Kirchhoff'S Law
Kirchhoff’s Law
Kirchhoff’s Law, named after the German physicist Gustav Kirchhoff, consists of two fundamental principles that govern electrical circuits.

Kirchhoff’s Current Law (KCL): This law states that the total current entering a junction in a circuit must equal the total current leaving the same junction. In other words, current cannot be created or destroyed.

Kirchhoff’s Voltage Law (KVL): This law states that the algebraic sum of the voltages around any closed loop in a circuit must equal zero. In simpler terms, the total voltage gained in a loop must be equal to the total voltage lost.
These laws provide the foundation for analyzing and understanding the behavior of electrical circuits, allowing engineers and scientists to calculate currents, voltages, and resistances within complex networks. Kirchhoff’s Laws are essential in circuit theory and form the basis for many circuit analysis techniques.
History about Gustav Robert Kirchhoff
Gustav Robert Kirchhoff (12 March 1824 – 17 October 1887) was a German physicist who contributed to the fields of spectroscopy, electricity, and heat radiation. He is best known for his formulation of Kirchhoff’s laws of spectroscopy, which describe the relationship between the emission and absorption of light by matter.
Early Life and Education: Gustav Kirchhoff was born on March 12, 1824, in Königsberg, Prussia (now Kaliningrad, Russia). He showed an early aptitude for mathematics and physics and studied at the University of Königsberg, where he earned his doctorate in 1847.
Spectroscopy: Kirchhoff’s most significant contributions were in the field of spectroscopy. In 1859, he published a paper titled “On the Relation between the Emission and Absorption of Light by Bodies,” which outlined his three laws of spectroscopy:
 Kirchhoff’s First Law: A hot object emits light of all wavelengths, with the intensity of emission depending on the wavelength and temperature of the object.
 Kirchhoff’s Second Law: A hot object absorbs light of the same wavelengths that it emits.
 Kirchhoff’s Third Law: The ratio of the emissivity (ability to emit light) to the absorptivity (ability to absorb light) of an object is the same for all wavelengths and is equal to the emissivity of a perfect blackbody.
These laws provided a fundamental understanding of the interaction between light and matter and laid the groundwork for the development of spectroscopy as a powerful analytical tool.
Electricity: Kirchhoff also made significant contributions to the field of electricity. In 1845, he published a paper titled “On the Motion of Electricity in Conductors,” which introduced the concept of electric current as the movement of positive and negative charges. He also developed a set of equations, known as Kirchhoff’s circuit laws, which describe the behavior of electric currents in circuits. These laws are still widely used today in the analysis and design of electrical circuits.
Heat Radiation: Kirchhoff’s work on heat radiation led to the development of the concept of blackbody radiation. A blackbody is an ideal object that absorbs all incident light and emits thermal radiation according to Planck’s law. Kirchhoff’s law of thermal radiation states that the emissivity of a blackbody is equal to its absorptivity for all wavelengths.
Legacy: Gustav Kirchhoff’s contributions to physics were profound and farreaching. His laws of spectroscopy revolutionized the study of light and matter, and his work on electricity and heat radiation laid the foundation for many important developments in these fields. He is considered one of the most influential physicists of the 19th century, and his legacy continues to inspire and guide scientific research today.
What Are Kirchhoff’s Laws?
Kirchhoff’s First Law or Kirchhoff’s Current Law
Kirchhoff’s First Law (Kirchhoff’s Current Law)
Kirchhoff’s First Law, also known as Kirchhoff’s Current Law (KCL), is a fundamental principle in electrical engineering and circuit analysis. It states that the total current entering a node in a circuit must equal the total current leaving the same node. In other words, current cannot be created or destroyed, it can only be redistributed.
Explanation:
Consider a simple circuit with a battery, a resistor, and a node where two wires meet. The current from the battery flows through the resistor and then splits at the node. Some of the current flows through one wire, while the rest flows through the other wire. According to KCL, the current entering the node (from the battery) must equal the total current leaving the node (through the two wires).
