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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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