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## Class 12 Physics (India)

### Course: Class 12 Physics (India) > Unit 3

Lesson 7: Kirchhoff's junction rule# Circuit terminology

Definition of circuit terms: element, component, node, branch, mesh, and loop . Created by Willy McAllister.

## Want to join the conversation?

- what is the difference between a mesh and a loop ?(17 votes)
- A loop is
**any**closed path around the circuit. You are allowed to visit each element only one time (you are not allowed to create a figure-8 by going through an element twice). A mesh is a loop, with the restriction that a mesh contains no other loops. We focus on meshes because the number of meshes is exactly right for getting the proper number of independent equations.(28 votes)

- Which form of current is more dangerous? AC or DC?

I have heard several contradictory things relating to this. Here is what I have heard:

"AC has a higher voltage and thus higher number of amps. Even 1 amp can kill you. AC is more dangerous."

"DC typically has a lower voltage and its amps are usually very small(milli or even microamps). However DC often leads to longer exposure and thus is more dangerous."

"AC is less dangerous because its sine wave makes your muscles twitch and force you to let go."

"DC is more dangerous because even though it takes more volts and amps to kill you than AC, it makes your muscles continuously contract and you might not be able to let go of the DC circuit."

So which one is more dangerous, AC or DC? I probably wouldn't have to worry about handling a DC circuit even at 12-24 volts because the amps are so small. I might get pain but I doubt I will get continuous contraction. I mean the resistance of human skin is anywhere from 1 million to 15 million ohms according to some of the sources I got this AC vs DC info from. And with amps that are already very small, that much resistance means practically nothing gets through me.(10 votes) - How do we know where to start and end the loop?(8 votes)
- Loops don't have a start or end, they are just loops. When you draw a loop on a circuit, you get to choose where to put your pencil down first. When I do it, my habit is to start in the lower left and draw in the clockwise direction. It's not a rule, just a habit. When you get to the circuit analysis method called the Mesh Current Method, you will learn how to "travel around a mesh". (A mesh is a kind of loop.) With this method, you have to select a starting point (the "start of the loop") and direction to travel. Again, my habit is the same: start at a point near the lower left and go around clockwise.(10 votes)

- At6:31the first loop is illustrated as extending to the end of the second mesh, but I don't understand why that is. My intuition is that there would only be 3 loops, but we were told to take the 3 loops given to us and add it to the number of mesh to get the loops. Why is this? It doesn't seem intuitive to understand, and I don't feel as if I was given really any information regarding what a loop is or even how to adequately count them. Thanks!(3 votes)
- Think of the meshes as minimal examples of loops, in the sense that they don't "contain" smaller loops inside of them. I think you may see that the meshes, being already covered, weren't also listed as loops until the end, where we we're told to count those three meshes in as well when counting loops.(7 votes)

- So in an ideal wire, the current is always infinite if the voltage is non-zero? (Ohm's law)(2 votes)
- The "ideal wire" is an abstract model that means the voltage on the wires is everywhere the same. The voltage difference between different points of an ideal wire is
**never**non-zero.(7 votes)

- What is the difference between a mesh and a loop(3 votes)
- A Loop is
**any**closed path around a circuit (not counting figure-8 paths).

A Mesh is a kind of loop. It is a little simpler. It has the property that it goes around the 'open windows' of a circuit.

Why do we make this distinction? Mesh vs Loop comes up in the section on DC Circuit Analysis. There is an important analysis technique called the Mesh Current Method, based specifically on the definition of a Mesh.

You will get to that in a little while. For the moment just remember these two words have similar meanings, and the small difference will become important later.(4 votes)

- At around 5 minutes, Willy starts the discussion about "mesh" and "loop". What is the practicality of knowing the number of meshes or loops?(2 votes)
- When we are doing mesh or loop analysis we need to have a certain number of equations to actually solve the system. It turns out in that you form a linear system with the loop/mesh equations and then you solve it. Usually we do this with linear algebra and matrices, but it can be done by hand with Gaussian elimination. You need as many linear equations as there are independent loops to solve the system and thus knowing the number of meshes (I believe the number of meshes = the number of independent loops) gives you one such system.

Have a look here:

https://www.khanacademy.org/science/electrical-engineering/ee-circuit-analysis-topic/ee-dc-circuit-analysis/a/ee-mesh-current-method

And in the two previous videos and the one following video to get more on this subject.(5 votes)

- what is the difference between nodes and branches?(1 vote)
- The way i see it, do not confused a "wire" with a "branch". A wire is arbitrary, it is basically a material that we need to make up a circuit. That said, once we have laid out how our circuit will look like, a branch is any path in the circuit that has a node at each end; a path connecting nodes.(4 votes)

- Is a voltage source a branch too ? if , it is i am confused about it because the teacher didn't said its the first branch .(1 vote)
- Yes, Problem 2 is a branch example and under "explain", you can see the voltage source is a branch too. Also, the definition of branch says, "A branch is an element (resistor, capacitor, source, etc.)" so a voltage source is included.(3 votes)

- Good morning! I really wish someone can answer my question: What is the difference between mesh and loop? Or is Loop just larger version of the mesh? Thank you!(1 vote)
- Loop – A loop is
**any**closed path going through circuit elements.

Mesh – A mesh is a loop that has no other loops inside it. There's one mesh for each "open window" of a circuit.

That means all meshes are loops, but not all loops are meshes. (Some loops have other loops inside them. Those are the ones that don't count as meshes.)

The reason we create these two terms that mean nearly the same thing is that later on, in the DC Circuit Analysis Tutorial, you will learn a really good way to analyze circuits, using only the meshes. It's called the Mesh Current Method.

This nearby article on Circuit Terminology explains the difference in writing, with more examples...

https://www.khanacademy.org/science/electrical-engineering/ee-circuit-analysis-topic/circuit-elements/a/ee-circuit-terminology(2 votes)

## Video transcript

- [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.