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 charges and materials and electric forces and fields.
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.
"I understood the differences between a conductor and an isolator. The polar molecule definition was also easy to understand from chemistry."
"Electrons move freely in a conductor but are still bound to atoms in a insulator but electrons will still orient to opposite charges on insulators. Polar molecules will also orient their opposing charge to a charged object, becoming aligned."
"Considering electrical charge, insulators result in outer electrons that are mostly fixed to their atomic location, while conductors allow electrons greater ability to move about the material. Both neutral conductors and insulators will be attracted to positively and negatively charged objects. Electrical forces and electrical fields will exert a force on a test charge."
"I think I understand Coulomb's law, it is a way to measure the force exerted from a source charge to a test charge. I understand it as the force an object is exerting on another object, but I am not entirely sure."
"Make sure to not have static electricity around a gas pump. I'm pretty sure I understand the differences between source charges and test charges as well as the electric force and electric field."
"From this section I learned that there are electric fields and electric forces that both act together to produce the force acted on something. With an electric field, the force applied to something is reliant on the type of electric field and the positive or negative test charge q."
"I quickly skimmed this and did not really understand anything. I didn't really get a good look at anything."
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.
"Not exactly sure about the insulator/conductor attractions."
"I had confusion with multiple aspects of this material starting with the definitions of isolators and conductors and what objects we might categorize as either insulators or conductors. Although, I'm proficient in the simple idea that opposite charges attract and same charges are repulsed from each other. I also was confused with the idea of electric fields and the interactions with electric forces. I feel like I'm on the verge of understanding in that the interactions of two charged objects--or possibly a single neutral object--creates an electric force either attracting or repulsing which in turn establishes an electric field in which that electric force occurs. But again, generally confused."
"The polarization aspect with insulators and conductors. It was confusing to understand the exact differences."
"I was very confused about the conductor and insulator concepts. I don't understand how the electrons come into play at all."
"I understand polarization but don't quite have a strong grasp on the characteristics of insulators and conductors. Also, fields are going over my head as a whole."
"I am kinda confused on what Coulombs law turns out to be unit-wise."
"I'm having trouble understanding electric forces and fields."
"The force model and field model did not make much sense to me."
"I found the concept behind the electric forces and field to be confusing. More specifically, the difference between the two."
"In the field model, I am confused by what is means for +q: F 'obeys' E, and for –q: F 'disobeys' E."
"Haven't got to this yet."
"I mean I honestly can't think of a specific question. I guess it would be helpful to go over as much as you can."
"I didn't find anything confusing in this topic."
"I think it is when she exits the car. I have heard of this occurring, but never understood what actually happened."
"Two insulators with different electron affinities are rubbed against each other. When she rubs her hand her shirt, it causes a reaction due to this difference in electron affinity."
"It was when she came into contact with the cloth on her seats in her car."
"The woman became electrically charged when she went back in her car and then touched the gas pump after getting out. She created friction by sliding in and out of her car, and the fire started after she touched the metal gas pump."
"When the woman stepped into her vehicle she created some exchange of electrons by rubbing tohether two different insulators with two different affinities for electrons. When she then reached for the pump, those electrons were sent toward the metal pump causing a spark."
"Static electricity allowed her to carry electrons making her negatively charged. When she got close to the pump, she transferred these electrons causing a spark."
"I am not super-positive, but I am going to guess that electrons were transferred by her rubbing her sweater and then touching the pump."
a positive. **** [4] a negative. *** [3] either a positive or a negative. *********************** [23] zero (neutral). * [1] (Unsure/lost/guessing/help!) ** [2]
a positive. *** [3] a negative. **** [4] either a positive or a negative. ********************** [22] zero (neutral). * [1] (Unsure/lost/guessing/help!) *** [3]
a positive. *** [3] a negative. **** [4] either a positive or a negative. ********************** [22] zero (neutral). * [1] (Unsure/lost/guessing/help!) *** [3]
Explain the difference between units for electric force, F, and the electric field, E.
"Force is in newtons, and electric field is in newtons per coulomb."Explain the conceptual difference between the electric force, F, and the electric field, E.
"N vs. N/C."
"The electric field units are taking into account two different charges that are exerting a force on each other (q1 and q1, both in coulombs) divided by the square of the distance between them (m2). k is a constant and has units of N·m2/C2. When you calculate the force of the electric field coulombs and meters cancel out leaving newtons. For the electric field, we once again have the constant k with units N·m2/C2 multiplied by the source charge Q (C) and divided by r2 (m2). When you plug into the formula, all units except N/C cancel."
"I don't understand the difference in units."
"An electrical force is created directly by a source charge Q and can affect a test charge q, where as an electric field is created by a source, and this electric field can cause an effect on a test."Explain the conceptual difference between a source charge (±Q), and a test charge (±q).
"Electric field refers to all the space around a source while electric force is the magnitude and direction of the field being exerted on the test charge."
"The electric force is more direct and is a one-step process. While the electric field is indirect and has two steps."
"The electric field surrounds the source charge. The electric force is what is exerted onto the test charge."
"I don't understand the difference."
"Will get to this later."
"I'm honestly not sure."
"A source charge Q creates an electric field, and a test charge q receives the force from the field."
"A source charge Q is a charge exerting a force onto a test charge q. The test charge is the charge which is being acted upon which will either be attracted to or repulsed by the source charge dependent upon whether those two charges have opposite (attract) or the same (repulsive) ± signs."
"A source charge Q is the charge that exerts the force on the test charge q, while the test charge is being exerted on by the source charge."
"Source charge exerts a force on the test chare. Source is giver, test is receiver."
"Q defines the direction of E and q is the relationship between F and E."
"This is what I need help with. I know a test charge has a very small magnitude because it shouldn't change the source charges. But don't understand why a test charge is necessary?"
(Only correct responses shown.)
Positive source charge (+Q): E field lines point outwards [75%]
Negative source charge (–Q): E field lines point inwards [70%]
along the E field direction. ******************** [20] opposite the E field direction. ****** [6] (Unsure/guessing/lost/help!) ******* [7]
A negative point charge (–q) is placed on an electric field line (due to some other source charge ±Q). This positive point charge will always experience an electric force F:
along the E field direction. ****** [6] opposite the E field direction. ******************** [20] (Unsure/guessing/lost/help!) ******* [7]
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"The reading said a neutrally charged insulator's electrons would turn away from a negatively charged external material. Why are they two materials still attracted?" (The insulator's positive nuclei would then be fractionally closer to the negatively charged external material, and this attractive force would be greater than then repulsive force between the insulator's negative electrons, which are fractionally farther away from the negatively charged external material.)
"With the examples given about induction and polarization, they all seemed to have some net attraction. Is there ever a net repulsion?" (No.)
"Very cool! A huge fan of the Star Wars analogy for this material."
"Can we please review the two-step field model? Thank you!"
"I don't quite understand the difference between the different qs?"
"I feel really lost, can you please explain field lines and test charges?"
"I have very little exposure to electricity so I am having a hard time understanding this chapter. There also seems to be a LOT of information!"
"This electricity stuff is hard :("
"Are we going to go over this a lot more?" (Yes.)
"I'll have to do some more reading at home. I thought I'd be able to do this during breaks at work but I stand corrected and confused."
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