20190311

Online reading assignment: electric potential energy

Physics 205B, spring semester 2019
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 electric potential energy.


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.
"Pushing together repelling charges and pulling apart attractive charges causes electric potential energy to increase and the opposite causes electric potential energy to decrease."

"A positive charge accelerates from higher electric potential region toward a lower electric potential region. The opposite happens for a negative charge."

"We know that a source charge can exert an electric force on a test charge, and now we know that a source charge can 'store' electric potential energy on a test charge! Whoa! Changes in this potential energy changes based on how far apart the source and test charge are from each other. More potential energy is stored when like charges are pushed together because this requires energy since these opposite charges will repel while the change in potential energy will be smaller when like charges are allowed to repel each other or opposite charges are allowed to attract towards each other."

"Electrical potential energy, EPE, can be observed using a direct approach (source Q stores EPE with test q) or a two-step approach (source Q creates Voltage potential which then stores EPE with test q). In both cases, electric field lines will point towards decreasing electric potential values. A positive test charge, q, will also point toward the direction of decreasing potential and decreasing potential energy. A negative test charge will point towards increasing potential but decreasing potential energy."

"Electric potential energy can be increased by pushing two same sign charges together because they will repel and that the same will happen when you pull apart opposite sign charges because they want to stay together. These both cases will cause EPE to become positive. When charges are able to do what they do naturally, which is to repel from same sign charges and move closer together to opposite sign charges, this will cause the EPE to become negative because it will decrease. I also learned that potential at a location in space is also referred to as voltage."
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 do not understand the concept of the equipotentials. I do not understand the relevance and meaning of the field lines and equipotential circles. I understand that it has to do with potential energy but that's about it."

"How is electric energy measured and observed in reality? What are the implications of high or low EPE? "

"How when we have a positive source charge and a positive test charge we get increasing potential energy but if they're left alone you get decreased potential energy. I also found the whole snowmobile analogy very confusing."

"So the potential of a point charge is a potential difference with a potential at infinity. I'm not quite sure what this is supposed to mean other than that if the point charge is positive that its potential is raised above the reference value of 0. So if the point charge is negative then its potential is lowered below the reference value of 0?"

"Sometimes it's hard to tell which direction stuff is pointing in different cases."

"I'm a bit confused with the concept for the two step approach to electric potential energy. SO the source charge won't directly affect the test charge instead it will create an electric field which will then exert a force onto the test subject. I guess what I don't quite understand is why does there need to be almost like a middle step to the direct approach."

"I didn't understand why electric field lines point towards decreasing electric potential values for both positive and negative charges."
Explain the difference between the units of electric potential V, and electric potential energy, EPE.
"V is measured in joules per coulomb and EPE is measured in joules."

"I didn't understand this."

"I think both of these are the same?"

Explain the conceptual difference between the electric potential V, and electric potential energy, EPE.
"A source creates a potential V, everywhere around it and the EPE is stored with the test charge. The source charge creates an electric potential everywhere around it on its own. EPE is stored when the source charge and test charge are brought closer together or farther apart. EPE is only made from both charges."

"EPE is a measure of energy. Electric potential (V) is a measure of energy per charge."

"Still a little foggy on this. It usually takes me seeing examples in class to fully understand the differences between these things and why they are different."

"I don't know"

Briefly summarize the difference (if any) between "voltage" and electric potential.
"There is none."

"I believe voltage is the same as electric potential."

"Potential and voltage are the same thing."

"Really not making sense."


Identify the changes in electric potential energy EPE (if any) for the following test charges (±q):
(Only correct responses shown.)
Positive test charge +q brought closer to a positive source charge +Q: increase [67%]
Negative test charge –q brought closer to a positive source charge +Q: decrease [61%]
Positive test charge +q brought closer to a negative source charge –Q: decrease [64%]
Negative test charge –q brought closer to a negative source charge –Q: increase [64%]

Identify the changes in electric potential V (if any) for the following test charges (±q):
(Only correct responses shown.)
Positive test charge +q brought closer to a positive source charge +Q: increase [44%]
Negative test charge –q brought closer to a positive source charge +Q: increase [36%]
Positive test charge +q brought closer to a negative source charge –Q: decrease [58%]
Negative test charge –q brought closer to a negative source charge –Q: decrease [31%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I am still not comfortable about the difference between electric potential and electric potential energy. I don't understand why work increases electric potential energy when two like charges are moved closer together or when two different charges are moved away from one another." (Two like charges repel each other, so it will take "effort" in trying to push them together. This "effort" is the work you are doing in increasing their electric potential energy (like compressing a spring raises its elastic potential energy). Likewise two opposite charges attract each other, so it will take "effort in trying to pull them apart. This "effort" is the work you are doing in increasing their electric potential energy (like stretching a spring raises its elastic potential energy.)

Can V be likened to a field if EPE can be likened to a force?" (Yes, as potential is measure of the energy that a charge could have if placed at a location near a source charge, while electric potential energy is energy two charges have with each other. Likewise electric field isa measure of the force that a charge could have if placed at location near a source charge, while electric is the force two charges exert on each other.)

"The book is difficult to read and the blogs are a bit hard to understand as well. It would be nice if actual problems where posted on the blogs with the answers and the step-by-step how to solve that way we would try and understand how to do these problems before coming to class. The blog is a bit vague and shows the formulas but posting actual examples like how we work examples in class would be very helpful." (Point taken; keep letting me know which problems from the homework reports you would like me to work out in class, while I will still go over other examples proactively in class (as time allows) before you do the subsequent homework report. Also the electric forces/fields/energy/potentials concepts and problems are very abstract, so keep at it, and we'll develop these ideas further as well get into magnetic forces and fields later.)

"Can you go over the snowmobile model in class?"

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