S13Phys2BaLec27B

5 apply loop equations junction equations ohms law

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5) Apply loop equations, junction equations, Ohm’s Law, power equations to search for desired variable.

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Circuits Kirchhoff’s Rules For the following circuit diagram, we eventually want to know the power dissipation by the 8 Ω resistor. I 1 I 2 I 3 We can identify the directions for currents at the top junction: Choose up for I 1 . To the left for I 2 . <-want To the left for I 3 . Now, by applying the junction rule we see that: I 3 = I 1 + I 2
Circuits I 1 I 2 I 3 We can make an inner left loop moving counterclockwise. Now, by applying the loop rule on left inner loop we get: Let’s choose voltage loop directions (need I 1 and I 2 ): Rise Drop Drop

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Circuits I 1 I 2 I 3 Rise Drop Rise We can make an inner right loop moving counterclockwise. Now, by applying the loop rule on right inner loop we get: V loop ! = " V 12 V + " V 4 # + " V 6 # + " V 4 V = 0 Drop
Circuits

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Clicker Question 27B-1 For the following circuit diagram (assuming all straight lines to be ideal wires), the current through the 10 Ω resistor will be: A) 0 Amps. B) Greater than 0 Amps but less than 1 Amp. C) Exactly 1 Amp. D) Greater than 1 Amp but less than 10 Amps. E) Greater than 10 Amps.
Circuits Back to our original circuit, what if we had chosen one of the current directions incorrectly when we first performed the Kirchhoff Junction Rule? It’s no problem. I 1 I 2 I 3 I 1 actually went down, not up (you will find a negative value for I 1 at the end).

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