Example:
In the circuit shown below, the current from the battery (I) flows through the resistor (R) and then splits at the node. The current through the upper wire is I1, and the current through the lower wire is I2. According to KCL, we have:
I = I1 + I2
This equation ensures that the total current entering the node (I) is equal to the total current leaving the node (I1 + I2).
Applications:
KCL is used extensively in circuit analysis and design. It is essential for determining the current distribution in a circuit, which is necessary for calculating voltage drops, power dissipation, and other circuit parameters. KCL is also used in the analysis of more complex circuits, such as those with multiple sources, capacitors, and inductors.
Summary:
Kirchhoff’s First Law (Kirchhoff’s Current Law) states that the total current entering a node in a circuit must equal the total current leaving the same node. This law is based on the principle of conservation of charge and is essential for circuit analysis and design.
Kirchhoff’s Second Law or Kirchhoff’s Voltage Law
Kirchhoff’s Second Law (Kirchhoff’s Voltage Law)
Kirchhoff’s Second Law, also known as Kirchhoff’s Voltage Law (KVL), states that the algebraic sum of the voltages around any closed loop in a circuit must equal zero. In other words, the total voltage supplied by a source must be equal to the total voltage consumed by the components in the circuit.
Mathematical Representation:
$$\sum V = 0$$
Where:
 ΣV represents the algebraic sum of the voltages.
 A closed loop refers to any continuous path in a circuit that starts and ends at the same point.
Explanation:
Kirchhoff’s Voltage Law is based on the principle of conservation of energy. In a closed loop, the energy supplied by the voltage sources must be equal to the energy consumed by the components. If this condition is not met, the circuit would not be in equilibrium, and currents would flow continuously without any energy loss.
Example 1: Simple Series Circuit
Consider a simple series circuit with a battery, a resistor, and a voltmeter. The battery supplies a voltage of 12 volts, and the resistor has a resistance of 6 ohms.
Using Kirchhoff’s Voltage Law, we can write the following equation:
$$V_{battery}  V_{resistor} = 0$$
Substituting the given values, we get:
$$12 V  6 V = 0$$
This equation confirms that the voltage supplied by the battery is equal to the voltage consumed by the resistor, satisfying Kirchhoff’s Voltage Law.
Example 2: Parallel Circuit
Now, consider a parallel circuit with two resistors, R1 and R2, connected to a battery. The battery supplies a voltage of 9 volts, R1 has a resistance of 4 ohms, and R2 has a resistance of 6 ohms.
Applying Kirchhoff’s Voltage Law to the loop containing the battery and R1, we get:
$$V_{battery}  V_{R1} = 0$$
Substituting the values, we have:
$$9 V  (4 ohms * I) = 0$$
Similarly, applying KVL to the loop containing the battery and R2, we get:
$$V_{battery}  V_{R2} = 0$$
Substituting the values, we have:
$$9 V  (6 ohms * I) = 0$$
Solving these two equations simultaneously, we find that the current I is 1.5 amps.
Therefore, the voltage drop across R1 is:
$$V_{R1} = I * R1 = 1.5 A * 4 ohms = 6 V$$
And the voltage drop across R2 is:
$$V_{R2} = I * R2 = 1.5 A * 6 ohms = 9 V$$
Adding these voltage drops, we get:
$$V_{R1} + V_{R2} = 6 V + 9 V = 15 V$$
This value is equal to the voltage supplied by the battery, satisfying Kirchhoff’s Voltage Law.
In summary, Kirchhoff’s Voltage Law is a fundamental principle in circuit analysis that ensures that the energy supplied by voltage sources is balanced by the energy consumed by circuit components. It is essential for analyzing and understanding the behavior of electrical circuits.
Kirchhoff’s Law Solved Example
Kirchhoff’s Current Law (KCL) states that the total current entering a junction must equal the total current leaving the junction. This can be expressed mathematically as:
∑Iin = ∑Iout
where:
 Iin is the current entering the junction
 Iout is the current leaving the junction
Example:
Consider the following circuit:
[Image of a circuit with a battery, two resistors, and a junction]
The current entering the junction is 10 A. The current leaving the junction is 5 A through resistor R1 and 5 A through resistor R2. This satisfies KCL, as the total current entering the junction (10 A) equals the total current leaving the junction (5 A + 5 A = 10 A).
