## 20180328

Physics 205B, spring semester 2018
Cuesta College, San Luis Obispo, CA

Students have a bi-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.
"The review of Kirckhoff's laws and how to apply them to various levels of complicated circuits. I also sort of understand ammeters."

"The equation for series and parallel, and the addition rules for them are pretty simple, especially after the lab."

"That series resistors are chain like and parallel resistors are when the current can divide up in a fork kind of way and portions of it can pass through two resistors simultaneously. I also understand the equations that come with these concepts."

"An effective voltmeter 'feels' the current without taking anything from it. We are reported the difference between the high and low potentials."

"According to the loop rule, the total change in rises equals the total change of drops."

"When measuring the amount of electrical potential used by a light bulb (or any other circuit element), the digital multimeter must be connected to both before and the current flows through the light bulb. This means that the wiring in the circuit is not modified in order to connect the digital multimeter to measure electric potential (making it an voltmeter), as it 'feels' the amount of electric potential before and after the light bulb, and reports the difference (whether a drop or rise)."

"The difference between an anameter and a voltmeter and how you need to wire them into a circuit to get the readings you want. I also understand the power equation given in the presentation and how you can substitute in Ohm's law and get different variations."

"Ammeters measure the amount of resistance in an item on a circuit like bulb or resistor and has a resistance of zero while voltmeters need a before and after and have a resistance of infinity."

"The circuit analysis presentation, especially after the review in class. The equivalent resistances for series and parallel circuits especially make sense to me: for series, you can simply add the resistances together since the current will only be able to take one path whereas with parallel circuits you have to add the inverses (and take the inverse of that amount) since the circuits have different junctions to go through."

"Joule heating is the rate of energy used per time. Joules per second is equivalent of watts. Ammeters must have resistance ideally zero. Voltmeter resistance ideally should be infinity."

"The power of a circuit is the product of the current and the voltage of that circuit."

"When a circuit is grounded, charge can flow through back to the circuit itself, where it can be released into the ground instead of running through a person's body. Household circuits are wired to have outlets in parallel, meaning that it's important not to overload them as the resistance could become too low."

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 was not sure where we were supposed to find the resistances of the list of devices below so I guessed."

"I would like an introduction on voltmeters. I think they are more confusing than ammeters."

"I don't understand the equations for the potential drops and rises."

"I am finding Kirchhoff's rules (especially the loop one) really confusing. In addition, I'm still not quite sure about voltmeters and ammeters. I don't know some examples using one or both of the ideas would I think help me better understand these ideas."

"I am still confused with the loop rule and would like more of an explanation on it in class."

"I found the section about runaway current and circuit breakers to be confusing. Also I would like to know how the resistance of a voltmeter is infinite."

"I have not read enough of the chapter to determine what I find confusing. At the moment almost all of it is confusing."

"Pretty chill lesson."

State the unit of electrical power, and give an equivalent definition in terms of other SI units.
"Watts, joules."

"Watt, one joule per second."

"Electric power is measured in watts, which equates to Joules per second when using other SI units to define watts."

"Watt (W), also amps times volts."

What are the resistances of these (ideal) devices?
(Only correct responses shown.)
Ideal light bulb: some finite value between 0 and ∞ [71%]
Burnt-out light bulb: ∞ [54%]
Ideal wire: 0 [58%]
Real (non-dead) battery: some finite value between 0 and ∞ [63%]
Ideal switch, when open: ∞ [33%]
Ideal switch, when closed: 0 [29%]

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) [58%]
Larger potential potential difference: light bulb R [46%]
More power used: light bulb R [75%]

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 [46%]
Larger potential potential difference: (there is a tie) [38%]
More power used: light bulb r [46%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"It will be better after spring break...hopefully!"

"Can we talk about Kirchhoff's loop rule, and power? I feel kind of lost."

"The potential drop is still not clear to me, reading isn't helping me understand it either." ("Potential" is just another term for voltage, so "potential drop" is the change in voltage ∆V, which measures how much voltage a resistor uses up while current is passing through it. If a Christmas light bulb is connected to an ideal 1.5 V emf, the emf provides 1.5 V to the current passing through it, and then the current as it passes through the light bulb uses up 1.5 V.)

"I didn't understand the upstream/downstream example. Will we be given a path direction and use that to check our answer? Or do we use our answer to determine the direction the current is traveling?" (For very complicated circuits it will not be apparent what direction(s) the current has, but for the scope of this course the direction(s) of current will be given, or will be apparent if you inspect the polarity (the +/− ends) of the emfs on the circuit.)