# lecture05 - Physics 102 Lecture 05 Circuits and Ohms Law...

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Circuits and Ohm’s Law Physics 102: Lecture 05

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Summary of Last Time Capacitors Physical C = ε 0 A/d C=Q/V Series 1/C eq = 1/C 1 + 1/C 2 Parallel C eq = C 1 + C 2 Energy U = 1/2 QV Resistors Physical R = ρ L/A V=IR Series R eq = R 1 + R 2 Parallel 1/R eq = 1/R 1 + 1/R 2 Power P = IV Summary of Today 5
Electric Terminology Current: Moving Charges Symbol: I Unit: Amp Coulomb/second Count number of charges which pass point/sec Direction of current is direction that + flows Power: Energy/Time Symbol: P Unit: Watt Joule/second = Volt Coulomb/sec P = VI 7

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Physical Resistor Resistance: Traveling through a resistor, electrons bump into things which slows them down. R = ρ L /A ρ : Resistivity: Density of bumps L : Length of resistor A : Cross sectional area of resistor Ohms Law I = V/R Cause and effect (sort of like a=F/m) potential difference cause current to flow resistance regulate the amount of flow Double potential difference double current I = (VA)/ ( ρ L) A L 10
Preflight 5.1 Two cylindrical resistors are made from the same material. They are of equal length but one has twice the diameter of the other. 1. R 1 > R 2 2. R 1 = R 2 3. R 1 < R 2 2 1 12 77% 3% 21% R = ρ L /A

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Comparison: Capacitors vs. Resistors Capacitors store energy as separated charge: U=QV/2 Capacitance: ability to store separated charge: C =
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## This note was uploaded on 01/25/2012 for the course PHYS 102 taught by Professor Lee during the Spring '08 term at University of Illinois, Urbana Champaign.

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lecture05 - Physics 102 Lecture 05 Circuits and Ohms Law...

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