## 20150306

### Online reading assignment: electric forces and fields

Physics 205B, spring semester 2015
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.
"Using Coulomb's law we can calculate the magnitude of the force that a source charge exerts on a test charge, separated by a distance which we call r. An electric field is depicted as filling in all space surrounding a source charge, and is drawn as field lines that point outwards away from Q when it is a positive source charge, and point in towards Q with a negative source charge."

"The source charge creates a field, which in turn the field exerts a force on test charge."

"A force will only be attractive if the source charge and test charge have opposite signs. If the source charge and the test charge have the same sign then they are repulsive."

"Coulomb's law has to do with how the electric force acts between points of non-zero electric charge. In regards to this force, the strength decreases as the separation increases."
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.
"Keeping all of the symbols and letters sorted in my brain."

"Could we please go over Coulomb's law in class just so I can get a better understanding? Please go over the field model in class if you have time because it seems pretty confusing to me on how they work."

"The field lines confused me a little. I understand that they will either point inwards for a negative source charge, or outwards for a positive source charge. Is the second step where the +q and -q test charges are introduced just to show which action the test charges will take depending on its charge relative to the field direction?"

"I found the images with all the lines everywhere confusing. I didn't quite understand what they were trying to represent or what they meant, going over this in class would be very 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. ******************************* [31] only the force from the closest charge. [0] (Unsure/guessing/lost/help!) ***** [5]

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

"Electric field has units of F/q."

"Electric force F would be in newtons, whereas electric field E is newtons per coulomb."
Explain the conceptual difference between the electric force, F, and the electric field, E.
"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."

"F is equal to E times q."

"The difference between electric force and electric field is that the electric force is a force directly exerted from a source on an object, where as an electric field is energy created by a source all around it, and that energy exerts force on an objects. The force energy is acted on something, and field energy is something around stuff."
Explain the conceptual difference between a source charge (±Q), and a test charge (±q).
"Q has to do with electric fields, q has to do with electric forces within that field. I think."

"The source charge is the source of the electric field while the test charge is used to measure the strength of the electric field."
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 [77%]
Negative source charge (–Q): E field lines point inwards [77%]

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. ******** [8] along the E field direction. ********************* [21] (Unsure/guessing/lost/help!) ********** [10]

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. ********************* [21] along the E field direction. ********* [9] (Unsure/guessing/lost/help!) ********* [9]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"What is the name of the first electrical detective?" (Sherlock Ohms.)

"Lots of clarification is needed please!"

"Maybe I am over thinking these questions, please elaborate in lecture."

"If many charges are in the same electric field created by a source charge, does that electric field weaken?" (Yes, but we will assume that the test charges are tiny compared to the source charge, such that we will ignore the puny electric field that create. This is the same assumption when we talk about Earth's gravitational field, even though everything else we encounter on an everyday basis creates their own gravitational field as well, their masses are so small in comparison that we ignore the puny gravitational fields they create.)

"I would like some clarification on the difference between the electric force and the electric field."

"Can we go over the equations please."

"My brain hurts."