over the IEC style. IMHO the IEC style just looks unremarkable and boring where the ANSI style looks distinctive and artistic. But that might be my American bias talking...

        • I agree. The basic schematic part symbols look very descriptive of their corresponding parts: a capacitor symbol looks like two parallel plates, and an inductor symbol looks like a coil of wire, while the ANSI zig-zag symbol looks like it would impede the flow of current with the extra distance and tight turns that the current would need to take while flowing through it. It makes sense, and looks good.

          On the other hand, the IEC symbol is a mysterious black box, with no hint to its function or construction. It makes sense to use a box for a complex integrated circuit where a pictorial representation of its function would be impossible or messy, but using a plain box for a resistor is unimaginative and uninspiring.

          Member #134773, who never seems to miss an opportunity to bash Eagle says:

          (That’s the “default” for Buzzard, er, Eagle, and one of the more minor reasons I strongly dislike that program!)

          No, that's not a "default" for Eagle, it's a function of the library you chose to use. There are many different libraries that come with Eagle, and many more that can be downloaded, like Sparkfun's own library, which has a good collection of common parts and symbols, including ANSI style resistors. Eagle has it's shortcomings, but I think it's a stretch to call this one of them.

          • I've been thinking about ShapeShifter's comment on my comment:

            Member #134773, who never seems to miss an opportunity to bash Eagle

            and have realized that I can actually praise Buzzard, er, Eagle: It beats the living daylights out of doing PC board layout with rubylith and a x X-acto knife. (Guess how I know!) (I was a bit amazed when just now I discovered you can still buy rubylith.)

          • First, I only bash Buzzard, er, Eagle, because it so richly deserves to be bashed. (I've used what I consider to be far better products that cost less. And yes, I've used both the "free" and "professional" versions of Buzzard, er, Eagle.)

            Well, as for "which library", I based that comment on what came with Buzzard, er, Eagle, when I downloaded and installed it from their website. To me, that IS a default. At that point, I saw absolutely no point in installing the library from SparkFun as for the project at hand involved a whopping ZERO parts from SparkFun. (If they'd offered me a choice between an IEC library and an ANSI library, rather than shoving IEC down my throat, I'd likely have selected ANSI, and that would have eliminated one of the more minor irritants about their product, but would have done nothing to make it any more "intuitive".

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