# lecture06 - Physics 102 Lecture 06 Kirchhoffs Laws Note...

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Kirchhoff’s Laws Physics 102: Lecture 06 Note opposite sign convention compared to your handouts!

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Last Time ... 3 2 1 + + + = R R R R eq Resistors in series: Resistors in parallel: ... 1 1 1 1 3 2 1 + + + = R R R R eq Current thru is same; Voltage drop across is IR i Voltage drop across is same; Current thru is V/R i Last Lecture Solved Circuits What about this one? Today 5
Kirchhoff’s Rules Kirchhoff’s Junction Rule (KJR): Current going in equals current coming out. Kirchhoff’s Loop Rule (KLR): Sum of voltage drops around a loop is zero. 12

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Using Kirchhoff’s Rules (1) Label all currents Choose any direction (2) Label +/- for all elements Current goes + - (for resistors) (3) Choose loop and direction (4) Write down voltage drops Be careful about signs R 4 + - + + + + - - - - + + + - - - R 1 ε 1 R 2 R 3 ε 2 ε 3 R 5 A B 17 I 1 I 3 I 2 I 4 I 5

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Loop Rule Practice R 1 =5 I ε 1 = 50V R 2 =15 ε 2 = 10V A B Find I: 22
Loop Rule Practice R 1 =5 I + ε 1 - IR 1 - ε 2 - IR 2 = 0 +50 - 5 I - 10 - 15 I = 0 I = +2 Amps ε 1 = 50V + - + - + - + - R 2 =15 ε 2 = 10V A B Find I: 22 Label currents Label elements +/- Choose loop Write KLR Opposite sign convention compared to your handouts!

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ACT: KLR
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