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KIRCHHOFF'S LAW


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. He coined the term "black body" radiation in 1862, and two sets of independent concepts in both circuit theory and thermal emission are named "Kirchhoff's laws" after him, as well as a law of thermochemistry. The Bunsen–Kirchhoff Award for spectroscopy is named after him and his colleague, Robert Bunsen.


Kirchhoff's Lawsen.wikipedia.org 
Kirchhoff's circuit laws are two equalities that deal with the conservation of charge and energy in electrical circuits, and were first described in 1845 by Gustav Kirchhoff. Widely used in electrical engineering, they are also called Kirchhoff's rules or simply Kirchhoff's laws.


These goals are very important.  If you can't write KVL equations and solve them, you may well be lost when you take a course in electronics in a few years.  It will be much harder to learn that later, so be sure to learn it well now.  (And, if you need to review the concept of voltage)

This fundamental law results from the conservation of charge. It applies to a junction or node in a circuit -- a point in the circuit where charge has several possible paths to travel. In Figure 1, we see that IA is the only current flowing into the node. However, there are three paths for current to leave the node, and these current are represented by IB, IC, and ID. Once charge has entered into the node, it has no place to go except to leave (this is known as conservation of charge). The total charge flowing into a node must be the same as the the total charge flowing out of the node.

The two Kirchhoff’s Laws tell us about the relationships between voltages and currents in circuits. Kirchhoff’s Current Law states that: ‘the algebraic sum of currents at a node is zero’. Two points might need further explanation: The second of Kirchhoff’s Laws, the Voltage Law, states that:  ‘the algebraic sum of voltages around a closed circuit loop is zero’. There’s the phrase ‘algebraic sum’ again, so we must recognise that the direction of voltages matters when using Kirchhoff’s Voltage Law.

Practice problems relating to kirchhoff's laws.

Learning to mathematically analyze circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way. You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the änswers" instead of a book or another person. For successful circuit-building exercises, follow these steps:

We saw in the Resistors tutorial that a single equivalent resistance, ( RT ) can be found when two or more resistors are connected together in either series, parallel or combinations of both, and that these circuits obey Ohm's Law. However, sometimes in complex circuits such as bridge or T networks, we can not simply use Ohm's Law alone to find the voltages or currents circulating within the circuit. For these types of calculations we need certain rules which allow us to obtain the circuit equations and for this we can use Kirchoff's Circuit Law.

Example: Three resistors are connected across a 50-volt source. What is the voltage across the third resistor if the voltage drops across the first two resistors are 25 volts and 15 volts? Solution: First, a diagram, such as the one shown in figure 3-23, is drawn. Next, a direction of current is assumed (as shown). Using this current, the polarity markings are placed at each end of each resistor and also on the terminals of the source. Starting at point A, trace around the circuit in the direction of current flow, recording the voltage and polarity of each component. Starting at point A and using the components from the circuit:

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