## 20140402

Physics 205B, spring semester 2014
Cuesta College, San Luis Obispo, CA

Students have a weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.
"I understand Kirchhoff's rules very well. In series the potential difference of the higher resistance will be larger because there are no junctions so the current is the same. In parallel the resistor with the lowest resistance will have the most current."

"What I understand from my reading is the concept of equivalent resistance, in the scenarios when we get a complicated circuit there are ways to make it easier to calculate by adding nodes that are in series and/or combining parallel nodes."

"I understand that the ammeter should ideally have zero resistance but shouldn't complete a second loop. it needs to be connected within the loop with the resistance."

"I understand how to use given equations to make substitutions for power dissipation. I understand that in order for an ammeter to measure a current that passes through a light bulb, the current must also pass through the ammeter. I also understand that a voltmeter must be connected to both before and after the current flows through the light bulb in order to tell the difference (rise or drop)."

"I have a good understanding of electrical safety; I also spend a lot of time with a multimeter for my hobby."

"I liked how a simple piece of gum wrapper could create fire when attached to a battery. The concept of ammeters and voltmeters is pretty straight forward and being able to relate this concepts to the lab and to normal home functions is pretty interesting."

"I'm still clearly understanding how to find Req. I also find that throughout this process I am getting a lot better at understanding the junction rule, where the current may split up, but at the end of the junction, the current will be the same as before."

"I think I just understood Ohm's law."

"Resistors in series are equivalent to their sum. Resistors in parallel are equivalent to the reciprocal of the sum of their reciprocals, Voltage drops with each resistor. Current is the same right after the power source as it is right before the power source...as well as at any single-path choke point in the circuit."

"Though I can do the math portion of this section fairly well, I cannot see this stuff conceptually, which is really confounding me."

"I understand Kirchhoff's two rules, the junction and the loop rules. Through this, in circuits energy is neither lost or gained. What goes in has to come out regardless of the type of circuit."

"Power for a circuit is the current times the potential difference. As electrons travel they collide with other molecules and lose kinetic energy, so a constant supply of energy must be provided to maintain a current. Learned a little bit about how ammeters/voltmeters work. The purpose of the third prong in may plugs (to ground it)."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.
"I am not confused by any of these topics. However, I could use a brief example using the power dissipation equation."

"The most confusing aspect of this chapter was understanding the effects of resistance."

"I am not sure how to apply the equations for power dissipation. I also don't understand how a circuit breaker works. I don't understand what the resistances of some of the ideal versus real circuit elements should be."

"I don't know much difference between voltmeters and ammeters besides the fact that ammeters have zero resistance and voltmeters have infinite resistance."

"I'm confused about the placements of the ammeters and voltmeters on a circuit. Do we put the voltmeters around the resistors and ammeters?"

"I am confused about the resistances of ideal devices. How can we tell what the resistance should be?"

"Just about everything! Haha, I think I am making a little bit of headway, but I seem to be inching along like a snail up the Cuesta Grade."

"I don't even know what I understood or didn't understand. I never understand until I come to class."

"I am still confused as to why the prison lighter works."

What are the resistances of these (ideal) devices?
(Only correct responses shown.)
Ideal light bulb: some finite value between 0 and ∞ [82%]
Burnt-out light bulb: ∞ [36%]
Ideal wire: 0 [68%]
Real (non-dead) battery: some finite value between 0 and ∞ [85%]

Two light bulbs with different resistances r and R, where r < R, are connected in series with each other to an ideal emf source. Select the light bulb with the greater quantity.
(Only correct responses shown.)
More current flowing through it: (there is a tie) [32%]
Larger potential potential difference: light bulb R [68%]

Two light bulbs with different resistances r and R, where r < R, are connected in parallel with each other to an ideal emf source. Select the light bulb with the greater quantity.
(Only correct responses shown.)
More current flowing through it: light bulb r [39%]
Larger potential potential difference: (there is a tie) [39%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"What is the difference between an ideal and real battery? Don't they both have resistors anyways?" (An ideal battery has zero resistance, and never dies. A real battery has a small amount of internal resistance even when new, and this internal resistance increases as it dies.)

"Can we go over this in class?" (Yes. From everyone's responses, we will definitely need to.)

"I'm still confused about some of the units and the basic definition of some of the variables. What is current? What is the potential difference--for what to do what?" (When I think of current (amps), potential (volts), and resistance (ohms), I think of this. Now that's what you think of, too.)