Online reading assignment: capacitors

Physics 205B, spring semester 2015
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.
"A capacitor is used to store charges, once build the amount of charge does not change. As the capacitor is charged the process slows down toward when it reaches capacity because of the separation of like charges. The energy stored can be quickly released."

"Capacitors store electric potential energy. They vary from batteries because they can release all of their energy very quickly."

"Once constructed, a capacitor's capacitance is fixed. But you can change the amount of charge it stores by hooking it up to a battery or some other power source. Charging happens quickly and easily at first, but as the positive plate becomes increasingly positive, it requires more energy to pull the negative electron away, and it takes more energy to force the electron onto the already negatively charged negative plate."

"Capacitors store potential energy. It takes work to do such a process."

"Once a capacitor is fully charged, the top and bottom value have same charge of +Q and -Q. Only way to change capacitance is by changing the area and/or the separation distance."

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.
"Things started to get a little dodgy with all the equations. It seems like some of them relate to each other and certain terms can be substituted into other equations, but I can't quite tell how."

"I am still unsure on the relationships involved. I guess I would benefit from some examples."

"I was understanding everything until I got to energy stored in an electric field."

"Ummmm...what exactly is a capacitor? How is it different than a battery? And how do I make a flux capacitor?'"

Describe two quantities that a capacitor is designed to store/hold.
"Positive and negative charges."

"Electric charge, and energy."

State the unit of capacitance, and give its definition in terms of other SI units.
"Farads, since C = Q/V, they're coloumbs over volts."

"F, farads or coulumbs-squared over joules."

"Could you please go over this in class?"


For a parallel-plate capacitor, ___________ the plate area and __________ the plate separation would increase its capacitance.
decreasing; decreasing.  * [1]
decreasing; increasing.  ***** [5]
increasing; decreasing.  ************************* [25]
increasing; increasing.  **** [4]
(Unsure/guessing/lost/help!)  **** [4]

For a parallel-plate capacitor, increasing the voltage (electric potential) difference applied to the capacitor would __________ the amount of charge stored in it.
decrease.  ** [2]
increase.  *********************** [23]
have no effect on.  ********** [10]
(Unsure/guessing/lost/help!)  **** [4]

Explain why increasing or decreasing the voltage (electric potential difference) of a capacitor cannot change the numerical value of its capacitance.
"Once a capacitor is built, its capacitance is fixed."

"Because the plate area and plate separation are fixed, the capacitance won't change."

"As the voltage increases or decreases so does charge. According to the equation C = Q/∆V, the capacitance would not change."

"Please go over this in class."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"No questions...yet."

"So in the Spider-Man comics, the Shocker is acting as a human capacitor?"

"I know that large capacitors have the potential to kill, even if they have been undisturbed for months."

"Are capacitors necessary?" (Depends on how you feel about the two questions above.)

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