Kirchoff'sLaw

KMUTNB Institute

Kirchoff's Law

TE Department

Table of Contents

• Gustav Robert Kirchhoff
• Reasons for using

Kirchhoff's law
• Definition of Kirchhoff's

Laws
• Kirchhoff’s current Law
• Kirchhoff's current

equation
• Kirchhoff's voltage Law
• Kirchhoff's voltage

equation
• Example Kirchhoff’s Law

Gustav Robert Kirchhoff

Ohm's law alone Circuit analysis is not very
convenient. Electrical circuits can sometimes be complicated.
Using Ohm's Law alone may be very difficult. which later
became the fundamental laws of physics of electronics.

Gustav Robert Kirchhoff (12 March
1824 – 17 October 1887) was a
German physicist who contributed
to the fundamental understanding
of electrical circuits, spectroscopy,
and the emission of black-body
radiation by heated objects.

Credit He coined the term black-body
COPYRIGHT UNKNOWN,COURTESY OF radiation in 1862. Several different
NATIONAL LIBRARY OF CONGRESS / sets of concepts are named
COLOURED BY SCIENCE PHOTO "Kirchhoff's laws" after him,
LIBRARYCIENCE PHOTO LIBRARY concerning such diverse subjects
as black-body radiation and
spectroscopy, electrical circuits,
and thermochemistry. The Bunsen–
Kirchhoff Award for spectroscopy
is named after him and his
colleague, Robert Bunsen.

Reasons for using Kirchhoff's law
- Used to solve problems in electrical circuits where Ohm's
law cannot be applied.
- Used when the total resistance in the circuit cannot be
determined.
- Used when there are more than 2 power sources in a circuit

Definition of Kirchhoff's Laws
Kirchhoff's Laws can be divided into two main ones:
1. Current Law (Kirchhoff's Current Law , KCL)
2. Voltage Law (Kirchhoff's Voltage Law, KVL)

Kirchhoff’s current Law

Said, “The current flowing into a point in an electrical circuit is
equal to the current flowing out of that point.”

Kirchhoff's current equation

� = � or � = 0

In electricity we prefer to have the amount of electricity
flowing into the point of interest to be + and flowing out of
the point of interest to be –, so Kirchhoff's current law (KCL)
can be written in general form.

example 1

Write Kirchhoff's current equation from the given circuit.

Kirchhoff's voltage Law

Said, "The algebraic sum of the voltages supplied to the
circuit is equal to the sum of the voltages, is equal to the sum
of the voltages across the resistance in that circuit.”

Equation
1 + 2 =

Kirchhoff's voltage equation

� = or � = 0

A loop (Loop) of an electric circuit refers to any path in an
electric circuit. If starting from a point along that path and
then able to return to that point again, it is called a loop
(Loop).

example 2

Write Kirchhoff's voltage equation from the given circuit.

Example Kirchhoff’s Law

example 1

KCL : 1 + 2 = 3 [1]
[2]
KVL : Loop1 ; 1 = +
1 = 1 1 + 3 3 [1]
1 = 1 1 + 3( 1 + 2) [2]
1 = 1 + 3 1 + 3 2
5 = 5 1 + 3 2

KVL : Loop2 ; 2 = +
2 = 2 2 + 3 3
2 = 2 2 + 3( 2 + 1)
2 = 3 1 + ( 2 + 3) 2
12 = 3 1 + 6 2

example 2

KCL : 1 = 2 + 3

KVL : Loop1 ; 1 = +
1 = 1 2 + 2 1
1 = 1 2 + 2( 2 + 3)
1 = 1 + 2 2 + 2 3
3 = 6 2 + 2 3

KVL : Loop2 ; 2 = +
1 − 2 = 1 1
1 − 2 = 1( 2 + 3)
1 − 2 = 1 2 + 1 3)
1 = 4 2 + 4 3