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.
"An ammeter must have a resistance that is ideally 0 while a voltmeter must have a resistance that is ideally infinity. Joule heating is when the power used by the circuit's resistance due to the about of current flowing through it."
"Power measures the rate of electric potential energy usage. It can be calculated as current squared times resistance or current times potential. When resistors are connected in parallel, resistance decreases and current increases. This increase in current can overload the circuit."
"Voltmeters measure the amount of electrical potential used by an element in the circuit and ammeters measure the amount of current passing through an element of a circuit."
"Ammeters measure the current of any circuit element that is "broken" open by measuring the current after it travels through the element. The ideal resistance of an ammeter is zero. Voltmeters measure the amount of electric potential used by any circuit potential, and measures the current of both before and after the element. The ideal resistance of a voltmeter is infinite."
"Series wiring is when devices are connected so that the same current runs through both devices. Parallel wiring is when devices are connected so that the same voltage is applied across each device. The junction and loop rules set the parameters through which current and voltage can move through a circuit."
"If there is too much current going through something, that something will heat up and it can become dangerous and lead to a runaway current which can cause fires and other damages. I also understand how series resistors and parallel resistors divide/use the current in a circuit. Whatever potential a battery adds to a circuit, the resistors will use up the same amount. Circuit breakers are designed to trip a current before it becomes dangerously high turns to a runaway current."
"That runaway currents can be prevented by circuit breaker that interrupts the current if it becomes to high due to excessive appliances being plugged into the same outlet. Each appliance plugged in is considered a resistor, so when many appliances are plugged into the same outlet the resistance becomes dangerously low and the current becomes dangerously high. Outlet overload if dangerous because it causes the wires to heat up thus melting the outer coating that insulates the current flowing through the wire. When the coating is melted off it can make contact with other surfaces and create a spark which could cause a fire."
"Honestly not a lot. Parallel circuits and resistors. Circuit breakers."
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'm going to need more help plugging in the numbers for the different equations."
"The equations may need more examples. Need extra practice on equations."
"I understood junction rule but could use another run through on it. Just in case."
"Can you describe more about the concept of parallel resistors?"
"The section on circuits wired in series and parallel. I am still trying to understand where one starts, the series or parallel."
"I do not really understand how a voltmeter makes measurements. I understand the equations involving power, but am unsure of how to apply the equations. I think it will make sense after practicing with lots of examples."
"I do not really understand why ammeters need to have a low resistance and voltmeters want to have a high resistance. Also (this might be from what we covered earlier) I do not understand why the resistance will decrease when multiple things are plugged into a circuit. This applies to my house because it happens fairly often and my circuit breaker stops the current."
"I am not sure why the resistance for an ideal ammeter is equal to zero while the resistance of and ideal voltmeter should be very high."
"I think I am kinda fuzzy on some of the terminology and how each of the aspects are used. I get the basic concepts of what is going on in each part of the presentation, but if I have to describe it using the right terminology, I am pretty lost."
"Not too confused after this chapter."
"It wasn't too confusing, but I need to read through it once more to get the concept a bit better."
"All pretty good I think! Maybe?"
What are the resistances of these (ideal) devices?
(Only correct responses shown.)
Ideal ammeter: 0 [75%]
Ideal voltmeter: ∞ [69%]
(Only correct responses shown.)
Equivalent resistance Req of circuit: decreases [78%]
Current I flowing through emf source: increases [56%]
current. ********************** [22] voltage. *** [3] (Both of the above choices.) ****** [6] (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.
"This subject can be shocking."
"I really liked the 'Twinkle, twinkle, little star, Power equals I-squared R' nursery rhyme. I'll definitely remember this."
"This was a super-interesting lecture because my house burnt down and there were suspicions that it was because of a faulty circuit breaker! So now I understand how that could be majorly bad. I wonder how likely that is?"
"I guess the electrons could just be looked at as the current (or the amount of electrons flowing through a circuit is equal to the current)?" (Yes, but remember that the direction of current is actually the reverse of the actual direction of electrons, due to the arbitrary assignment of negative charge to electrons.)
"Why should a voltmeter have an ideal resistance of infinity?" (When current flows through a resistor, the current doesn't get "used up" (the same amount that goes in comes back out), but the potential/voltage/energy per charge gets "used up." So a voltmeter that measures how much voltage gets "used up" by a resistor is attached on either side of the resistor to compare how much voltage there is in the current before entering the resistor, to how much voltage there is in the current after exiting the resistor. You don't want current to actually flow through the voltmeter when taking these measurements (as you don't want it to affect the current flowing through the resistor that it's measuring), so it should ideally have a resistance of infinity in order to "block" current flowing through itself.)
"What is more dangerous volts/voltage or current? Are they the same?" (As for which is more dangerous, current is the actual (backwards) flow of electrons, and too much of that going through your body is bad. Voltage by itself is just the measure of how much energy per charge there is that current could potentially use to flow through you. A very crude analogy is that a bowling ball dropped on your foot is bad. A lot of bowling balls continuously dropped on your foot is very bad. (This is analogous to the amount of current.) Whether a bowling ball starts from an inch above your foot, or from up on a very high shelf above you is a measure of how much potential energy there is should it drop. (This is analogous to the amount of voltage.) So lots of current with a high voltage is dangerous, but high voltage by itself (with no current) is not dangerous, as long as you don't touch anything that would conduct current through yourself.)
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