Circuit terminology | Circuit analysis | Electrical engineering | Khan Academy

In this video, we're going to talk about some terminology that we use to talk about how circuits are put together. In previous videos, we've talked about the components or elements that are used to make up circuits. So, for example, a resistor, capacitor, and inductor are circuit components. We also call those elements. In addition, we have some sources, like a voltage source or, for example, a current source. Those would be the components or elements of a circuit.

Now, we're going to start assembling these things into circuits, and we need a few more words to talk about. Here are some circuit components that are laying out on the tabletop, and we're going to connect those up with a wire. For example, I could connect this one to this one with this yellow ideal wire. An ideal wire has zero resistance, and it's perfect everywhere. This forms a junction between these two components, and that is called a node.

Node is the word for junction, meaning the same thing. That's what a node is. If I connect up these two other components, I still have one node because I have one junction that's the same voltage everywhere. So, that's what a node is. We're going to go over to this circuit here, and we'll identify the nodes. This is a little more normal-looking tidy circuit. Here's a junction right here between a resistor and this voltage source, so that's one node.

If I move over here, I see resistors connected together by ideal wires, so that forms one single node like that. So, that's our second node. And down below, same sort of thing: I see an ideal wire connecting the resistors and the source. So, I can color that in, and that's node number three. So, this circuit has three nodes.

Now, what's connecting the nodes? The thing that's connecting the nodes is called a branch, and a branch is the same thing as an element. We'll count the branches or elements in this thing. This voltage source connects the third node to the first node, so that's one element. This resistor connects node one and node two, so that's the second branch. This resistor connects node two and node three, so there's the third branch.

And this resistor with a separate current also connects those two nodes, so that's the fourth branch. So, this circuit has one, two, three, four elements in it, and it also means it has four branches. Four branches. So, that's what a branch and a node are. I'm gonna move the picture over a little bit so that we can do this again on a little more fancy circuit.

So, first thing we're going to do again, just to repeat the process, we're going to count the nodes. Here's a junction between a resistor and a source. Here's three resistors connected by a perfect wire, so that's the second node. Here we find three more resistors connected by a perfect wire; there's the third. Down here, we have a junction between two resistors, so that's our fourth node. And finally, we have this node here connecting these four elements with one node.

This is sometimes called a distributed node when it's all spread out on the page like that, but it's still just one node. So, this circuit has five nodes, and if we count up the elements, that tells us how many branches there are: one element, two, three, four, five, six, seven. Seven elements. All right, there's our two keywords: elements and nodes.

Now, I'm going to quickly move again down, bring in another circuit here, and we're going to talk about the idea of a mesh. The other thing we're going to talk about is the word loop. The word mesh comes from screen doors or screens that you put on your windows to keep the bugs out. If I draw a screen like this, this is what it looks like—a bunch of crossing wires—and this little space right here, that little gap, is called a mesh. That's what that word comes from.

So, we're going to find the meshes of our circuit, and what we look for are the branches. The mesh is a kind of a loop that fills up this open space. This circuit has one mesh, two mesh, three meshes. That's how that looks. To draw a mesh, you start on a node, you go through elements until you come back to where you started. That's how we did those three, and they fill the open windows of the circuit. So, this circuit has three meshes.

Now that a mesh is a loop, we can have other kinds of loops too; they don't have to be just the ones that fill the windows. So, in general, this circuit has other loops, and we'll identify some of those. Let's just start at one of these nodes here and go around like that, and this is a loop. I can draw other loops in here; we'll make them all different colors. There's a loop. If I start right here, I can draw a loop through these elements.

And finally, if I have a sharp eye, there's one more loop in this circuit. If I start, let's just start right here, and it actually goes all the way around the outside. So, this circuit has three—actually, if I add them together, this has three loops that I drew here plus the three loops that were the meshes. So, this circuit has six loops.

Circuits always have a lot of loops, and so usually we don't talk about these more often. It's more organized and straightforward to talk about how many meshes are in a circuit. All right, so that does it for this video. We got mesh and loop. We talked about components and elements, and we finished up with the idea also of nodes and branches. So, that'll do it. There's our new vocabulary for talking about circuits.