## 20140305

### Online reading assignment: electric forces and fields

Physics 205B, spring semester 2014
Cuesta College, San Luis Obispo, CA

Students have a 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 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 understand that in a positive point charge will have electric field lines moving outwards and away from the point. A negative point charge will have electric field lines moving away from the point. I also understand that the strength of the electric force of an object will decrease as separation increases."

"I understand that a field occupies empty space around and between the source object. If the source charge is positive it points outwards and if the source charge is negative the field points inwards."

"When analyzing the attractive or repulsive force between two objects with electrical charge, there are two ways to look at it. The electric force model gives an equation for force between two objects. This seems to cause philosophical problems about interaction between objects across distance so another model was created in which a source object generates a field that is supposed to exist in the space between the two charged objects and then the field interacts with the test object."
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 not sure how to apply Coulomb's law in equations to find the magnitude of electric force."

"Even after the example I don't know how to differentiate between a source and the test object. The source exerts the force on the test object but is the field is exerted on both. How do F and E change with more than two electric forces on the point charge?"

"I understand that objects can be moved at a distance because of the electric force which is caused by an electric field. I'm kind of lost on this stuff though an in class discussion would be really helpful."

"How to use force models and coulombs law in a problem. A better understanding between electric forces and electric fields and their purposes."
If there are two more more electric forces acting on the same point charge, the point charge will experience:
 only the force with the greatest magnitude. ***  the vector addition of all the forces. ****************************  only the force from the closest charge. *  (Unsure/guessing/lost/help!) ****** 

Explain the difference between units for electric force, F, and the electric field, E.
"The electric field is electric force per unit charge."

"F is in units of newtons and E is in newtons/coulomb."

"Electric force is measured in V (voltage), while electric field is not."
Explain the conceptual difference between the electric force, F, and the electric field, E.
"Electric force F is the force exerted on q2 which is either attractive when test charge is opposite signs and when the test charge is repulsive they have the same sign. The F is found using Coulomb's law. As for E field, source Q creates it. The sign of the electric field depends on the source charge being either positive or negative, which determines whether or not the field points inward or outward."

"F quantifies the magnitude of the force, while E finds the direction of the electric field."

"The electric force describes how a charged particle attracts or repels another charged particle. The electric field describes the force a charged particle will experience due to its location in space."
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Can we please go over these concepts and do practice problems as a class?" (Yes.)

"All around, feeling fairly confident with the entire electric field section. Guess we'll see if this confidence lasts by tomorrow's lecture."

"I don't understand how to tell whether or not an object is small enough that its internal structure is of no importance (thus becoming a point charge) or how it changes in different contexts." (A point charge is anything that has charge with a size so small that it's shape doesn't matter. Protons can be pretty well approximated as positive point charges, unless you push two protons so close that they begin to undergo fusion; while electrons are truly point-like negative charges (as far as we know), as two electrons never really seem to come into contact with each other, no matter how hard you try to push them together.)

"It's a trap!"