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Voltage, Current, Power, and Energy


If you’re first starting to learn about basic circuits or basic electronics, it’s best to take a brief few minutes to understand the basics of electricity and some of the fundamental terms. We’ve created multiple tutorials that talk about the underlying physics principles of their operation but that’s not really necessary to start working with circuits. Of course, if you have the time, we recommend you going over those tutorials as well to give you a better intuitive understanding.

But first things first, we need to understand what voltage and current is. Throughout all of the circuits courses you take, the majority of your efforts will be focused on finding either the voltage, the current, or both, of circuits. Occasionally, you’ll be asked to find the power as well and we’ll touch on energy just to clarify its role as well. Let’s break these down at a high level:

Summary of Terms

  • Voltage - the electric potential between one place and another. How much the electricity wants to move from one point to another. Measured in volts.
  • Current - the current flow from one point to another, literally based on how many electrons are moving per second. Measured in amps
  • Power - work that is being done per second. In circuits, this usually means the amount of heat given off by a circuit. Measured in watts, or joules per second.
  • Energy - total amount of work done. There is no time component for this, which is the differentiator between power and energy. Measured in joules. These are clarified more later in this tutorial.

Voltage and Current

For decades, the most common examples to illustrate how electricity works and the difference between voltage and current is by using water as an example. This is because, while not perfect, it’s surprisingly similar and quite effective.

Imagine voltage is like water in a lake at the top of a hill. It wants to flow downhill, and if it has the opportunity to do so, it will. This desire of the water to flow downhill is like voltage, it doesn’t represent movement and, in itself, is static. If water does begin to flow, this flow of water is the current. And the size of the channel that leads from the top of the hill to the bottom of the hill is the resistance. These three items are all directly related and understanding that relationship is both a fundamental portion of circuit analysis and also the topic of our next tutorial.

To expand on that analogy, though, you’ll notice that with voltage, it doesn’t matter how high that hill is - if there’s no opening for the water to flow, it’ll just sit there. If the hill is a mountain three miles high, there’s a lot of potential there, but still no flow unless there’s a path or pipe. That being said, a lake three miles high with a pipe will be pushing a lot more water through that pipe than a lake 3 feet high with the same size pipe. This is how the voltage (the potential) affects the current (the flow). Keeping the resistance (the size of the pipe) the same, you can increase current by increasing the voltage.

Similarly, if you increase the size of the pipe (decrease the resistance), without changing the height of the potential, you’ll still get more flow. Conversely, if you decrease the size of the pipe (increase the resistance), you’ll get less flow. This is how the resistance (the pipe size) affects the current (the flow). Typically, in a circuit, you can control the voltage and the resistance, so the height of the potential and the pipe size, to give you the flow you want. Resistance is easily changed with something like the high-power rheostats (also known sometimes as a variable resistor) from our Friend of CircuitBread Ohmite.

Example of a High-Power Resistor

There are many types of power resistors but this one goes up to 1000W of power dissipation.

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One final thing about voltage

Note that difference between one potential and another is relative. For example, the top of the hill is obviously higher than the bottom of the hill. But what if we dug a hole at the bottom of the hill and made the bottom even lower? Or what if there were a mountain next to the hill? The hill is lower than the mountain, so there is a potential between the mountain and the hill, much as the bottom of the hill is a higher potential than a hole dug at the bottom. It is the same with voltage - when we talk about voltage, we’re talking about the electric potential between two points in relation to each other. We typically assume that the lowest point is “0” or what we call “ground” as a reference. But then sometimes you get negative voltages, which just means that the electric potential at that point is below what we established as our “ground” potential. It may seem odd to do that sometimes but once you get some experience with circuits and electricity, negative voltages make a lot of sense. It makes even more sense when you realize that, since everything is relative, you can flip your perspective and invert the sign of the voltage. It may be 10 volts from the top to the bottom, but it’s also -10 volts from the bottom to the top, so vab = -vba. This will come in handy on occasion.

Power Versus Energy

Let’s focus on power and energy again. Saying that the relationship between power and energy is only a matter of the time component is unsatisfying and not very clear. Let’s do a quick example that may make things simpler. Imagine you need to lift a box 10 feet. You can throw it directly up in 1 second or lift it slowly over 10 seconds. The amount of energy needed to move the box from 0 to 10 feet is the same but the first option, throwing it directly upwards requires 10 times the power as lifting it slowly. In the vast majority of circuits applications and problems, we only care about power and ignore energy, but when discussing energy sources like batteries and capacitors, this distinction becomes critical.

“Batteries are more energy dense than capacitors but capacitors are more power dense than batteries. Expanding on the box example and using some arbitrarily chosen numbers, what this means is that a capacitor can lift a box 100 feet in the air in one second, while a battery the same physical size can only lift a box 10 feet in the air in one second. But, given equal physical sizes, the battery can lift a box a total of 5,000 feet before running out of energy but a capacitor can lift a box a total of 300 feet before running out of energy.”

Electric power, mathematically, is simply current times voltage, so is a factor of both flow and potential. Going back to the water analogy, you can have a small flow from a great height produce a lot of power. Or you can have a very large flow from a relatively low height create a lot of power. But if you have too little of one or the other, there isn’t much power. Much like a raindrop falling won’t create a usable amount of power, a huge voltage without any current won’t produce much power. Or water spilling out of a cup on a table may have flow but there’s no potential behind it to do any work. It’s the combination that creates the power.

When dealing with the theoretical side of circuits, power is often ignored, particularly at first. As a power resistor manufacturer, Ohmite specializes in resistors of all power levels. If you check out their website, you'll see one portion that is dedicated to power resistors of all types, but if you use their part selection filter, you can see that the amount of power a load can take varies significantly. Some of these are ideal for high power surges - lots of current at high voltages - for short periods of time, while smaller resistors can handle comparatively smaller loads.

This should lay the foundation of understanding the basic terms needed to start solving circuits. Next, let’s learn about the relationship between voltage, current, and resistance with Ohm’s Law.

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