## 20170327

Physics 205B, spring semester 2017
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.

The following questions were asked on reading textbook chapters and reviewing presentations on circuit analysis and previewing 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.
"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 after where 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 more appliances that are plugged in an outlet, the resistance will decrease This can be very dangerous. Circuit breakers are to prevent current levels from reaching a dangerously high level--if the circuit breaker is triggered, this turns off the electricity so that the wires in the house do not become a danger to one's household."

"All household electrical outlets provide 120 volts of electric potential and operate independently of each other. The outlets are wired in parallel to each other to the same 120 volt electromotive source."

"An ideal ammeter has zero resistance because it measures the current within a circuit. Resistance in the ammeter is added to the circuit, which causes a reduction in current (since it is added in series). This would result in an artificially low measurement of current within a circuit."

"Right now this makes sense, currents are measured with anmeters and it tells you how much is running though the circuit. Voltmeters are used to measure the change in EPE of the item that you are using."

"A voltmeter measures the amount of volts running through a circuit. It should be hooked up to the first part of the circuit and the last part of the circuit so that it can read the change in potential when the voltage goes through the light bulb or resistor. There should be a really good resistor in there so that all the voltage is used up as it goes back to the start of the circuit. An ammeter measures current that passes through a light bulb. Since the current must pass through the digital multimeter the wiring of the circuit must be opened up so that the ammeter can read the current going through the circuit. There should be no resistance in the ammeter because you want to measure the original amount of current running through it."

"I learned that ammeter measures current and must be 'broken into' the circuit in order for the current to flow through it and a voltmeter measures the voltage and does not need to be 'broken in' since it measures a potential differences. The resistance for a voltmeter is infinite to avoid any current going into the voltmeter where as with the ammeter, the current is zero to allow the current to flow freely."

"In this homework assignment, it explains how multimeters must be used correctly to measure accurately because they are breaking a circuit when used as an ammeter. When used as a voltmeter, a multimeter must be connected to both before and after the current flow through a light bulb."

"I love all of your mnemonics! That is the best way for me to remember things so it makes less work for me to have t come up with little rhymes! "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.
"What I found confusing a from this reading assignment was the many different equations. I believe I understood that the different terms are able to be substituted to be able to relate to some of the other equations. When applying the equations was confusing."

"What I found a little confusing was the difference between a voltmeter and an ammeter. I'm still not entirely sure what the difference is except that an ideal voltmeter should have a high resistance and the ideal ammeter should have almost no resistance."

"I don't think I could figure out which equations to use."

"I don't understand how P can equal change in ∆V2/R or I2 times R."

"I'd like some more explanation on how multimeters measurements as ammeters and voltmeters differ. Also, how they relate to the equations."

"Nothing was confusing in the reading/lecture material. It was all pretty straightforward."

What are the resistances of these (ideal) devices?
(Only correct responses shown.)
Ideal ammeter: 0 [92%]
Ideal voltmeter: ∞ [80%]

State the unit of electrical power, and give an equivalent definition in terms of other SI units.
"The unit for power is watts (W), or joules (J) per second (s)."

"Not sure."

"Joules = newton·meters?"

"Volts?"

"Coulombs/second or joules/second?"

"Amps?"

Determine what will happen to the following parameters when additional electrical appliances are plugged in and turned on in the same household circuit.
(Only correct responses shown.)
Equivalent resistance Req of circuit: decreases [56%]
Current I flowing through emf source: increases [8%]

A fuse or circuit breaker is designed to prevent too much __________ in household wiring.
 current. ******************** [20] voltage. ** [2] (Both of the above choices.) ** [2] (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.
"Why does a circuit breaker trip when you have too many things plugged into outlets at your house?" (In a given room in your house, all the outlets are wired in parallel. Plugging in more appliances (each with a given resistance) means that they are all connected in parallel with each other; each additional appliance plugged in means that the equivalent resistance goes down, and thus the current going through the main wire to that part of the house goes up. A wire carrying a lot of current will tend to heat up, and if it is more current that it is rated for, then there is a fire risk, unless the circuit breaker trips.)

"It is scary to see the damage from a runaway current, that's why I'm not an electrician, haha! Circuit breakers and surge protectors must save a lot of buildings and lives!"

"When the resistance is low, the current is as fast as Usain Volt ; - )" (Well, not necessarily fast (all levels of currents travel at the same speed through a given wire); a better analogy would be that when resistance is low, current is as big as a football offensive line.)

"If power is the rate of electric potential energy (joules) per time, does that make it a second order derivative of electric potential?" (No, electric potential energy is not a derivative of electric potential ∆V, EPE (joules) is merely charge q (coulombs) times ∆V (joules/coulomb). So voltage is a ratio of energy per charge, rather than a "per time" derivative.)

"Why must a voltmeter have a resistance that is ideally infinity? I'm confused on this and what it means to have voltage." (If the voltmeter did not have an infinite resistance, then current would flow through it, and the voltmeter would then be measuring how much voltage is lost by the current flowing through itself, rather than measuring how much voltage is lost by what it is connected to (say, a disconnected battery, or a light bulb that has a given amount of current flowing through it.)

"Two parallel resistors double the resistance whereas two series resistors will halve the resistance." (#wut)

"Spring break!"