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 Tutorial EE101 - Part 1: Terms and Units filter_list Linear Mode Threaded Mode View a Printable Version Author Message
EE101 - Part 1: Terms and Units #1
So, I'm certain all of you have looked at the back of a power adapter or a breaker box, or the fuse panel in your car or whatever by now. Likewise, I'm certain you've heard of a Watt, Volt, Ampere, etc. Those are pretty common units, but many people have absolutely no idea what they are or mean. In this tutorial, we're going to define and look at the relationships between the following units
• Ampere
• Ohm
• Volt
• Watt
• Joule

Now, let's start off pretty simple. What is power? Many of you would likely respond with "electricity", which is somewhat correct but not really. Power is the capacity to do useful work (keep this in mind for the end). For the scope of this thread, we'll be focusing only on electrical power. Let's start off with the easy one.

Current.
We'll stick with the old high school analogy of a ball you're holding in the air. That ball always has some energy, but as long as it's stored as potential energy it can't do any useful work, and therefore it has no power. Once you release the ball, you convert that stored potential energy into kinetic energy. The same concept is true for current. Current is simply the electrical equivalent of kinetic energy. It's simply the measure of how much electricity is flowing. This is measured in Amperes (amps). The amp itself is a derived unit, where the following is true:
Code:
`A=C/s`
The C there is not capacitance (don't even go there). It's a Coulomb, which is the base unit for electrical charge. What this means is that the Ampere is the amount of charge flowing through a conductor per second. Because of this, you can think about the amp as the velocity of the falling ball. Thus, we call this current

Now that we understand what the electrical form of the moving ball is, let's talk about what makes that ball come to a stop. In the physical case, this is friction. You may think it's impact, but think, if the ball impacted feathers or if it impacted iron, it still has the same energy. The difference is that the iron has more internal stress and binding. We're going to loosely say this is due to friction. The electrical (DC) equivalent to this, is resistance. I shouldn't really have to explain this one too much more, so I'm just going to give out one more example. Let's say we're holding the very same ball in the air again, but rather than dropping it through air or a vacuum we're going to drop it through water. The same energy is released, but due to friction (ignoring surface tension), we end up with slower movement. Thus, resistance resists the flow of current. This is measured in Ohms (I will not explain that unit here). A key thing to note here. Just because the ball moved more slowly does not mean it had less energy. Remember that the Ampere is simply the amount of charge that moves per second. If we make it take more seconds, we just change the current, NOT the energy.

Now, let's get into what some consider a more abstract unit (it isn't): voltage. I know all of you understand what the numbers are, and that certain numbers are dangerous, but do you know why? Moving back to our ball example, but with a slight shift. We know that the ball has energy, and we know it wants to release it because we can feel that it's heavy. Of course, in the physical world, this weight is an effect of gravity. Even though it may be disconnected in your mind from the ball, it is still acting on it with a force. This is what voltage is, it's the measure of how much a given charge wants to be released. This is basic entropy, if we increase the entropy on our battery, we're just increasing the voltage.

Now, how do they relate? well to put it simply, by Ohm's law. Known more simply to most as:
Code:
`I=V/R`
Deconstructed, this is simply that the current is the ratio between the voltage and resistance. Knowing this, we can make a very important conclusion. You can only change two of the 3 factors that determine the throughput of a circuit (notice how I didn't say power). We can go over this in a later tutorial if you want, or you can wiki it.

I want to get to the fun stuff....derrived units. Anybody that has ever used a vape, or read their electrical meter is familiar with a Watt (W) or KiloWatt-Hour (kWh). What does any of this stuff mean? Well, a Watt is a unit of power. In an electrical sense, we get
Code:
`W=IV`
or in English, the Current multiplied by the Voltage

Now, what about that pesky kW/h? Let's come back to that one in a second. First let's define what we're even talking about here.

We know that the Watt is the power we have, but power isn't too terribly useful without something to supply it to. This is why putting 2 9v batteries opposed to each other creates a heater. All of their energy must go to thermal radiation because there is nothing to use the power. This is of course, known as a short circuit. If you don't have a short circuit, you are doing work, and that work is using energy.
We know that the work done on something is the force (power) over the distance, but electrical distance doesn't manifest itself here, that's manifested in the form of resistance. So what's our distance measurement? Time.

Just as current is a factor over time, so is the common electrical measurement for houses. We simply measure how many kiloWatts (1,000 Watts) go through the circuit in one hour. There you go, but that's actually not the end of it. The kWh isn't the universal energy unit, that's actually the Joule. I'll let you do the math for figuring it out, but 1J = 1W/s.

Anyways, I want to keep this short, and I'm getting progressively more drunk as I write this, so enjoy. Let me know what you want to learn about next.

RE: EE101 - Part 1: Terms and Units #2
Thanks for the lesson! I've learned about this in science classes and they've lacked a good explaination. The common man sees many units of measurement for one thing "electricity" and gets confused. I see watts in commercial advertisements, amps in diy tutorials, but this really helped me understand the different between them. In school we didn't focus on them, just conversions and stuff to pass exams.

RE: EE101 - Part 1: Terms and Units #3
(08-04-2018, 06:34 AM)skrtja Wrote: Thanks for the lesson! I've learned about this in science classes and they've lacked a good explaination. The common man sees many units of measurement for one thing "electricity" and gets confused. I see watts in commercial advertisements, amps in diy tutorials, but this really helped me understand the different between them. In school we didn't focus on them, just conversions and stuff to pass exams.

Commercial products tend to use Watts for a number of reasons:

1. the current draw is not stable. This can happen when the power output is fixed, meaning that either the voltage or the resistance must change. You see this sort of thing in light bulbs, which have a fixed power output (usually 60W) and a fixed voltage (usually 120V). Since power is volts * amps, we know the resistance needs to change. You can see this change in resistance because the bulb glows as the resistive wire heats up and gets more resistance.
2. it's a universal unit of work. You may have noticed that european cars aren't rated in horsepower anymore, they're rated in kilowatts. That's watts as a mechanical unit, which is pretty easy to calculate. See image below note that in that, P is power in Watts, W is work, F is force, R is distance, and v is velocity

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