Physics quiz question: finding amount of unknown charge

Physics 205B Quiz 3, spring semester 2020
Cuesta College, San Luis Obispo, CA

A –4.5 nC point charge and an unknown positive point charge are held a distance of 3.0 cm apart. The magnitude of the electric force of the –4.5 nC charge on the unknown positive charge is 7.6×10^–5 N. The amount of the unknown positive charge is:
(A) 1.7×10^–9 C.
(B) 5.6×10^–8 C
(C) 1.7×10⁴ C.
(D) 4.5×10⁴ C.

Correct answer (highlight to unhide): (A)

The magnitude of the force on the unknown positive point charge is given by:

|F_{1 on 2}| = k·|q₁|·|q₂|/(r²),

where q₁ is the –4.5 nC point charge, and q₂ is the unknown positive charge. Then the absolute value of the unknown charge can then be solved for:

|q₂| = |F_{1 on 2}|·(r²)/(k·|q₁|),

|q₁| = |7.6×10^–5 N|·(0.030 m)²/((8.99×10⁹ N·m²/C²)·|–4.4×10^–9 C|),

|q₁| = 1.6907675195...×10^–9 C,

or to two significant figures, the amount of the unknown (positive) charge is (+)1.7×10^–9 C.

(Response (B) is |F_{1 on 2}|·r/(k·|q₁|); response (C) is |F_{1 on 2}|/|q₁| (which is the electric field of the unknown positive charge at the location of the –4.5 nC charge); and response (D) is k·|q₁|/r² (which is the electric field of the –4.5 nC charge at the location of the unknown positive charge).)

Sections 30882, 30883
Exam code: quiz03Cv1d
(A) : 28 students
(B) : 1 student
(C) : 4 students
(D) : 1 student

Success level: 82%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.18

Physics quiz archive: interference, electrostatics

Physics 205B Quiz 3, spring semester 2020
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz03Cv1d



Sections 30882, 30883 results

0- 6 :
7-12 :
13-18 :	**** [low = 15]
19-24 :	***************
25-30 :	*************** [mean = 24.2 +/- 4.2] [high = 30]

Online reading assignment: electric potential energy

Physics 205B, spring semester 2020
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 potential energy.


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.

"EPE is equal to the k constant times the source charge q₁ times the test charge q₁divided by a distance denoted as r. Voltage (potential) is equal to the k constant times the source charge Q divided by a distance from the source charge r."

"Electric potential energy can change when moving a source charge and test charge closer together, or moving them farther apart. One direction will give a positive change in EPE when pushing like signs together or pulling two opposite signs apart."

"Electric potential energy can be increased by pushing together like-sign charges or pulling apart opposite-sign charges. This results in a change in electric potential energy because it requires work to be done. Work is done by charges that are allowed to do what they want."

"I understood EPE can be increased by pushing like charges together or pulling apart opposite charges. While, allowing things to act without using 'work' results in a decreased EPE. Also, I believe to calculate ∆EPE, one must calculate the EPE at different locations and subtract."

"EPE utilizes a field of equipotentials, similar to electric fields."

"I get there is a difference between electric potential energy and electric potential, but I don’t really know what it is."

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.

"Maybe the images on the blog explaining when something was negative rather than positive."

"I did not understand voltage, electric potential energy and what the peaks or wells mean."

"The difference between electric potential and electric potential energy is a bit confusing. I'm also getting lost on how the two-step approach is implemented in finding the potential energy."

"I had a hard time with the end of the presentation because I was confused about the circular equipotentials. Also, are EPE and potential are the same? I don't know why they have different labels in the equations."

"I am a little unclear with how to determine when something is a two-step approach versus a direct approach."

"The textbook does not have good examples of these types of problem. Can you please go over in class?"

"Nothing at this time. Possibly application."

Explain the difference between the units of electric potential V, and electric potential energy, EPE.

"EPE is in joules and potential is in joules per coulombs."

"The electric potential energy is an energy and is measured in joules. The electric potential is an energy per unit charge and is measured in joules per coulomb, or volts."

