Last time…

1. Last time… • Combining capacitors • Begin circuits Today… • Resistor circuits • Start resistor-capacitor circuits Physics 208 Lecture 13

2. Resistors Circuits Physical layout Schematic layout Physics 208 Lecture 13

3. 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 13

4. 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 13

5. Work and energy in a circuit • What happens as charge q moves? • Uqc=qVc • Uqd=qVd • Ohm’s law: • Charge has lost potential energy • Energy lost Elost = qIR • Energy dissipated in resistor as • Heat (& light in bulb) • Power dissipated in resistor = Joules / s = Watts Physics 208 Lecture 13

6. 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 13

7. 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 13

8. R R 2R = Resistors in Series • Potentials add • V = DV1 + DV2 = IR1 + IR2 = = I (R1+R2) • The equivalent resistance Req = R1+R2 • I1 = I2 = I 2 resistors in series: R  L Like summing lengths Physics 208 Lecture 13

9. Quick Quiz What happens to the brightness of the bulb B when the switch is closed? Gets dimmer Gets brighter Stays same Something else Battery is constant voltage,Vacross bulbBdoesn’t changeso I through bulb B doesn’t change Physics 208 Lecture 13

10. R R Resistors in Parallel • DV = DV1 = DV2 • I = I1 + I2 (lower resistance path has higher current) • Equivalent Resistance R/2 Add areas Physics 208 Lecture 13

11. 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 13

12. 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 13

13. 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 13

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

15. 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 13

16. DV V I R R IA A e e Ammeter Electrical measurements • 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 13

17. Kirchoff’s loop law R1 I1 e R2 R3 • Conservation of energy I2 I3 Physics 208 Lecture 13

18. Resistor-capacitor circuit • Why does QC (charge on capacitor) depend on time after switch is closed? • Qc decreases: does Qc change faster, slower, same? • R increases: does Qc change faster, slower, same? • C increases: does Qc change faster, slower, same? Physics 208 Lecture 13

19. RC discharge • RC time constant time t Physics 208 Lecture 13