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Last time… Today… Combining capacitors Begin circuits

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1 Last time… Today… Combining capacitors Begin circuits
Resistor circuits Start resistor-capacitor circuits Tue. Oct. 14, 2008 Physics 208 Lecture 13

2 Resistors Circuits Physical layout Schematic layout Tue. Oct. 14, 2008
Make point here that energy is dissipated in the resistor due to collisions. Say it is dissipated as heat in resistors, light in light bulbs. Will see later that it is not directly proportional to current, but more current -> more dissipation -> hotter resistor or brighter light. Need this for the demos and next quizzes. Schematic layout Tue. Oct. 14, 2008 Physics 208 Lecture 13

3 Kirchoff’s junction law
I1=I2+I3 Charge conservation Iin Iout I2 I3 I1 I1+I2=I3 Iout = Iin Tue. Oct. 14, 2008 Physics 208 Lecture 13

4 Quick Quiz Which bulb is brighter? A B Both the same I
Say that light bulbs are like resistors, and that more current flowing through the bulb makes the bulb brighter. Current through each must be same Conservation of current (Kirchoff’s current law) Charge that goes in must come out Tue. Oct. 14, 2008 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 Tue. Oct. 14, 2008 Physics 208 Lecture 13

6 Light bulbs and power Household voltage is 120V Cost 60 Watt
24 hours on requires MG&E ~ 13¢ / kWatt-hour 19¢ / day Tue. Oct. 14, 2008 Physics 208 Lecture 13

7 Two different bulbs I b a R1 c e R2 Current same through each d
Power dissipated different Brightness different Tue. Oct. 14, 2008 Physics 208 Lecture 13

8 Resistors in Series = I1 = I2 = I Potentials add
V = DV1 + DV2 = IR1 + IR2 = = I (R1+R2) The equivalent resistance Req = R1+R2 R = 2R 2 resistors in series: R  L Like summing lengths Tue. Oct. 14, 2008 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, V across bulb B doesn’t change so I through bulb B doesn’t change Tue. Oct. 14, 2008 Physics 208 Lecture 13

10 Resistors in Parallel DV = DV1 = DV2
I = I 1 + I 2 (lower resistance path has higher current) Equivalent Resistance R/2 Add areas Tue. Oct. 14, 2008 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 Tue. Oct. 14, 2008 Physics 208 Lecture 13

12 Question As more and more resistors are added to the parallel resistor circuit shown here the total current flowing I… …. R1 R2 R3 R4 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 Tue. Oct. 14, 2008 Physics 208 Lecture 13

13 Question You use one power strip to plug in your toaster, coffee pot, microwave. Toaster Coffee Pot Microwave 10 A 5 A 12 A 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 Tue. Oct. 14, 2008 Physics 208 Lecture 13

14 More complicated circuits
Both series & parallel Determine equivalent resistance Replace combinations with equivalent resistance Tue. Oct. 14, 2008 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. Tue. Oct. 14, 2008 Physics 208 Lecture 13

16 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 A R e Ammeter I IA DV DVV Voltmeter e V IV R 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 Tue. Oct. 14, 2008 Physics 208 Lecture 13

17 Kirchoff’s loop law e I1 Conservation of energy I2 I3 R1 R2 R3
Tue. Oct. 14, 2008 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? Tue. Oct. 14, 2008 Physics 208 Lecture 13

19 RC discharge RC time constant time t Tue. Oct. 14, 2008
Physics 208 Lecture 13


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