## 20160304

### Online reading assignment: electric forces and fields

Physics 205B, spring semester 2016
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.
"That an electric field is depicted as filling in all space surrounding a source charge."

"A source charge is one that exerts force on a test charge, and the force is attractive if the two charges have opposite signs."

"Coulomb's law is used in the direct model of electrical forces, in which a source charge (q1) exerts a force on a test charge (q2). The direct method does not explain the 'action-at-a-distance" problem though. The field model is the two-step approach to electrical forces. In this model a source charge (Q) creates an electric field, and this electric field exerts a force on a test charge (q)."

"If the source charge is positive, its electric field vector directions will be outwards. If the source charge is negative, then its electric field vector direction will be inward."

"Whenever a positive point charge is placed on an electric field line. The positive point charge will always experience an electric force F along the E field direction."

Coulomb's law calculates the magnitude of the force that source charges exerts on test charges separated by a distance r. In this calculation the sign charges do not matter when calculating magnitude of force. The constant k also cancels out the units and convert the units to the proper units of newtons."
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 didn't find anything particularly confusing about the material, I just would like to see some problems worked out in class."

"I'm having difficulty applying this to real life."

"Charges and electric fields and forces."

"The physical representation of an electrical field--like, what is it? I understand that it is the force on an electric test charge divided by the tea charge itself, but I am having a hard time visualizing what a field represents."

"The concept of the electric field."

"Field model seems quite confusing to comprehend."

"What I find difficult is remembering what all the letters and symbols mean. There are just so many that it's hard to remember which one is which and what it means. I just need to put more work into this class."
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. [0] the vector addition of all the forces. ****************************** [30] only the force from the closest charge. ** [2] (Unsure/guessing/lost/help!) *** [3]

Explain the difference between units for electric force, F, and the electric field, E.
"Electric force units are in N and electric field units are in N/C."

"F is a force magnitude, E is a force magnitude per unit charge."

"Not sure what the difference is."
Explain the conceptual difference between the electric force, F, and the electric field, E.
"Electric force is the force applied by the electric field onto an object."

"Electric force is when an object exerts a direct force on another object. While the electric field is created by an object and the electric field exerts the force on the other object. I totally felt like there was a difference when I was doing the reading, but now that I'm typing it out they seem like the same thing except that a field spreads out in all directions."

"The electrical field is what a source charge creates around itself, and this is where the electric force comes from when applied to the test source."

"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."

"I'm still a little confused on this."
Explain the conceptual difference between a source charge (±Q), and a test charge (±q).
"A source charge exerts the force and the test charge reacts to it."

"A source charge creates an electric field whereas an Electric field exerts the force on the test charge."

"The source charge is the source of the electric field while the test charge is used to measure the strength of the electric field."

"I honestly do not understand the difference between a source charge and a test charge."
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 [74%]
Negative source charge (–Q): E field lines point inwards [71%]

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. ********* [9] along the E field direction. ******************** [20] (Unsure/guessing/lost/help!) ****** [6]

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. ******************* [19] along the E field direction. ********* [9] (Unsure/guessing/lost/help!) ******* [7]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"From lab when we illuminated the CD tracks with a laser: (a) Why don't we see a series of dots? (b) Isn't the width of the laser wider than multiple tracks on the CD? (c) Also why don't we see any light between the dots?" ((a) You are seeing a series of dots (maxima) in different directions, but the maxima θ angles are quite large. (b) The laser needs to illuminate many multiple tracks on the CD such that light will pass through them like passing through multiple slits. (c) You don't see any light between the maxima dots, because between the maxima θ angles are the minima θ angles, where destructive interference occurs.)

"I can't wait until we all evolve to have superpowers." (Then life would become boring, like the TV show Heroes.)

"My brain hurts. Conceptual stuff is not my thing."

"I can't wait for this semester to be over!" (Me, too. #squadgoals)

"Quiz 3 will be easier!" (#jedihandwave Quiz 3 will be easier... #jedihandwave)