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Last time…. Begin circuits. Today…. Resistor circuits Start resistor-capacitor circuits. Resistors. Circuits. Physical layout. Schematic layout. I. I. I. I. Quick Quiz. Which bulb is brighter?. A B Both the same. Current through each must be same

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  1. Last time… • Begin circuits Today… • Resistor circuits • Start resistor-capacitor circuits Physics 208 Lecture 12

  2. Resistors Circuits Physical layout Schematic layout Physics 208 Lecture 12

  3. I I I I Quick Quiz Which bulb is brighter? A B Both the same Current through each must be same Conservation of current (Kirchoff’s current law) Charge that goes in must come out Physics 208 Lecture 12

  4. 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? • qVd – qVc • qVc – qVd • qVd + qVc • zero Physics 208 Lecture 12

  5. Quick Quiz What is the change in kinetic energy as it moves from c to d? • qVd – qVc • qVc – qVd • qVd + qVc • zero Physics 208 Lecture 12

  6. 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 Physics 208 Lecture 12

  7. 60 Watt Light bulbs and power • Household voltage is 120V • Cost • 24 hours on requires • MG&E ~ 13¢ / kWatt-hour 19¢ / day Physics 208 Lecture 12

  8. I b a I R1 c I e R2 d I Two different bulbs • Current same through each • Power dissipated different • Brightness different Physics 208 Lecture 12

  9. I2 Iin I1 I3 I1=I2+I3 I1 I3 Iout I2 I1+I2=I3 Kirchoff’s junction law • Charge conservation Iout = Iin Physics 208 Lecture 12

  10. Quick Quiz What happens to the brightness of the bulb A when the switch is closed? Gets dimmer Gets brighter Stays same Something else Physics 208 Lecture 12

  11. I …. R1 R2 R3 R4 Question As more and more resistors are added to the parallel resistor circuit shown here the total current flowing I… Increases if each Ri getting bigger Increases if each Ri getting smaller Always increases Always decreases Stays the same Each resistor added adds V/Ri to the total current I Physics 208 Lecture 12

  12. 10 A 5 A 12 A Toaster Coffee Pot Microwave Question You use one power strip to plug in your toaster, coffee pot, microwave. Everything works great until you plug in your space heater, then you smell smoke. This is because The resistance of the circuit is too high The voltage in the circuit is too high The current in the circuit is too high Physics 208 Lecture 12

  13. More complicated circuits • Both series & parallel • Determine equivalent resistance • Replace combinations with equivalent resistance Physics 208 Lecture 12

  14. Quick Quiz The circuit below contains three 100W light bulbs. The emf e = 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. Physics 208 Lecture 12

  15. DV V I R R IA A e e Ammeter 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 DVV Voltmeter IV IR DVA I • The internal resistance of the • ammeter must be very small • I = IA • = DV+DVA = RI + rAI  RI for rA 0 The internal resistance of the voltmeter must be very large I = Iv+IR DVV = e Physics 208 Lecture 12

  16. Kirchoff’s loop law R1 I1 e R2 R3 • Conservation of energy I2 I3 Physics 208 Lecture 12

  17. Resistor-capacitor circuit • What happens to charges on the capacitor after switch is closed? • Why does the charge flow through the resistor? • Why does the charge on the capacitor change in time? Physics 208 Lecture 14

  18. Time t = 0: t increases: Time t = : Charging a capacitor • Again Kirchoff’s loop law: Looks like resistor & battery: uncharged cap acts like short circuit VC increases, so VR decreases Fully charged capacitor acts like open circuit Physics 208 Lecture 14

  19. C B A D Discharging the capacitor • Kirchoff’s loop law Charges in the current I come from capacitor: Physics 208 Lecture 14

  20. RC discharge • RC time constant Physics 208 Lecture 14

  21. Charging a capacitor Physics 208 Lecture 14

  22. Question The circuit contains three identical light bulbs and a fully-charged capacitor. Which is brightest? A B C A & B All equally bright Physics 208 Lecture 14

  23. Question The circuit contains three identical light bulbs and an uncharged capacitor. Which is brightest? A B C A & B All equally bright Physics 208 Lecture 14

  24. RC discharge • RC time constant time t Physics 208 Lecture 12

  25. RC analysis • Kirchoff loop law: • I related to QC Physics 208 Lecture 12

  26. RC analysis Physics 208 Lecture 12

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