Kirchhoff’s Voltage Law (KVL) states that the sum of the voltages around a closed loop must equal zero. This can be expressed mathematically as:
∑V = 0
where:
 V is the voltage around a closed loop
Example:
Consider the following circuit:
[Image of a circuit with a battery, two resistors, and a closed loop]
The voltage around the closed loop is 10 V. This satisfies KVL, as the sum of the voltages around the closed loop (10 V) equals zero (0 V).
Kirchhoff’s laws are fundamental laws of electricity that can be used to analyze circuits. They are used to determine the current and voltage in a circuit, as well as to design circuits that meet specific requirements.
Frequently Asked Questions – FAQs
State Kirchhoff’s Current Law
Kirchhoff’s Current Law (KCL) states that the total current entering a junction must equal the total current leaving the same junction. In other words, current cannot be created or destroyed.
To understand KCL, consider a simple circuit with a battery, a resistor, and a switch. When the switch is closed, current flows from the battery, through the resistor, and back to the battery. The current entering the junction at the positive terminal of the battery is equal to the current leaving the junction at the negative terminal of the battery.
Another example of KCL is a parallel circuit. In a parallel circuit, the current from the source divides between the different branches of the circuit. The total current entering the junction at the source is equal to the sum of the currents in each branch of the circuit.
KCL is a fundamental law of electricity that is used to analyze and design circuits. It is also used to troubleshoot circuits by identifying points where current is not flowing properly.
Here are some additional examples of KCL:
 In a series circuit, the current is the same throughout the circuit.
 In a parallel circuit, the current divides between the different branches of the circuit.
 In a complex circuit, the current can be found by using Kirchhoff’s Current Law and Kirchhoff’s Voltage Law.
KCL is a powerful tool that can be used to analyze and design circuits. It is a fundamental law of electricity that is essential for understanding how circuits work.
What is Kirchhoff’s First Law also known as?
Kirchhoff’s First Law, also known as Kirchhoff’s Current Law (KCL), states that the total current entering a junction must equal the total current leaving the same junction. This law is based on the principle of conservation of charge, which states that electric charge cannot be created or destroyed.
In other words, KCL states that the net current at any point in a circuit must be zero. This can be understood by considering a simple circuit with a battery, a resistor, and a voltmeter. When the circuit is closed, the battery pushes electrons through the resistor, causing the voltmeter to register a voltage. However, the number of electrons entering the resistor must be equal to the number of electrons leaving the resistor, or else the voltmeter would read an infinite voltage.
KCL can be used to analyze more complex circuits by breaking them down into smaller loops. For each loop, the sum of the currents entering the loop must equal the sum of the currents leaving the loop. This can be used to determine the current flowing through each component in the circuit.
Here are some examples of how KCL can be used to analyze circuits:
 In a series circuit, the current is the same throughout the circuit. This can be seen by applying KCL to a series circuit with a battery, a resistor, and a voltmeter. The current entering the resistor is equal to the current leaving the resistor, and the current entering the voltmeter is equal to the current leaving the voltmeter.
 In a parallel circuit, the current divides between the different branches of the circuit. This can be seen by applying KCL to a parallel circuit with a battery, two resistors, and a voltmeter. The current entering the junction of the two resistors is equal to the sum of the currents leaving the junction, and the current entering the voltmeter is equal to the current leaving the voltmeter.
 In a more complex circuit, KCL can be used to determine the current flowing through each component by breaking the circuit down into smaller loops. For example, consider the circuit shown in the figure below.
[Image of a circuit with a battery, two resistors, and a voltmeter]
To analyze this circuit, we can break it down into two loops. The first loop consists of the battery, resistor R1, and the voltmeter. The second loop consists of the battery, resistor R2, and the voltmeter.
Applying KCL to the first loop, we get:
I_battery  I_R1  I_voltmeter = 0
Applying KCL to the second loop, we get:
I_battery  I_R2  I_voltmeter = 0
Solving these two equations simultaneously, we can find the current flowing through each component in the circuit.
KCL is a fundamental law of circuit analysis and is used to analyze a wide variety of circuits.
