# Why the potential difference between a point and

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Unformatted text preview: t be zero. WHY? The potential difference between a point and itself is zero ! Physics 212 Lecture 10, Slide 6 Kirchoff’s Current Rule ∑I in = ∑ I out Kirchoff's Current Rule states that the sum of all Current currents entering any given point in a circuit must equal the sum of all currents leaving the same point. WHY? WHY? Electric Charge is Conserved Electric Physics 212 Lecture 10, Slide 7 Kirchhoff’s Laws (1) Label all currents Choose any direction (2) Label +/- for all elements R1 A + I1 + Current goes + ⇒ - (for resistors) (3) Choose loop and direction Must start on wire, not element. 3 + B 1 - First sign you hit is sign to use. I2 I3 - + 2 (4) Write down voltage drops + R2 R3 R5 - + I4 - R4 + + I5 (5) Write down node equation Iin = Iout We’ll do calculation first today It’s actually the easiest thing to do !! Physics 212 Lecture 10, Slide 8 17 Preflight 3 DROP GAIN BB N AI G A B C D E 60 50 40 30 20 With the current Against the current VOLTAGE DROP VOLTAGE GAIN 10 0 Physics 212 Lecture 10, Slide 9 1Ω 2Ω 1Ω Calculation 2V 1V I2 In this circuit, assume Vi and Ri are known. What is I2 ?? 1V • Conceptual Analysis: – Circuit behavior described by Kirchhoff’s Rules: • KVR: ΣVdrops = 0 • KCR: ΣIin = ΣIout •Strategic Analysis – – – Write down Loop Equations (KVR) Write down Node Equations (KCR) Solve Physics 212 Lecture 10, Slide 10 10 V1 R1 ++ - -+ + R3 - - V2 R2 I2 In this circuit, assume Vi and Ri are known. -...
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