"The units."

"I don't understand the difference in units for the different energy potentials."

Explain the conceptual difference between the electric potential V, and electric potential energy, EPE.

"EPE is measured in joules because it is an energy. The volt is measured in joules per unit."

"Electric potential energy is an energy while electric potential is an energy per unit charge."

"Electric potential is created by a source charge. EPE is the energy stored in electric potential. I think this is why I'm so confused. I get one is an energy and the other one is an energy per charge but I don’t get how they relate to each other, or if one created the other..."

"Not really sure."

Briefly summarize the difference (if any) between "voltage" and electric potential.

"They are the same."

"A volt is the way to measure electric potential."

"There isn't a difference. Electric potential is measured in volts at a location in space."

"They are interchangeable."

Identify the changes in electric potential energy EPE (if any) for the following test charges (±q):
(Only correct responses shown.)

Positive test charge +q brought closer to a positive source charge +Q: increase [67%]
Negative test charge –q brought closer to a positive source charge +Q: decrease [70%]
Positive test charge +q brought closer to a negative source charge –Q: decrease [70%]
Negative test charge –q brought closer to a negative source charge –Q: increase [73%]

Identify the changes in electric potential V (if any) for the following test charges (±q):
(Only correct responses shown.)

Positive test charge +q brought closer to a positive source charge +Q: increase [48%]
Negative test charge –q brought closer to a positive source charge +Q: increase [48%]
Positive test charge +q brought closer to a negative source charge –Q: decrease [42%]
Negative test charge –q brought closer to a negative source charge –Q: decrease [45%]

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

"Pleeeeease review the difference of potential and EPE."

"Thrilled to see the energy equation back with a new member added to it."

"That was electrifying."

"I am having a hard time with the last question, are you going to go over it in class?"

"Would like to review the last question above."

"You weren't lying when you said this material was THICK."

"Do the concepts in this section relate to how magnets work? (Not yet. Too soon.)

Online reading assignment: charges, electric forces and fields

Physics 205B, spring semester 2020
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.

"I understood the differences between a conductor and an isolator. The polar molecule definition was also easy to understand from chemistry."

"Electrons move freely in a conductor but are still bound to atoms in a insulator but electrons will still orient to opposite charges on insulators. Polar molecules will also orient their opposing charge to a charged object, becoming aligned."

"Considering electrical charge, insulators result in outer electrons that are mostly fixed to their atomic location, while conductors allow electrons greater ability to move about the material. Both neutral conductors and insulators will be attracted to positively and negatively charged objects. Electrical forces and electrical fields will exert a force on a test charge."

"I think I understand Coulomb's law, it is a way to measure the force exerted from a source charge to a test charge. I understand it as the force an object is exerting on another object, but I am not entirely sure."

"Make sure to not have static electricity around a gas pump. I'm pretty sure I understand the differences between source charges and test charges as well as the electric force and electric field."

"From this section I learned that there are electric fields and electric forces that both act together to produce the force acted on something. With an electric field, the force applied to something is reliant on the type of electric field and the positive or negative test charge q."

"I quickly skimmed this and did not really understand anything. I didn't really get a good look at anything."

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.

"Not exactly sure about the insulator/conductor attractions."

"I had confusion with multiple aspects of this material starting with the definitions of isolators and conductors and what objects we might categorize as either insulators or conductors. Although, I'm proficient in the simple idea that opposite charges attract and same charges are repulsed from each other. I also was confused with the idea of electric fields and the interactions with electric forces. I feel like I'm on the verge of understanding in that the interactions of two charged objects--or possibly a single neutral object--creates an electric force either attracting or repulsing which in turn establishes an electric field in which that electric force occurs. But again, generally confused."

"The polarization aspect with insulators and conductors. It was confusing to understand the exact differences."

"I was very confused about the conductor and insulator concepts. I don't understand how the electrons come into play at all."

"I understand polarization but don't quite have a strong grasp on the characteristics of insulators and conductors. Also, fields are going over my head as a whole."