State Kirchhoff’s voltage law
Kirchhoff’s voltage law (KVL) states that the algebraic sum of the voltages around any closed loop in a circuit must be equal to zero. In other words, the total voltage supplied by a source must be equal to the total voltage dropped across the components in the circuit.
Here’s an example to illustrate KVL:
Consider a simple circuit with a battery, a resistor, and a voltmeter. The battery supplies a voltage of 12 volts, and the resistor has a resistance of 6 ohms. The voltmeter is used to measure the voltage across the resistor.
According to KVL, the voltage across the battery must be equal to the voltage across the resistor. In this case, the voltmeter reads 6 volts, which means that the voltage across the resistor is 6 volts.
This can be verified by calculating the current flowing through the circuit using Ohm’s law (I = V/R). The current is 2 amps (12 volts / 6 ohms). The power dissipated by the resistor is then calculated using the formula P = I^2 * R. The power is 12 watts (2 amps * 6 ohms).
The power supplied by the battery is also 12 watts (12 volts * 2 amps). This shows that the total voltage supplied by the battery is equal to the total voltage dropped across the components in the circuit, which is in accordance with KVL.
KVL is a fundamental law of circuit theory and is used to analyze and design electrical circuits. It can be applied to any circuit, regardless of its complexity.
Who put forth Kirchhoff’s laws?
Kirchhoff’s Laws
Kirchhoff’s laws are two fundamental laws that describe the behavior of electrical circuits. They were formulated by the German physicist Gustav Kirchhoff in 1845.
Kirchhoff’s Current Law (KCL)
Kirchhoff’s current law states that the total current entering a junction must equal the total current leaving the junction. This law is based on the principle of conservation of charge, which states that charge cannot be created or destroyed.
Kirchhoff’s Voltage Law (KVL)
Kirchhoff’s voltage law states that the sum of the voltages around a closed loop must equal zero. This law is based on the principle of conservation of energy, which states that energy cannot be created or destroyed.
Examples
Example 1: A simple circuit consisting of a battery, a resistor, and a switch.
[Image of a simple circuit]
In this circuit, the current flows from the positive terminal of the battery, through the resistor, and back to the negative terminal of the battery. The current is the same at every point in the circuit.
Example 2: A more complex circuit consisting of multiple batteries, resistors, and switches.
[Image of a more complex circuit]
In this circuit, the current flows through multiple paths. However, the total current entering each junction is equal to the total current leaving the junction. Also, the sum of the voltages around each closed loop is equal to zero.
Applications
Kirchhoff’s laws are used in a wide variety of applications, including:
 Circuit analysis
 Circuit design
 Troubleshooting
 Power systems
 Electronics
Kirchhoff’s laws are essential for understanding how electrical circuits work. They are a powerful tool for analyzing and designing circuits.
Kirchhoff’s second law is also known as?
Kirchhoff’s second law, also known as the loop rule or the voltage law, states that the algebraic sum of the voltages around any closed loop in a circuit must be equal to zero. In other words, the total voltage supplied by the sources in a circuit must be equal to the total voltage dropped across the resistors and other components in the circuit.
Here’s an example to illustrate Kirchhoff’s second law:
Consider a simple circuit with a battery, a resistor, and a voltmeter. The battery supplies a voltage of 12 volts, and the resistor has a resistance of 6 ohms. The voltmeter is used to measure the voltage across the resistor.
According to Kirchhoff’s second law, the algebraic sum of the voltages around the loop in this circuit must be equal to zero. In other words, the voltage supplied by the battery must be equal to the voltage dropped across the resistor.
In this case, the voltage dropped across the resistor is given by Ohm’s law: V = IR, where V is the voltage, I is the current, and R is the resistance. Since the current in the circuit is 2 amps, the voltage dropped across the resistor is 12 volts.
Therefore, the algebraic sum of the voltages around the loop in this circuit is 12 volts  12 volts = 0 volts, which satisfies Kirchhoff’s second law.
Kirchhoff’s second law is a fundamental principle of circuit analysis and is used to calculate the voltages and currents in circuits. It can also be used to determine the power dissipated by components in a circuit.