# Tue. Oct. 13, 2009Physics 208 Lecture 121 Last time… Begin circuits Resistor circuits Start resistor-capacitor circuits Today…

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Tue. Oct. 13, 2009Physics 208 Lecture 121 Last time… Begin circuits Resistor circuits Start resistor-capacitor circuits Today…

Tue. Oct. 13, 2009Physics 208 Lecture 122 Resistors Schematic layout Circuits Physical layout

Tue. Oct. 13, 2009Physics 208 Lecture 123 Quick Quiz Which bulb is brighter? A.A B.B C.Both the same Current through each must be same Conservation of current (Kirchoff’s current law) Charge that goes in must come out I I I I

Question When current flows, charge moves around the circuit. Suppose that positive charge carriers flow around the circuit. What is the change in potential energy of a positive charge as moves from c to d? Tue. Oct. 13, 2009Physics 208 Lecture 124 A.qV d – qV c B.qV c – qV d C.qV d + qV c D.zero

Quick Quiz What is the change in kinetic energy as it moves from c to d? Tue. Oct. 13, 2009Physics 208 Lecture 125 A.qV d – qV c B.qV c – qV d C.qV d + qV c D.zero

Tue. Oct. 13, 2009Physics 208 Lecture 126 Power dissipation (Joule heating) Charge loses energy from c to d. Ohm’s law: Energy dissipated in resistor as Heat (& light) in bulb Power dissipated in resistor = Joules / s = Watts

Tue. Oct. 13, 2009Physics 208 Lecture 127 Light bulbs and power Household voltage is 120V Cost 24 hours on requires MG&E ~ 13¢ / kWatt-hour 60 Watt 19¢ / day

Tue. Oct. 13, 2009Physics 208 Lecture 128 Two different bulbs Current same through each Power dissipated different Brightness different R1R1 R2R2 a b c d e I I I I

Tue. Oct. 13, 2009Physics 208 Lecture 129 Kirchoff’s junction law Charge conservation I in I out I out = I in I1I1 I2I2 I3I3 I 1 =I 2 +I 3 I2I2 I3I3 I1I1 I 1 +I 2 =I 3

Tue. Oct. 13, 2009Physics 208 Lecture 1210 Quick Quiz What happens to the brightness of the bulb A when the switch is closed? A.Gets dimmer B.Gets brighter C.Stays same D.Something else

Tue. Oct. 13, 2009Physics 208 Lecture 1211 Question As more and more resistors are added to the parallel resistor circuit shown here the total current flowing I… …. R1R1 R2R2 R3R3 R4R4 I A.Increases if each R i getting bigger B.Increases if each R i getting smaller C.Always increases D.Always decreases E.Stays the same Each resistor added adds  V/R i to the total current I

Tue. Oct. 13, 2009Physics 208 Lecture 1212 You use one power strip to plug in your toaster, coffee pot, microwave. ToasterCoffee PotMicrowave 10 A5 A12 A Everything works great until you plug in your space heater, then you smell smoke. This is because Question A.The resistance of the circuit is too high B.The voltage in the circuit is too high C.The current in the circuit is too high

Tue. Oct. 13, 2009Physics 208 Lecture 1213 More complicated circuits Both series & parallel Determine equivalent resistance Replace combinations with equivalent resistance

Tue. Oct. 13, 2009Physics 208 Lecture 1214 Quick Quiz The circuit below contains three 100W light bulbs. The emf  = 110 V. Which light bulb(s) is(are) brightest ? A. A B. B C. C D. B and C E. All three are equally bright.

Tue. Oct. 13, 2009Physics 208 Lecture 1215 Measurements in a circuit A multimeter can measure currents (as an ammeter), potential difference (as a voltmeter) Electrical measuring devices must have minimal impact in the circuit R  V Voltmeter The internal resistance of the ammeter must be very small I = I A  =  V+  V A = RI + r A I  RI for r A  0 VAVA A R  Ammeter I IAIA VV The internal resistance of the voltmeter must be very large I = I v +I R  V V =  VVVV IVIV IRIR I

Tue. Oct. 13, 2009Physics 208 Lecture 1216 Kirchoff’s loop law Conservation of energy R1R1 R2R2 R3R3  I1I1 I2I2 I3I3

Thur. Oct. 16, 2008Physics 208 Lecture 1417 Resistor- capacitor circuit What happens to charges on the capacitor after switch is closed? Why does the charge on the capacitor change in time? Why does the charge flow through the resistor?

Thur. Oct. 16, 2008Physics 208 Lecture 1418 Charging a capacitor Again Kirchoff’s loop law: Time t = 0: Looks like resistor & battery: uncharged cap acts like short circuit t increases: V C increases, so V R decreases Time t =  : Fully charged capacitor acts like open circuit

Thur. Oct. 16, 2008Physics 208 Lecture 1419 Discharging the capacitor Kirchoff’s loop law A B C D Charges in the current I come from capacitor:

Thur. Oct. 16, 2008Physics 208 Lecture 1420 RC discharge RC time constant

Thur. Oct. 16, 2008Physics 208 Lecture 1421 Charging a capacitor

Thur. Oct. 16, 2008Physics 208 Lecture 1422 Question The circuit contains three identical light bulbs and a fully-charged capacitor. Which is brightest? A.A B.B C.C D.A & B E.All equally bright

Thur. Oct. 16, 2008Physics 208 Lecture 1423 Question The circuit contains three identical light bulbs and an uncharged capacitor. Which is brightest? A.A B.B C.C D.A & B E.All equally bright

Tue. Oct. 13, 2009Physics 208 Lecture 1224 RC discharge RC time constant time t

Tue. Oct. 13, 2009Physics 208 Lecture 1225 RC analysis Kirchoff loop law: I related to Q C

Tue. Oct. 13, 2009Physics 208 Lecture 1226 RC analysis

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