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 previewing presentations on capacitors.
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.
"A capacitor consists of two conducting plates of the same geometry and with opposite charges of the same magnitude close together but not touching. The space between capacitors is sometimes filled with a non-conductive dielectric substance. When a dielectric is used, the capacitance (the proportional constant of a capacitor) increases and the electric field between the plates decreases."
"Capacitors store EPE and charges."
"Capacitance is fixed once constructed. The capacitor has a bigger capacitance with a bigger area and a smaller separation distance."
"They capacitance of a capacitor is fixed once it's built, but you can change the amount of potential applied. Applying a high potential will allow it to store more charge, if you apply less potential then it it stores less charge."
"Plate capacitors work similar to batteries in that they store electric charges and create 'pressure' that causes a circuit to function."
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 don't quite understand the whole idea of changing the amount of charge that each capacitor can hold. I just don't quite understand how changing the potential difference would change the amount of charge a capacitor."
"How is electricity initially stored in the capacitor and how is it released evenly?"
"What I don't understand is why the first electron moves quickly and the last moves the slowest. I don't understand the concept of 'start-up' and 'end-cost.'"
"I'm a bit confused on how to use the equations for capacitor energy storage."
"Can we go over substituting terms in the EPE equations?"
"Why there are three equations for capacitor energy storage and why they're all necessary."
"I'm confused about the units that is being used in this chapter. Id like to have explanation on this chapter little bit more time."
"The equations are confusing."
Describe two quantities that a capacitor is designed to store/hold.
"Charge and electric potential energy."
"Capacitors are designed to store/hold electric potential energy by storing a given amount of charge."
"Coulombs and joules."
"Positive and negative charges?"
State the unit of capacitance, and give its definition in terms of other SI units.
"The unit of capacitance is farads, which are coulombs2 per joule. It measures the units for charge per potential."
"The farad (F) named for Michael Faraday. The units are coulombs/volt."
"The unit of capacitance is farads (F). As far as defining in terms of other SI units I am confused. I battle with units and conversion. Wut? Confused guy emoji."
decreasing; decreasing. [0] decreasing; increasing. ***** [5] increasing; decreasing. ******************** [20] increasing; increasing. *** [3] (Unsure/guessing/lost/help!) ** [2]
For a parallel-plate capacitor, increasing the voltage (electric potential) difference applied to the capacitor would __________ the amount of charge stored in it.
decrease. [6] increase. **************** [16] have no effect on. ***** [5] (Unsure/guessing/lost/help!) *** [3]
Explain why increasing or decreasing the voltage (electric potential difference) of a capacitor cannot change the numerical value of its capacitance.
"Because the capacitance is fixed once the capacitor is built."
"With the voltage change the amount of charge also changes."
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Knowing that 'endless amounts of amusement await us when doing capacitor energy problems' is good to know. Now were prepared and understand that this is something we really need to figure out how to do right."
"It was interesting to learn that the 'ones' and 'zeros' in computers are stored/delineated through in and with transistor/capacitor combinations (in the millions) in RAM chips. This was such an abstract idea for me for a long time. It is odd to start understanding how it works now."
"I am not really familiar with capacitors, so I'm not entirely sure what they are or what they do (in a practical sense), like what are they used for? I get the whole process of making them, how they work, etc., but I don't really know what they are in other than defibrillators." (As in defibrillators, capacitors are really good at storing energy (and charge) that can be released quickly, for things like camera flashes and tasers. But they can also take in energy (and charge) very quickly--so they can regulate and control surges in circuits--and can be found in surge protectors, chargers, and in audio systems.)
"Does the material of a capacitor matter?" (For the plates, no, as long as they are made of any conductive metal. However, the material between the plates does matter, whether you use a gap of air between them, or some other insulating material (a "dielectric") such as a layer of glass or plastic, or even certain types of oil or solvents, these materials will typically raise the capacitance value more than having just air between the plates.)
"A lot of the online presentation was not loading for me last night. It would have been nice to see the photos because I was a little confused on what the presentation was talking about." (It wasn't just you; the presentation slides are hosted by Google, which had connectivity issues last night.)
"I would like the 'KRIF' image example to be explained a little more." (Capacitors are commonly used in sound systems to regulate current surges, so high-capacitance capacitors are sold at a premium to people who are willing to pay those prices for their audio systems. This means there are unscrupulous manufacturers who deliberately mislabel and repackage their low-capacitance capacitors as having much higher capacitance values. Caveat emptor.)
"Electricity is scary conceptually and in actuality."
"More practice on equations please."
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