## 20170303

### Online reading assignment: electric forces and fields

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 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.
"The electrostatic force that stationary charges exert on each other depends on the amount of charge on each objects and the distance between them; and that the greater the charges and the closer together they are, the greater the force."

"I understand there are two ways to think about the force electric charge exerts on another: the force model, which can directly measure the force exerted; and the field model, which assumes that the force exerted by the electric field is all around."

"A source charge Q creates an electric field everywhere; the magnitude of this electric field can be calculated for a location at a distance r from the source charge. If the charge is positive the field lines point outward; if it is negative the field lines point inward."

"I understand that in order for the Force to actually work, you have to get into a Jedi state of mind, and basically have an electric field coming off of you otherwise you can't pull the lightsaber without touching it."

"Positively charged test particles move along an external electric field line while negatively charged particles move against it."

"That a source Q creates its own electric field around itself and that electric field exerts a force on the test q. I was able to follow the models on the blog and they seem to make sense by themselves, but honestly after I read the book I was kind of confused after."

"I will need help in class."
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 don't understand certain variables. Depending on what's given, sometimes I input the wrong info into the formula."

"I'm not totally sure when the forces are repulsive. In the example it uses the same signs. Opposites attract right?"

"Coulomb's law is a little confusing, but it is probably just because it is new and has so many unfamiliar variables."

"I am not really too comfortable with these calculations. Also can there be more than one charge going on? Some examples on the calculation in class would be really helpful."
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. *** [3] the vector addition of all the forces. ***************** [17] only the force from the closest charge. * [1] (Unsure/guessing/lost/help!) **** [4]

Explain the difference between units for electric force, F, and the electric field, E.
"Electric force is measured in newtons (N), and electric field is measured in newtons per coulomb (N/C). Since electric field is defined as a force per charge, its units would be force units divided by charge units so it would be newtons per coulomb. For electric force, it is represented in just newtons."

"F is in Newtons, and E is a vector and the units are (N/C)."

"F is the combination of the source charge and test charge's force; while E is a separately generated from a source charge."

"Not sure."
Explain the conceptual difference between the electric force, F, and the electric field, E.
"I think the electric force is just based on if the charges associated with the atoms are repulsive or attractive. The electric field is created by a source and that field then exerts a force on another charged particle."

"The electric force happens inside the electric field because the field exerts some force on an object."

"Electric fields are capable of exerting a force, but not vice versa."
Explain the conceptual difference between a source charge (±Q), and a test charge (±q).
"The source charge is what is exerting an electric force upon the test charge."

"Q creates the E which can attract or oppose q depending on the charge of q."

"F is equal to E times q."
Indicate the direction of electric field lines for these ±Q source charges:
(Only correct responses shown.)
Positive source charge (+Q): E field lines point outwards [88%]
Negative source charge (–Q): E field lines point inwards [88%]

A positive 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:
 opposite the E field direction. ****** [6] along the E field direction. ****************** [18] (Unsure/guessing/lost/help!) * [1]

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:
 opposite the E field direction. ****************** [18] along the E field direction. ****** [6] (Unsure/guessing/lost/help!) * [1]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"So electric forces happen in electric fields?" (Yes, if you place a charged particle in an electric field.)