## 20150403

### Online reading assignment: advanced electricity (review)

Physics 205B, spring semester 2015
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.

The following questions were asked on re-reading textbook chapters and reviewing presentations on advanced electricity concepts.

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.
"Isn't this the same reading assignment? I understand basically everything except when you were talking about that circuit last time in class."

"A fuse is put in series with the circuit and is designed to melt—due to I2R heating—if the current to the circuit exceeds a given value. The melted fuse is an open switch, interrupting the circuit and stopping the current."

"I understand Kirchhoff's rules a lot better, but I think the labs are really helping me understand all of this cause I feel like electrical stuff really isn't my strong point."

"'Joule heating' is the historical term for the power (or rate of energy used per time) continuously used by a circuit element of resistance R due to the amount of current I flowing through it, as in those radiating coils."

"I understand that ammeters must be wired in series while voltmeters must be set up in parallel. If ammeters are wired in parallel the current will take the path of least resistance (zero for an ideal ammeter) instead of going through the resistor which would result in a dangerous amount of current in the circuit. If a voltmeter is wired in series no current can flow through the circuit due to its infinitely high resistance that would zero out the current. The remaining voltage would be a useless reading."

"'Twinkle twinkle little star, power equals I squared r."

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 had some difficultly understanding the loop rule."

"I found power confusing. Wiring both voltmeters and ammeters seems tricky still."

"I get confused with the difference between voltage and current. I don't understand (yet) how to read or figure out electric potential differences. It felt like in lab you could get different readings depending on where you put the leads, but I'm not sure.'

"The most confusing part of the readings and also apart of the labs is understanding how the ammeters, ohmmeters and voltmeters are used unless they are first explained to me."

"I'm confused by the law for circuits and what it is actually used for and how to manipulate the variables."

"I'm not exactly sure how runaway currents work. Additionally I dont understand how the circuit breaker is implemented to prevent runaway currents."

"I still get confused with electric potential, electrical potential energy, wells and peaks, and the direction of decreasing and increasing values based on the sign of the charge. I also get confused when reducing circuits to find total current and then building them back up again and applying the rules of a circuit in series versus a circuit in parallel. Especially with the more confusing circuit arrangements."

What are the resistances of these (ideal) devices?
(Only correct responses shown.)
Ideal light bulb: some finite value between 0 and ∞ [61%]
Burnt-out light bulb: ∞ [37%]
Ideal wire: 0 [48%]
Ideal (non-dead) battery: 0 [45%]
Real (non-dead) battery: some finite value between 0 and ∞ [61%]
Ideal switch, when open: ∞ [42%]
Ideal switch, when closed: 0 [42%]

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) [42%]
Larger potential potential difference: light bulb R [45%]
More power used: light bulb R [34%]

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 [50%]
Larger potential potential difference: (there is a tie) [26%]
More power used: light bulb r [32%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"If I was in prison, I would just study physics all day." (No better way to get pumped up.)

"Will try not to forget this over spring break."

"I think I need an intervention on this conceptual stuff and resistances for real versus ideal devices."

"It all seems simple enough, until it gets confusing."