Kirchhoffs Circuit Laws, Physics tutorial

Introduction to Kirchhoff's Circuit Laws:

There are quite a few simple relationships between currents and voltages of various branches of an electrical circuit. Such relationships are determined by a few fundamental laws which are termed as Kirchhoff laws or more particularly Kirchhoff Current and Voltage laws. Such laws are extremely helpful in finding out the equivalent electrical resistance or impedance (that is, in case of AC) of a complex network and the currents flowing in different branches of the network. Such laws are first introduced by Guatov Robert Kirchhoff and therefore these laws are as well termed as Kirchhoff Laws.

Kirchhoff laws are necessary for resistor network theory. They were formulated through the German scientist Gustav Kirchhoff in the year 1845. The laws explain the conversation of energy and charge in the electrical networks. They are as well termed as Kirchhoff's circuit laws. Kirchhoff contributed as well to other fields of science, thus the generic term Kirchhoff law can encompass different meanings. Both the circuit laws, the Kirchhoff Current Law (or KCL) and the Kirchhoff Voltage Law (or KVL), will be described in detail.

Brief history on Kirchhoff's Circuit Laws:

Gustav Kirchhoff born at Konigsberg on March 12, 1824, Gustav Kirchhoff undertakes studies at a college from his own society. Gustav Kirchhoff's very first study topic was on the conduction of electricity. In the year 1845, Kirchhoff published the laws of closed electric circuits whereas he was still a university student. Nowadays, we now know that the Kirchhoff's Current and Voltage laws that were pass on read to as subsequent to their author. Kirchhoff's Current and Voltage laws are necessary laws that cover all virtually electrical circuits; it truly is of great worth that one has to be knowledgeable of these laws to be capable to know how an electric circuit works. Gustav may have been immortalized by these laws however genuinely; he as well had a great deal of contributions in some other regions. It absolutely was furthermore Gustav Kirchhoff who had been the very first person to verify that an electrical impulse traveled at the speed of light. In addition, Gustav as well contributed a lot in the study of spectroscopy. And in the year 1887, Kirchhoff died in Berlin.

In the year 1845, German physicist Gustav Kirchhoff first specified two laws which became basic to electrical engineering. The laws were simplified from the work of Georg Ohm. Kirchhoff s laws could be derived from the Maxwell's equation; however they were designed well before Maxwell's work has been founded.

Kirchhoff's Laws provides the given specifications that deduce a consistent current. The laws should be used for a time dependent process that takes the momentary current beneath consideration for alternating electric currents.

Kirchhoff's Current Law:

Kirchhoff's Current Law or KCL, defines that the 'net current or charge entering a junction or node is precisely equivalent to the charge leaving the node as it consists of no other place to go apart from to leave, as no charge is lost in the node'. In another words the arithmetical sum of all the currents entering and leaving a node should be equivalent to zero, I(exiting) + I(entering) = 0. This thought by Kirchhoff is generally termed as the Conservation of Charge.

Mathematically, this can be defined as:

k = 1n Ik = 0

Here, 'n' is the total number of branches having currents flowing towards or away from the node.

792_kirchhoffs current law.jpg

In this, the three currents entering the node, I1, I2, I3 are all positive in value and the two currents leaving the node, I4 and I5 are negative in value. Then this signifies we can as well rewrite the equation as:

I1 + I2 + I3 - I4 - I5 = 0

The word Node in an electrical circuit usually refers to the connection or junction of two or more current carrying paths or elements like cables and components. As well for current to flow either in or out of a node a closed circuit path should exist. We can make use of Kirchhoff's current law whenever analyzing the parallel circuits.


The matrix version of Kirchhoff's current law is the base of most of the circuit simulation software, like SPICE. Kirchhoff's current law joined with Ohm's Law is employed in nodal analysis.

KCL is valid to any lumped network irrespective of the nature of the network; whether active or passive, unilateral or bilateral and linear or non-linear.

Kirchhoff's voltage Law:

Kirchhoff's Voltage Law or KVL, defines that 'in any closed loop network, the net voltage around the loop is equivalent to the sum of all the voltage drops in the similar loop' that is as well equivalent to zero. In another words the arithmetical sum of all voltages in the loop should be equivalent to zero. This theory by Kirchhoff is as well termed as the Conservation of Energy.

Mathematically, the Kirchhoff's voltage law can be defined as:

k =1n Vk = 0

Here, 'n' is the net number of voltages measured.

1534_kirchhoffs voltage law.jpg

VAB + VBC + VCD + VDA = 0

Beginning at any point in the loop continue in the similar direction noting that the direction of all the voltage drops, either negative or positive, and returning back to the similar starting point. This is significant to maintain the similar direction either clockwise or anti-clockwise or the final voltage sum will not be equivalent to zero. We can make use of Kirchhoff's voltage law whenever examining the series circuits.

If analyzing either DC circuits or AC circuits employing Kirchhoff's Circuit Laws a number of definitions and terminologies are employed to explain the portions of the circuit being examined like: node, paths, branches, loops and meshes. Such words are employed frequently in circuit analysis therefore it is significant to comprehend them.

Kirchhoff's voltage law is mainly based on the conservation of energy given or taken or energy taken through potential field excluding energy taken through dissipation. A charge that consists of completed a closed loop doesn't gain or lose energy for a particular voltage potential. It simply goes back to initial potential level.

