## 20140331

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.
"Even though wires have a low resistance, a large enough current can overheat and melt the wire. Circuit breakers combat that by cutting off the current if it gets too large."

"An ammeter needs to be wired differently than a voltmeter. 'Twinkle twinkle little star, power equals i squared r.'"

"Power is the rate energy conversion takes place and is the product of current and potential difference. A voltmeter measures the potential difference across the resistor. An ammeter measures the current through the resistor. Both voltmeter and ammeter are used to measure the resistance in a circuit."

"Ammeters, are placed within the circuit and approach zero resistance. Voltmeters are placed on a circuit and approach infinite resistance (so that they don't operate as an in parallel conduit). Household circuits have all of their plugs wired in parallel, with more resistors in parallel, more current in drawn leading to potential overheating, thus requiring a circuit breaker to guard against this."

"The power dissipation equations do look very familiar to the potential energy equations. It makes a little more sense why households have parallel rather than series wiring because the voltage will be same throughout."

"An ammeter breaks into a circuit, whereas a voltmeter feels the current running through the circuit. In this class, we'll consider resistance to be unaffected by temperature."

"I understand that the voltmeter measures the change in voltage so it must be connected before and after the current flows through a resistor. I also understand that the current increases when more parallel resistors are added."

"Even though wires have a low resistance, a large enough current can overheat and melt the wire. Circuit breakers combat that by cutting off the current if it gets too large."

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.
"Not quite sure about the difference between an ammeter and voltmeter."

"What I find confusing is the new material. I understand that a multimeter can be used as a ammeter (current) and voltmeter (change in electric potential), but I am having a hard time understanding the 'break-in' connection for the ammeter set up. I am also having a hard time putting all the different concepts together to understand power dissipation. I am not sure when and how to use all the different equations."

"There are lots of equations to interpret."

"I didn't find anything to be too confusing that I couldn't figure out on my own going through the presentations and the section in the book once more."

"I'm not really following what the power dissipation stuff is. I'm not seeing how it is specifically used in the types of problems we've been doing."

"I don't understand how power dissipation is applied to a situation."

"I'm a little confused in the blog presentation about the joule heating. What is actually going on there and why is that significant to currents."

"I'm still not feeling great about what all the variables represent in these equations. Current, voltage, electric potential energy, etc."

"I didn't really understand the concept of joule heating. Is it just the same thing as electrical power?"

"I don't understand how power relates to everything yet a brief overview in class would be great. How does power relate to current voltage and resistance conceptually, the equations are okay."

"I'm confused about the junction rule and the loop rule. I understand as far as looking at them in a series or parallel but I don't understand WHEN to apply them. Seems like one deals with current and the other voltage."

I'm just slightly confused about when to use a certain power equation. If there is a rise in electric potential, should the preferred equation be the one where the potential difference is squared?"

"Rises and drops in emf, the loop rule, and the junction rule are coming to me like sludge moving upwards through a pipe. More explanations this Monday would be very much in order."

State the unit of electrical power, and give an equivalent definition in terms of other SI units.
"The watt (symbol: W). The unit, defined as one joule per second, measures the rate of energy conversion or transfer."

"Amps?"

"J = N·m?"

What are the resistances of these (ideal) devices?
(Only correct responses shown.)
Ideal ammeter: 0 [88%]
Ideal voltmeter: ∞ [81%]

Plugging in and using additional electrical appliances in the same household circuit would increase the total __________ in the wiring inside the house.
 current. *********************** [23] voltage. *** [3] (Both of the above choices.) ** [2] (Neither of the above choices.) ** [2] (Unsure/guessing/lost/help!) ** [2]

A fuse or circuit breaker is designed to prevent too much __________ in household wiring.
 current. ******************************* [31] voltage. [0] (Both of the above choices.) [0] (Neither of the above choices.) [0] (Unsure/guessing/lost/help!) * [1]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Can we please go over power dissipation and how to use the equations? (And generally go over all the new material.)"

"I'm still not confident with the more complex circuit problems and how you can find each value after you solve for the total circuit current."

"The midterm wasn't too hard."

"I don't have any questions right now."