Kirchhoff’s first law | Kirchhoff’s Current Law (KCL) — Explained & derived

Gustav Kirchhoff, an eminent German physicist is the person behind Kirchhoff’s first law dealing with electric current. This law is also known as Kirchhoff’s Current Law or KCL. The KCL states that, for any point or node in an electrical circuit, the sum of currents into that point is equal to the sum of currents out of that point.

State Kirchhoff’s first law or KCL | Explain Kirchhoff’s first law with a diagram

Kirchhoff’s first law or current law (KCL) states that, for any point in an electrical circuit, the sum of currents into that point is equal to the sum of currents out of that point.

The diagram above will help to understand the first law of Kirchhoff. The law can be written as Σi in=ΣI out where Σ denotes ‘sum of’.
Σi in is the sum of the current into a point and ΣI out is the sum of current out of that point.

In diagram(1-a), 9 Ampere current is flowing into the node and then again (5+4) Ampere i.e. 9 Ampere current is coming out of the same node.

In diagram (1-b) total (6+4) Ampere = 10 Ampere current is coming into a node and again 10 Ampere current comes out from the node.

Lastly, in diagram (1-c) total current coming into the shown node is (5 +2 ) Amp = 7 Amp.

And total current coming out is also (4+3) Amp = 7 Amp. All these examples help to understand the first law of Kirchoff (KCL).

Deriving Kirchhoff’s first law from the Conservation of Charge Law

Kirchhoff’s first law can be derived from the Conservation of Charge Law. The charge is a fundamental physical property and one of the properties that must be conserved. Here we will first study about Conservation of charge and then utilize that to derive the KCL.

What is the Conservation of Charge Law?

Conservation of charge states that electric charge can neither be created nor destroyed. The total amount of electric charge in the universe is constant.

Derive KCL or Kirchhoff’s first law from the Conservation of Charge Law

Kirchhoff’s first law is based on the Conservation of charge, where the charge (coulombs) is the product of the current (in amperes) and the time (in seconds). Charge can’t be destroyed, so the charge carriers entering a point in a given time must equal the total number of charge carriers leaving that same point during that time.

Mathematically, we can write the following equation to express the conservation of charge: ………

Read the complete post on my physics blog: Derivation & Application of KCL

Parallel Circuit and Application of KCL

Let’s take a parallel circuit and find out how to use KCL for it.

As per KCL, we can write the equations and solve them to find out individual current components flowing through different resistors.

Read the complete post on my physics blog: Derivation & Application of KCL

Originally published at https://physicsteacher.in on December 24, 2020.

NIT Grad & IT prof. Writes and teaches HS physics. A motivational speaker and an active blogger with multiple niches. With lots of stories to tell.

NIT Grad & IT prof. Writes and teaches HS physics. A motivational speaker and an active blogger with multiple niches. With lots of stories to tell.