Online reading assignment: capacitors

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.

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.
"For some reason I didn't connect until now that electrons are the only ones that move in a conductor. Capacitance: the relationship between how much voltage is applied and how much charge is actually stored."

"I understand everything that is on the quiz because I studied for that instead of doing the reading."

"From what I read, it talked about the storage of capacitors. Capacitors are like batteries that store energy."

"Capacitors store electric potential energy by storing separated positive and negative charges. The parallel plate capacitor is the simplest form of capacitor. Two plates are spaced a distance d apart and the plates are of equal area A. Electrons are the only mobile charges in a conductor making them move from the top plate to the bottom plate. The first electron moves easily showing the energy cost of effectively zero and the last electron since it is being pulled away from all the positive charges and towards all the negative charges it takes a lot of work, and has a high energy cost."

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 did not understand why voltage does not affect capacitance."

"I'm sure this will all be confusing for me in class tomorrow."

"To be honest, I did not know what a capacitor does. I'm still a little confused."

"I'm still very cloudy on what potential energy is and electric potential energy is."

Describe two quantities that a capacitor is designed to store/hold.
"Charge and energy."

"A capacitor stores electric charges and it stores energy in the form of those electric charges."

"Negative and positive charges."

State the unit of capacitance, and give its definition in terms of other SI units.

"Coulombs per volt: F (farad)."

"Farads: coulombs2 per joule."

For a parallel-plate capacitor, ___________ the plate area and __________ the plate separation would increase its capacitance.
decreasing; decreasing.  * [1]
decreasing; increasing.  **** [4]
increasing; decreasing.  ************************** [26]
increasing; increasing.  [0]
(Unsure/guessing/lost/help!)  *** [3]

For a parallel-plate capacitor, increasing the voltage (electric potential) difference applied to the capacitor would __________ the amount of charge stored in it.
decrease.  **** [4]
increase.  *************** [15]
have no effect on.  ************ [12]
(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.
"I'm not sure."

"Because it is a constant."

"Because once a capacitor is built the capacitance is fixed. The capacitance is based on the area of the plates and the distance apart."

"The electric potential (∆V) is not part of the capacitance equation."

"Because it is a positive value in the equation."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I'm confused about why atomic nuclei in a conductor remain fixed. I thought that subatomic particles are constantly moving. Does this mean that electrons are transferring from the nuclei of one atom to the nuclei of another?" (The outer valance electrons transfer from nuclei to nuclei, but the more massive nuclei, surrounded by their inner shell electrons are relatively stationary, held in place by each other.)

"Why do capacitors explode when they get too many volts?" (Many capacitors have oil (or an oil-impregnated fabric sheet) between its plates to boost its capacitance value. Some of the energy put into a capacitor (as with all circuit components) is converted into heat. You do the math.)

"A formal lecture would really help tie everything together, a lot of us extremely lost. Thank you."

"I want to learn how to electrocute people legally." (Consider either a medical or law enforcement career.)

"Your 'endless hours of amusement' comment is freaking me out."

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