20170306

Online reading assignment: electric potential energy

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 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.
"Electric potential energy increases when two like sign charges are pushed together or when two opposite sign charges are pulled apart. EPE decreases when two like sign charges are moved farther apart or when two opposite sign charges are moved closer together."

"This section was on electric potential energy. A direct approach to electric potential energy is a source charge that can store electric potential energy on with a separate test charge charge. A two-step approach to electric potential energy is that a source charge first creatse a potential everywhere around itself."

"I understand the difference between joules and joules per coulomb (volts), so I use that to help conceptualize everything."

"when we are dealing electric potential energy there are a couple of ways to deal with it. The first way is known as direct approaches. This method looks at whether or not the source charge Q exerts a force on, or stores EPE on test charge q. The equation for this method that is EPE = k·Q·q/r. The second way to deal with electric potential energy is known as a two-step approach. This approach looks at how the source charge Q creates a V potential around it, that then stores EPE with a test charge q. This particular method has two separate equations. One of the equations deals with determining the electric potential of the source charge Q, the second way is to be able to use the magnitude from the potential one has calculated from the first step to be able to find the value of electric potential energy of a test charge q."

I understood that there are two different approaches to looking at electrical potential energy. I also mostly followed the equations, but really the concepts were confusing for me."

"Nothing."
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'm still confused with the difference between electric potential and electric potential energy."

"I do not yet understand the differences in voltage compared to electric potential energy."

"I am not 100% certain about the effect that moving a test charge closer to or farther from a source charge will have on EPE and V. The effect is pretty clear when the source charge is positive, but I am having difficulty imagining the effects when the source charge is negative."

"I am confused about equipotentials. For a positive source charge do the circles (equipotentials) closest to the center of the charge have a greater or smaller potential value?"

"What I found confusing was the stuff on equipotentials and how they have an effect on electric fields. I also had a hard time understanding how potential energy is affected by the peaks and wells."

"I am confused about the section on equipotentials. The explanation was confusing to me, and I don't believe I fully grasped the concept that was being conveyed."

"A lot."

"Everything."
Explain the difference between the units of electric potential V, and electric potential energy, EPE.
"The units for electric potential V are measured in joules per coulomb or volts. The units for electric potential energy are measured in joules."

"Potential V = volts; EPE = joules."

"Volts are electric potential which have units of joule/coulomb, this is the electric potential energy per unit charge, furthermore, electric potential energy simply has units of joules."

"EPE is in Joules and electric potential V is in J/C (which is volts, which is the amount of potential it has). To be honest I am not sure if I can explain the difference other than that."

Explain the conceptual difference between the electric potential V, and electric potential energy, EPE.
"Electric potential energy is a type of potential energy that results from electrostatic forces. It is measured in joules, and depends on the positioning of charged particles relative to one another, as well as the magnitude of their respective charges."

"A source charge Q can create an electric potential V everywhere around itself. This potential stores electric potential energy with a separate test charge q."

"Electric potential is an energy per unit charge, while electric potential energy is the actual energy."

"You get EPE when you multiply V with q."

"I am looking for that answer because this is where it really isn't clear for me."

Briefly summarize the difference (if any) between "voltage" and electric potential.
"There isn't a difference."

"The terms are used interchangeably."

"The amount of electric potential is measured in volts."

"The difference between voltage and electric potential is something I do not know!"

"I have no idea."

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 [75%]
Positive test charge +q brought closer to a negative source charge –Q: decrease [71%]
Negative test charge –q brought closer to a negative source charge –Q: increase [75%]

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 [50%]
Negative test charge –q brought closer to a positive source charge +Q: increase [25%]
Positive test charge +q brought closer to a negative source charge –Q: decrease [54%]
Negative test charge –q brought closer to a negative source charge –Q: decrease [29%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"When discussing peak and wells, and source and test charges, don't EPE and V have to change in the same way?" (Not necessarily, but EPE and q·V need to change the same way.)

"Would a source Q in the potential model also have an electric field around it?" (Yes, the equipotentials it creates around itself is definitely related to the field lines that it makes around itself.)

"Are higher voltage batteries large because they need more distance to create a greater change in EPE?" (Well, probably more sets of chemical cells, as a chemical reaction only creates a set amount of potential difference (say, 1.5 V). So in order to create a 12 V difference between terminals, a battery would need to stack eight of these 1.5 V chemical cells in a row.)

"I think working through potential model examples would be beneficial."