"I am kinda confused on what Coulombs law turns out to be unit-wise."

"I'm having trouble understanding electric forces and fields."

"The force model and field model did not make much sense to me."

"I found the concept behind the electric forces and field to be confusing. More specifically, the difference between the two."

"In the field model, I am confused by what is means for +q: F 'obeys' E, and for –q: F 'disobeys' E."

"Haven't got to this yet."

"I mean I honestly can't think of a specific question. I guess it would be helpful to go over as much as you can."

"I didn't find anything confusing in this topic."

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

"I think it is when she exits the car. I have heard of this occurring, but never understood what actually happened."

"Two insulators with different electron affinities are rubbed against each other. When she rubs her hand her shirt, it causes a reaction due to this difference in electron affinity."

"It was when she came into contact with the cloth on her seats in her car."

"The woman became electrically charged when she went back in her car and then touched the gas pump after getting out. She created friction by sliding in and out of her car, and the fire started after she touched the metal gas pump."

"When the woman stepped into her vehicle she created some exchange of electrons by rubbing tohether two different insulators with two different affinities for electrons. When she then reached for the pump, those electrons were sent toward the metal pump causing a spark."

"Static electricity allowed her to carry electrons making her negatively charged. When she got close to the pump, she transferred these electrons causing a spark."

"I am not super-positive, but I am going to guess that electrons were transferred by her rubbing her sweater and then touching the pump."

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

a positive.	**** [4]
a negative.	*** [3]
either a positive or a negative.	*********************** [23]
zero (neutral).	* [1]
(Unsure/lost/guessing/help!)	** [2]

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

a positive.	*** [3]
a negative.	**** [4]
either a positive or a negative.	********************** [22]
zero (neutral).	* [1]
(Unsure/lost/guessing/help!)	*** [3]

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

a positive.	*** [3]
a negative.	**** [4]
either a positive or a negative.	********************** [22]
zero (neutral).	* [1]
(Unsure/lost/guessing/help!)	*** [3]

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

"Force is in newtons, and electric field is in newtons per coulomb."

"N vs. N/C."

"The electric field units are taking into account two different charges that are exerting a force on each other (q1 and q1, both in coulombs) divided by the square of the distance between them (m²). k is a constant and has units of N·m²/C². When you calculate the force of the electric field coulombs and meters cancel out leaving newtons. For the electric field, we once again have the constant k with units N·m²/C² multiplied by the source charge Q (C) and divided by r² (m²). When you plug into the formula, all units except N/C cancel."

"I don't understand the difference in units."

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

"An electrical force is created directly by a source charge Q and can affect a test charge q, where as an electric field is created by a source, and this electric field can cause an effect on a test."

"Electric field refers to all the space around a source while electric force is the magnitude and direction of the field being exerted on the test charge."

"The electric force is more direct and is a one-step process. While the electric field is indirect and has two steps."

"The electric field surrounds the source charge. The electric force is what is exerted onto the test charge."

"I don't understand the difference."

"Will get to this later."

"I'm honestly not sure."

Explain the conceptual difference between a source charge (±Q), and a test charge (±q).

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

"A source charge Q is a charge exerting a force onto a test charge q. The test charge is the charge which is being acted upon which will either be attracted to or repulsed by the source charge dependent upon whether those two charges have opposite (attract) or the same (repulsive) ± signs."

"A source charge Q is the charge that exerts the force on the test charge q, while the test charge is being exerted on by the source charge."

"Source charge exerts a force on the test chare. Source is giver, test is receiver."

"Q defines the direction of E and q is the relationship between F and E."

"This is what I need help with. I know a test charge has a very small magnitude because it shouldn't change the source charges. But don't understand why a test charge is necessary?"

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 [75%]
Negative source charge (–Q): E field lines point inwards [70%]

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:

along the E field direction.	******************** [20]
opposite the E field direction.	****** [6]
(Unsure/guessing/lost/help!)	******* [7]

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:

along the E field direction.	****** [6]
opposite the E field direction.	******************** [20]
(Unsure/guessing/lost/help!)	******* [7]

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

"The reading said a neutrally charged insulator's electrons would turn away from a negatively charged external material. Why are they two materials still attracted?" (The insulator's positive nuclei would then be fractionally closer to the negatively charged external material, and this attractive force would be greater than then repulsive force between the insulator's negative electrons, which are fractionally farther away from the negatively charged external material.)

