What is a node in a circuit

Knot rule and mesh rule (Kirchhoff)

In this section we explain Kirchhoff's laws to you. This means the knot rule and the mesh rule. What this is all about and how to use it, you will find out in this article.

Before we go directly into the rule of stitches and knots, you should know the basic terms of electrical engineering and Ohm's law. If you still have problems with this, you should first read through our articles. Because this knowledge is needed to understand the Kirchhoff rules.


Knot rule (1. Kirchhoff rule)

Let's start with the knot rule, often referred to as Kirchhoff's first rule. The statement behind this is actually quite simple: The current that flows into a node must be just as large as the current that flows out. First take a look at the following graphic, underneath you will find explanations.

The arrows pointing to the node mean: The current flows into the node. The arrows away from the node say: The current flows out of the node. The knot rule says that the currents that flow in are as large as the currents that flow out. For our example this means:

The sum of the incoming flows is the same as the sum of the outgoing flows. If you know three of the four currents in our example, you can calculate the fourth.


Mesh rule (2nd Kirchhoff rule)

The 2nd Kirchhoff rule is also known as the mesh rule. It can be used to provide information on the tension in a mesh. Here, too, we take a simple example in order to explain the facts as simply as possible. First take a look at the following graphic:

In our example we have a voltage source and three resistors. The rule of mesh says that the voltage from the source is the same as the voltage dropped across the three resistors. It looks like this:

  • UVoltage source = UResistance 1 + UResistance 2 + UResistance 3


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