S13Phys2BaLec27A

# V i key watch path direction wrt current v downstream

• Notes
• 25

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V = I Key: watch path direction w/r.t. current V = (“downstream”) (“upstream”) I –IR +IR

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Emf rises/drops V = + Key: watch battery terminals (ignore current dir.) V = + (“power-up”) (“penalty”) + ε ε
Kirchhoff’s Rules Let’s analyze the upper loop using Kirchhoff’s Loop Rule in the following circuit (say we want to solve for the power dissipated by the 9 Ω resistor). There are only two circuit elements in this loop:

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Kirchhoff’s Rules Kirchoff’s Loop Rule is nice, since you can get valuable information from an embedded part (most likely in parallel) of the circuit without breaking down the rest of the circuit. Warning: You may not be given the direction of the currents in a circuit. In this case you have to make a choice. Your choice of current directions is up to you, just be consistent throughout the problem. Remember that all you need is a closed loop to use this rule.
Kirchhoff’s Rules Kirchhoff’s Junction Rule The sum of the currents entering any junction must equal the sum of the currents leaving. There is a Kirchhoff rule that applies to currents in a circuit (in addition to loop rule). I in = I out This is a statement of Conservation of Charge. For the diagram: I 1 = I 2 + I 3

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Kirchhoff’s Rules Going back to our previous example from last time (where we wanted power dissipated by 9 Ω resistor, let’s now apply Kirchhoff’s Junction Rule.
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• Spring '08
• schuller
• Physics, Resistor, Series and parallel circuits, loop rule, Kirchhoff’s Junction

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