Online reading assignment: charges, electric forces and fields

Physics 205B, spring semester 2018
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.

"Negative and positive charges are attracted to each other. Also, net neutral objects can attracted to charged objects."

"In insulators electrons are fixed while in conductors electrons are free to move around the material. Polar molecules are more positive/negative at different parts of the molecule due to the electrons being at different parts of the atom more."

"I understand that if a negative source charge interacts with an insulator, the electrons will be repelled to the other side of the insulator, such that there will be a net attractive force on the insulator."

"Opposite charges are attracted to each other. Coulomb's law is used to find the magnitude of force that a source charge exerts on a test charge."

"I understand the electric field and how charges exert electric forces on other charges. Coulomb's law quantifies the magnitude of the force that a source charge exerts on a test charge, separated by a distance r."

"I understand a little about Coulomb's law. The value of q₁ represents the source charge and the value of q₂ represents the test charge. In addition k is a constant, r is the distance they are apart and if the q values are opposite in signs the charges are attracted and if the q values are the same in signs they are repulsed."

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.

"Thankfully this is chemistry review so not much review is needed other than the application of it in physics."

"I am having a hard time wrapping my head around electric forces and fields. I'm sure some examples in class would help me make a connection."

"I'd like some in class clarification on electric forces and fields. I'm lost."

"What is a field?"

"The field model."

"Why is the electric force along the electric field direction on a positive test charge, and the against the electric field direction for a negative charge?"

"I definitely need some help with this material. It was difficult to decipher."

"I'm having some issues with the electric field models. In the online lectures I'm not really sure as to why the two field models are mathematically different. I mean I understand them from problem solving perspective but I don't really get it from a conceptual perspective."

"The fields were a little confusing overall, but I think with some in class problems (which I'm sure we are going to do) it will make more sense."

"Nothing was too confusing."

"I liked this lesson."

A gas pump fire is inadvertently caused by the sudden discharge of an electrical spark. Describe when and how the woman became electrically charged.

"The woman transferred electrons between her hands and her shirt when she adjusted it."

"When she sat down in her car she may have become positively or negatively charged (doesn't matter which). She may have some sort of cloth car interior."

"The woman has some sort of felt seats. When she got in and out of the car, without electrically grounding herself by touching the metal car, she became electrically charged."

"Static built from her shirt or the wind was stored in her body and released when she touched the gas pump."

Electrically neutral polystyrene "packing peanuts" would be attracted to a cat that has __________ charge.

a positive.	* [1]
a negative.	*** [3]
either a positive or a negative.	*********************** [23]
zero (neutral).	* [1]
(Unsure/lost/guessing/help!)	[0]

An electrically neutral aluminum soda can would be attracted to a balloon that has __________ charge.

a positive.	**** [4]
a negative.	* [1]
either a positive or a negative.	*********************** [23]
zero (neutral).	[0]
(Unsure/lost/guessing/help!)	[0]

Electrically neutral polar molecules in a water stream would be attracted to a comb that has __________ charge.

a positive.	**** [4]
a negative.	*** [3]
either a positive or a negative.	******************** [20]
zero (neutral).	* [1]
(Unsure/lost/guessing/help!)	[0]

Explain the difference between units for electric force, F, and the electric field, E.

"E has the units of N/C while electric force F has the units of N."

"Electric force F is in units of newtons (N). Electric field E is in units of newtons/coulomb (N/C)."

"They are both newtons?"

"Not sure."

"I'm lost!!!"

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 exerts a force on the test charge. The source charge is responsible for creating an electric field. The presence of a test charge causes an electric force to be exerted."

"Q is the charge that is creating the field, and q is how we can test what direction the field is."

"A source charge is the what creates an electric field, and a test charge is what receives the force from the field."

"The source charge creates an electric field anywhere around it. This electric field exerts a force on the test charge."

"Source charge creates E field. The test charge obeys or disobeys the E 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 [82%]
Negative source charge (–Q): E field lines point inwards [79%]

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.	**************** [16]
along the E field direction.	********* [9]
(Unsure/guessing/lost/help!)	[3]

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.	******* [7]
along the E field direction.	****************** [18]
(Unsure/guessing/lost/help!)	*** [3]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"The change of subjects is kinda scary now... I am still trying to understand double-slit interference..."

"Please go over the positive/negative source charges and test charges. Thanks!"

"Hey P-dog, can we go over electric fields? I'm not entirely sure what is going on."

"This presentation made my brain hurt..."

"This unit looks supercool!"