Kirchhoff's junction rule
- [Voiceover] In this video, we're gonna 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 the elements, that are used to make up circuits. So, for an example, 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's some circuit components, that are laying out on the tabletop, and we're gonna 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. That is called a node. Node is the word for junction, mean 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. That's what a node is. We're gonna 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. That's our second node. Down below, same sort of thing. I see an ideal wire, connecting the resistors, and the source, so I can color that in. That's node number three. This circuit has three nodes. Now what's connecting the nodes? The thing that's connecting the nodes, is called a branch. A branch is the same things 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 resister 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. This circuit has one, two, three, four elements in it, and it also means, it has four branches. Four branches. 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. First thing we're gonna do, again, just to repeat the process, we're gonna 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, there's a junction, between two resistors, so that's our fourth node. Finally, we have this node here, connecting these four elements, with one node. This is sometimes called a distributed, 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. If we count up the elements, that will tell us how many branches there are. One element, two, three, four, five, six, seven? Seven elements. Alright, there's our two key words. Elements and nodes. Now, I'm gonna quickly move again, down. Bring in another circuit here. We're gonna talk about the idea of a mesh. The other thing we're gonna 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. This little space right here, that little gap, is called a mesh. That's what that word comes from. We're gonna find the meshes of our circuit. What we look for, here's the branches, and the mesh, it 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. This circuit has three meshes. A mesh is a loop, and we can have other kinds of loops, too. They don't have to be just the ones that fill the windows. 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. This is a loop. I could 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. Finally, if I have a sharp eye, there's one more loop in this circuit. Let's just start right here, and it actually goes all the way around the outside. 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. 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. Alright. 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. That'll do it. There's our new vocabulary, for talkin' about circuits.