Online reading assignment: capacitors

Physics 205B, spring semester 2017
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.
"This section was on capacitors. We can calculate the capacitance of a given parallel-plate capacitor from its area A and separation distance d. The capacitance of a capacitor is fixed once it is constructed."

"I understand that capacitors use two metal plates, one positively charged and one negatively charge, to hold electric charge for later use, given a potential. Capacitance is a measure of how well it can do it, with some parameters being the surface area and the distance between the plates."

"The construction of the capacitors. They are build with two metal plates with same area and a space between the whether it be air, vacuum, or some type of dielectric."

"Even though capacitors come in many different shapes and sizes, carefully taking one apart will demonstrate certain common features. In the case of this cylindrical capacitor, after removing the casing, and unrolling its layers, there are two parallel metal sheets."

"It is interesting that the same system is used in defibrillators and camera flashes. Such different processes, yet they utilize the same technology."

"In contrast to batteries capacitors can release electric potential energy stored much quicker and with more energy due to its epe being stored. Electric energy that is traveling through our body can be considered electric potential energy just as that of a capacitor. Capacitance equation are interchangeable with which variable you chose to solve for. ∆V (or potential value) and Q the charge of an atom or electron is what makes up electric potential energy."

"A capacitor's capacitance is fixed once it is constructed. One is able to change the amount of charge it can store by connecting it up to a battery or another power source. The charge can occur fast and easy at first, but when the positive plate start to become 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."

"The capacitance of a capacitor is a fixed value based on the area of two metal plates and the distance between the two plates. When a capacitor is charged by a voltage source, the initial cost of charging is minimal, and the end cost of charging is significant. The difference in the cost of charging is caused by the extent to which the plates are no longer neutral; highly charged plates will continue to charge at a higher cost."

"Capacitors! Capacitors! Capacitors!"

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 am slightly confused how capacitors store electric potential energy. I understand it is when connected to an energy source but does it matter how much energy?"

"How the two parameters Q and ∆V in the electric potential energy equation can each be substituted out, yielding two other equivalent equations for electric potential energy EPE."

"I'm confused by how we can calculate the capacitance of a given parallel-plate capacitor from its area A and separation distance d. "

"Still unsure how the equations relate, and why the parameters can be swapped out."

"There are a lot of variables and terms in this lesson, so I need more practice with solving problems using these equations."

"A lot, honestly. I read through the blogs twice but im still a bit confused. probably because its hard to keep in mind what's meant by all the terms, as they are all somewhat new to me."

"Electricity is confusing."

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

"The capacitor is designed to store electrical charges and energy."

"Electric potential energy and electrons."

"Charge and energy."

State the unit of capacitance, and give its definition in terms of other SI units.
"Farads. The capacitance of a capacitor in which one coulomb of charge causes a potential difference of one volt."

"The unit of capacitance is Farad which is (coulombs2)/joules."

"Potential energy, V?"

For a parallel-plate capacitor, ___________ the plate area and __________ the plate separation would increase its capacitance.
decreasing; decreasing.  [0]
decreasing; increasing.  *** [3]
increasing; decreasing.  ********************** [22]
increasing; increasing.  [0]
(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.  ***** [5]
increase.  ******************* [14]
have no effect on.  ******* [7]
(Unsure/guessing/lost/help!)  * [1]

Explain why increasing or decreasing the voltage (electric potential difference) of a capacitor cannot change the numerical value of its capacitance.
"It's independent of the voltage."

"The capacitance is dependent on the area of the plate and the distance of air between the plates, once that is set (the capacitor is built) it cannot be changed. So if you bring in the outside source of voltage the capacitor will only be able to handle whatever it was built for."

"The only way the numerical value of a capacitor's storage can be changed is by manipulating the build parameters: area and or separation distance."

"The capacitance is a fixed value. The only things that can be shifted in a capacitor is the voltage or the amount of charge inside."

"When the voltage increases or decreases the charge does so as well. According to our equation C = Q/∆V, the capacitance will not change.When a capacitor is built, the capacitance is fixed."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Are capacitors used in the KERS (Kinetic Energy Recovery Systems) for Formula One cars?" (A KERS car converts kinetic energy that would be normally lost to braking and instead stores that energy mechanically in flywheels (weighted rotating disks) or electrically in lithium-ion batteries. Energy could instead be stored in capacitors, but right now their storage efficiency is only about 5% that of lithium-ion batteries.)

"I'm not sure if Q and EPE are the same thing?" (Charge and electrical potential energy are not the same thing, but they're related. If you move two opposite sign charges away from each other, then you doing work separating them, and this means that there is now electrical potential energy stored. So, for capacitors, if you start with both plates being neutral, and separate out positive and negative charges so they're stored on each plate, then you did work to sort them out this way (remember, positive and negative charges are attracted to each other and don't like being apart from each other), and so this shows up as the electrical potential energy stored in the capacitor. Later on, if you let the charges flow back to each others' plates, then the electrical potential energy is released as an electrical current.)

"I don't fully understand how a capacitor 'stores' energy, I get that charges get separated to each plate, but how do they just stay there?" (You can either leave the capacitor connected to a battery (which keeps "pushing" on the charges to keep them from flowing back to each others' plates), or you can disconnect the battery, leaving each plate separated and isolated from each other. If you did connect a wire from one plate to the other, or touched both plates, then the charges can flow back to each others' plates, "discharging" the capacitor and releasing energy in the process.)

"How is a capacitor different from a battery?" (A capacitor stores energy by the separation of opposite charges, this energy can be released later when the charges are free to flow back to each others' plates. A battery stores energy by the separation of different chemicals/materials (which have different affinities for electrons), this energy can be released later when electrons are allowed to flow from one chemical/material to the other.)

"Can capacitors still store energy when they aren't connected to a battery?" (Yes, although capacitors in properly-designed power supplies are meant to discharge in a reasonable amount of time after they are turned off. Still, to be safe, electricians will exercise caution by manually discharging a capacitor before handling it.)

"Remember to turn your clock forward an hour this weekend!" (#springfoward.)

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