Welcome Back to Physics 1308 Circuits Gustav Robert Kirchhoff 12 March 1824 17 October 1887
Announcements Assignments for Thursday, October 4th: - Reading: Chapter 27.3-4 - Watch Video: https://youtu.be/ytoe-rwi8us Lecture 13 - Simple and Complex Circuits Homework 7 Assigned - due before class on Thursday, October 11th. No office hours or class during Fall break Monday, October 8th and Tuesday, October 9th due to Fall break. Midterm Exam 2 will be in class on Tuesday, October 16th. It will explicitly cover chapters 24-26. However, these chapters build on the previous material and you will be expected to apply all concepts/information from the beginning of class to this point to any problem or question that you encounter on the exam. Dr. Cooley will be out of the office the week of October 15th. Her office hours are cancelled that week. If you need to reach her, please send an email.
Review Question 1 A parallel-plate capacitor is attached to a battery that maintains a constant potential difference V between the plates. While the battery is still connected, a glass slab is inserted so as to just fill the space between the plates. The stored energy A) increases. B) decreases. C) remains the same. When the glass slab is between the plates, it becomes polarized, thereby decreasing the magnitude of the electric field. In order for the battery to maintain a constant potential difference across the plates, it must do work to deposit more charge on the plate. Thus, work is done on the system and the stored energy increases.
Review Key Concepts V = ir
Key Concepts Junction Rule: Traversing the LH loop in a CCW direction from point b: Traversing the RH loop in a CCW direction from point b: Applying the loop rule in the CCW direction to the big loop from b: This is the sum of the RH and LH loops!
Key Concepts Resistors in Parallel: Resistors in parallel have the same potential difference. Applying the junction rule and note that V is the same: Thus,
Key Concepts
Lightbulb Game
Lightbulb Game: Phase 1 Get the Resistance Filament (a resistor) Cap (connects to low electric potential, e.g. ground ) Contact (connects to high electric potential)
Lightbulb Game: Phase 1 Get the Resistance 40W Bulb Rated at 120V R40 = 100W Bulb Rated at 120V R100 =
Lightbulb Game: Phase 2 Lightbulbs in Parallel 40 V The light bulbs are in parallel. What is the same for both of them? What is different for both of them? Instructor measurement of property that is the same for both:
Lightbulb Game: Phase 2 Lightbulbs in Parallel 40 What is the total resistance? Rtotal = V What is the total current flowing through the circuit? Itotal = [predicted] Itotal = [measured]
Lightbulb Game: Phase 2 Lightbulbs in Parallel 40 What is the current in the 40W bulb? I40 = [pred] I40 = [meas] V What is the current in the 100W bulb? I100 = [predicted] I100 = [measured]
Lightbulb Game: Phase 2 Lightbulbs in Parallel 40 What is the power emitted by the 40W and 100W bulbs? P40 = V P100 =
Lightbulb Game: Phase 3 Lightbulbs in Series V The light bulbs are in series. What is the same for both of them? What is different for both of them?
Lightbulb Game: Phase 3 Lightbulbs in Series What is the total resistance? Rtotal = V What is the total current flowing through the circuit? Itotal = [predicted] Itotal = [measured]
Lightbulb Game: Phase 3 Lightbulbs in Series What is the voltage in the 40W bulb? V40 = [pred] V40 = [meas] V What is the voltage in the 100W bulb? V100 = [predicted] V100 = [measured]
Lightbulb Game: Phase 3 Lightbulbs in Series What is the predicted power emitted by the 40W and 100W bulbs at their predicted voltages? V P40 = P100 = Even though the predictions of current and voltage are not within a reasonable distance of the measurements, the gross prediction that the 100W bulb will glow much more faintly than the 40W bulb is consistent with the observation of the system.
Research Questions In parallel... we made pretty reliable predictions with Ohm's Law, consistent with the data collected from the system. In series... we made unreliable predictions. Specifics were way off but the gross details were OK. What happened? what testable and falsifiable conjectures can we make to explain these results?
What can we learn from the data? From Data: 144 Ω 360 Ω Resistance CHANGES with voltage! Questions: Can light bulb resistances really change so dramatically? How do we figure this out?
Summary from Wikipedia article, Incandescent light bulb (http://en.wikipedia.org/wiki/incandescent_light_bulb): The actual resistance of the filament is temperature-dependent. The cold resistance of tungsten-filament lamps is about 1/15 the hot-filament resistance when the lamp is operating. For example, a 100-watt, 120-volt lamp has a resistance of 144 ohms when lit, but the cold resistance is much lower (about 9.5 ohms). Data from Prof. John Fattaruso (currently teaching PHY3340, Computational Physics) for a 4W, 115V bulb:
Conclusions Ohm's Law works great when resistance is independent of current and voltage When the bulbs are used within the manufacturer's specifications (e.g. in parallel), we can make reliable predictions of current, voltage, power, etc. Beware in the real world of non-ohmic situations Begin by assuming that Ohm's Law applies GENERAL LESSON: Check your assumptions if calculations go wrong never stick to the premise if it's delivering wrong results.
Biological Effects of Current
The End for Today! https://xkcd.com/356/