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13h_kirchoff - Lesson 13h: Kirchoff's Rules We sometimes...

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Lesson 13h: Kirchoff's Rules We sometimes encounter a circuit that is too complicated for simple analysis. Maybe there is a weird mix of series and parallel, or more than one power source. To deal with such complicated circuits, we use Kirchhoff’s rules, invented by G.R. Kirchhoff (1824-1887). His rules are just convenient applications of the laws of conservation of charge and energy. In the previous section, you were already (sort of) using his rules, even if you didn’t know it. The First Rule: The Junction Rule Kirchhoff's first rule is based on the conservation of charge, and we already used it in deriving the rule for parallel resistors. “At any junction point, the sum of all currents entering the junction must equal the sum of all currents leaving the junction.” This just means that when current reaches the branches in a parallel circuit, it will split up and take different routes. When the branches come back together, the currents will add back together too. 11/14/2010 © studyphysics.ca Page 1 of 5 Illustration 1: G.R. Kirchhoff 7.1 A 7.1 A 3.0 A 2.3 A 1.8 A

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The Second Rule: The Loop Rule This one is based on the conservation of energy. “The algebraic sum of the changes in potential around any closed path of a circuit must be zero.” This just means that when you look at the resistors in series, the drop in voltage across all of them will be equal to whatever the source is. The importance of these rules comes when you need to analyze a more complicated circuit that has a
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This note was uploaded on 12/02/2011 for the course PHYSICS 235 taught by Professor Staff during the Fall '08 term at Rutgers.

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13h_kirchoff - Lesson 13h: Kirchoff's Rules We sometimes...

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