The law is applicable even with energy dissipating resistance in the circuit as electrical charges don't return to their preliminary potential due to energy dissipation however just terminate at the negative terminal rather than positive terminal. This signifies all the energy given by potential difference is been completely dissipated by resistance in the form of heat.

Kirchhoff's voltage law is the law associating to potential produced by voltage sources in spite of the electronic components that are present in the circuit whereby the loss or gain in energy given by the potential field should be zero if a charge completes a closed loop.

Application of Kirchhoff's Laws to Circuits:

The current distribution in different branches of a circuit can simply be found out by applying the Kirchhoff Current law at various nodes or junction points in the circuit. After that Kirchhoff Voltage law is applied, each and every possible loop in the circuit produces algebraic equation for each and every loop. By resolving all such equations, one can simply find out diverse unknown currents, voltages and resistances in the circuits.

Some well-liked conventions we usually make use of throughout applying KVL:

A) The resistive drops in a loop due to current flowing in the clockwise direction should be taken as the positive drops.

B) The resistive drops in a loop due to current flowing in an anti-clockwise direction should be taken as negative drops.

C) The battery emf causing the current to flow in clockwise direction in a loop is assumed as positive.

D) The battery emf causing the current to flow in anti-clockwise direction is termed as negative.

Kirchhoff law example:

The Kirchhoff laws form the base of network theory. Joined with Ohm's law and the equations for resistors in series and parallel, more complicated networks can be resolved. Some of the illustrations of resistor circuits are given to describe how Kirchhoff can be employed.

Illustration 1: The bridge circuit

The Bridge circuits are a much common tool in electronics. They are employed in measurement, transducer and switching circuits. Assume that the figure of bridge circuit is as shown below. In this instance we have to show, how to use Kirchhoff's laws to find out the cross current I5. The circuit consists of four bridge sections having resistors R1 - R4. There is one cross bridge connection having resistor R5. The bridge is subject to the constant voltage V and I.

1598_bridge circuit.jpg

The Kirchhoff's first law defines that the sum of all currents in one node is zero. This outcome in:

The first Kirchhoff law defines that the sum of all currents in one node is zero. This outcomes in:

I = I1+ I2

I = I3+ I4

I1= I3+ I5

The second Kirchhoff law defines that the sum of all voltages across all elements in a loop is zero. This directs to:

0 = R1I1 + R3I3 - V

0 = R1I1+ R5I5 - R2I2

0 = R3I3+ R5I5 - R4I4

The six sets of equations above can be rewritten by employing normal algebra to determine the expression for I5 (that is, the current in the cross branch):

0 = I5 = [V (R2R3- R1R4)]/ [R5 (R1 + R3)(R2+ R4)+ R1R3(R2+ R4)+ R2R4 (R1 +R3)]

The equation represents that the bridge current is equivalent to zero the bridge is balanced:

0 = R1R4 = R2R3

Illustration 2: The star delta connection

Kirchhoff's laws can be employed to transform a star connection to a delta connection. This is frequently done to resolve complex networks. A broadly used application for star delta connections is to limit the beginning current of electric motors. The high starting current causes high voltage drops in the power system. As a solution, the motor windings are joined in the star configuration throughout starting and then change to the delta connection.

1938_star delta connection.jpg

The star connection is as represented in the figure above, consists of the similar voltage drops and currents as the delta connection represented on the right side, only when the given equations are valid:

R1 = (R31R12)/(R12+R23+R31)

R12 = R1 + R2 + (R1R2/R3)

R2 = (R12R12)/(R12+R23+R31)

R23 = R2 + R3 + (R2R3/R1)

R3 = (R23R31)/(R12+R23+R31)

R31 = R3 + R1 + (R3R1/R2)

Limitations of Kirchhoff's Circuit Laws:

KCL and KVL both based on the lumped element model being applicable to the circuit in question. If the model is not applicable, the laws don't apply.

KCL, in its general form, is dependent on the supposition that the current flows only in conductors and that whenever current flows into one end of a conductor it instantly flows out the other end. This is not a safe supposition for high-frequency AC circuits, where the lumped element model is no longer applicable. This is frequently possible to enhance the applicability of KCL by considering parasitic capacitances distributed all along the conductors. Significant violations of KCL can take place even at 60Hz that is not a very high frequency.

In another words, KCL is valid only when the total electric charge, Q, remains constant in the area being considered. In practical situations this is for all time so when KCL is applied at a geometric point. If investigating a finite area, though, it is possible that the charge density in the area might change. As charge is conserved, this can merely come about through a flow of charge across the region boundary. This flow represents a total current, and KCL is violated.

KVL is mainly based on the supposition that there is no fluctuating magnetic field linking the closed loop. This is not a safe supposition for high-frequency (that is, short-wavelength) AC circuits. In the presence of a changing magnetic field, the electric field is not a conservative vector field. Thus the electric field can't be the gradient of any potential. That is to state, the line integral of the electric field around the loop is not zero, directly denying the KVL.

It is regularly possible to enhance the applicability of KVL by considering the parasitic inductances (comprising mutual inductances) distributed all along the conductors. These are treated as imaginary circuit elements that generate a voltage drop equivalent to the rate-of-change of the flux.

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