Online reading assignment: capacitors

Physics 205B, spring semester 2018
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.
"The capacitance of a capacitor is fixed given its separation distance and area. Capacitance gives us an idea of the potential energy a capacitor can carry."

"The capacitance of a capacitor is fixed once it is constructed, in order to change the capacitance the area or the separation distance must first be changed."

"A capacitator has two metal plates held at a fixed distance apart, and an equal area. You charge them up by connecting them to a voltage source."

"Capacitance is fixed once a capacitor is constructed. And I understand that with a bigger area and a smaller separation distance the capacitor has a bigger capacitance."

"That once built, a capacitor can't change it capacitance. This is because its parameters are inherently fixed, and they just serve as a way to hold a charge."

"Capacitance is fixed once a capacitor is made, the only way to change its value is to reconstruct the capacitor itself. Capacitance is a measure of 'charge-storing efficiency.'"

"You can store electrical potential energy in a capacitor for extended periods of time. I also understand that you can calculate the amount of energy a capacitor can store."

"Once constructed a capacitor has a fixed amount of storage, and capacitors store potential electric energy that can be released very quickly unlike a battery."

"Capacitors can hold an electrical charge that dumps over time. When a machine is turned off then it still can have electrical charge due to the capacitors."

"Capacitors store electric potential energy by transferring electrons between plates. The first electrons to be transferred do so "easily", while the last electrons are more "difficult" and that is how the capacitor reaches its maximum storage."

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 would like clarification on what capacitance actually is. What are farads?"

"How the positive charge knows to go to one end of the capacitor. I understand electrons can move, but aren't the protons technically moving too? I am confused."

"I would like to actually see in class examples of how to use the capacitor energy storage equations. Both the equations were hard for me to understand."

"Pretty much all of the reading was confusing. Especially the construction section."

"The formulas that you use to determine a capacitors ability to store energy."

"I am struggling with using the equations from this chapter."

"I think I would need more time before deciding on what topics I need more clarification for."

"Most of this."

"Nothing for now."

"I think I'm understanding this topic correctly."

Describe two quantities that a capacitor is designed to store/hold.
"A capacitor is designed to hold voltage and electric potential energy."

"Coulombs and volts."

"Charges and EPE."

"Positive and negative charges."

"Capacitors are designed to store charge and electric energy or voltage. The charge and energy comes from the electrons that it is storing."

"Electrons and electric potential energy."

"Area and separation distance?"

State the unit of capacitance, and give its definition in terms of other SI units.
"The unit of capacitance is the farads (F)."

"C2/J, also known as farads."

"The unit of capacitance is the farad (F) which is coulombs/volt."

"Farads (F). Would like clarification on this."

"Not sure yet."


For a parallel-plate capacitor, ___________ the plate area and __________ the plate separation would increase its capacitance.
decreasing; decreasing.  * [1]
decreasing; increasing.  ***** [5]
increasing; decreasing.  **************** [16]
increasing; increasing.  **** [4]
(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.  ******** [8]
(Unsure/guessing/lost/help!)  ** [2]

Explain why increasing or decreasing the voltage (electric potential difference) of a capacitor cannot change the numerical value of its capacitance.
"Because capacitance is defined by the capacitor's construction, not its charge."

"Because there is only a certain amount of capacitance that a capacitor can have based on its plate area and the separation of the plates."

"Capacitance is fixed once the capacitor is made."

"The capacitance of a capacitor is fixed once it is constructed, as the only way to change the capacitance is to change its 'build' parameters: the area A and/or the separation distance d."

"The capacitance of a capacitor is fixed once it is constructed. The only way to increase/decrease its capacitance is to change the area (A) and/or the separation distance (d) of its parallel metal plates. Increasing/decreasing the voltage only creates a potential difference between the plates."

"You are not changing the 'build' of the capacitor, only the EPE of it."

"No clue. It should make the capacitor's capacitance value go up and down?"

"Not sure yet."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Could you please explain why increasing or decreasing the voltage of a capacitor cannot change the numerical value of its capacitance?" (We'll go over this in more detail in class today, but the capacitance of a capacitor depends on how you built it--area of the plates, separation distance between the plates, etc. How you use the capacitor (by applying a certain voltage to it, by putting a certain amount of charges on it, by storing a certain amount of energy in it) does not change how it was built.)

"Could you go over how the electrons move while the protons essentially stay fixed? The GIF animation in the presentation was pretty confusing for me." (In a conductor, the protons are all in nuclei of the metal atoms that are fixed (because we're talking about solids, right?), while the valence electrons are free to move from atom to atom.)

"Hey P-dog!" (Holla.)

"Ugh... I am so behind. :("

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