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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 ?
• 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.
• 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.
• How do we know where to start and end the loop?
• 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.
• At the 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!
• 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.
• So in an ideal wire, the current is always infinite if the voltage is non-zero? (Ohm's law)
• 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.
• What is the difference between a mesh and a loop
• 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.
• At around 5 minutes, Willy starts the discussion about "mesh" and "loop". What is the practicality of knowing the number of meshes or loops?
• 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:

And in the two previous videos and the one following video to get more on this subject.
• 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.
• 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.
• 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...