"With the examples given about induction and polarization, they all seemed to have some net attraction. Is there ever a net repulsion?" (No.)

"Very cool! A huge fan of the Star Wars analogy for this material."

"Can we please review the two-step field model? Thank you!"

"I don't quite understand the difference between the different qs?"

"I feel really lost, can you please explain field lines and test charges?"

"I have very little exposure to electricity so I am having a hard time understanding this chapter. There also seems to be a LOT of information!"

"This electricity stuff is hard :("

"Are we going to go over this a lot more?" (Yes.)

"I'll have to do some more reading at home. I thought I'd be able to do this during breaks at work but I stand corrected and confused."

Physics quiz archive: interference, electrostatics

Physics 205B Quiz 3, spring semester 2019
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz03b3Am



Sections 30882, 30883 results

0- 6 :	** [low = 3]
7-12 :	**
13-18 :	*********
19-24 :	***************** [mean = 21.5 +/- 6.0]
25-30 :	************ [high = 30]

Online reading assignment: electric potential energy

Physics 205B, spring semester 2019
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 potential energy.


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.

"Pushing together repelling charges and pulling apart attractive charges causes electric potential energy to increase and the opposite causes electric potential energy to decrease."

"A positive charge accelerates from higher electric potential region toward a lower electric potential region. The opposite happens for a negative charge."

"We know that a source charge can exert an electric force on a test charge, and now we know that a source charge can 'store' electric potential energy on a test charge! Whoa! Changes in this potential energy changes based on how far apart the source and test charge are from each other. More potential energy is stored when like charges are pushed together because this requires energy since these opposite charges will repel while the change in potential energy will be smaller when like charges are allowed to repel each other or opposite charges are allowed to attract towards each other."

"Electrical potential energy, EPE, can be observed using a direct approach (source Q stores EPE with test q) or a two-step approach (source Q creates Voltage potential which then stores EPE with test q). In both cases, electric field lines will point towards decreasing electric potential values. A positive test charge, q, will also point toward the direction of decreasing potential and decreasing potential energy. A negative test charge will point towards increasing potential but decreasing potential energy."

"Electric potential energy can be increased by pushing two same sign charges together because they will repel and that the same will happen when you pull apart opposite sign charges because they want to stay together. These both cases will cause EPE to become positive. When charges are able to do what they do naturally, which is to repel from same sign charges and move closer together to opposite sign charges, this will cause the EPE to become negative because it will decrease. I also learned that potential at a location in space is also referred to as voltage."

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 do not understand the concept of the equipotentials. I do not understand the relevance and meaning of the field lines and equipotential circles. I understand that it has to do with potential energy but that's about it."

"How is electric energy measured and observed in reality? What are the implications of high or low EPE? "

"How when we have a positive source charge and a positive test charge we get increasing potential energy but if they're left alone you get decreased potential energy. I also found the whole snowmobile analogy very confusing."

"So the potential of a point charge is a potential difference with a potential at infinity. I'm not quite sure what this is supposed to mean other than that if the point charge is positive that its potential is raised above the reference value of 0. So if the point charge is negative then its potential is lowered below the reference value of 0?"

"Sometimes it's hard to tell which direction stuff is pointing in different cases."

"I'm a bit confused with the concept for the two step approach to electric potential energy. SO the source charge won't directly affect the test charge instead it will create an electric field which will then exert a force onto the test subject. I guess what I don't quite understand is why does there need to be almost like a middle step to the direct approach."

"I didn't understand why electric field lines point towards decreasing electric potential values for both positive and negative charges."

Explain the difference between the units of electric potential V, and electric potential energy, EPE.

"V is measured in joules per coulomb and EPE is measured in joules."

"I didn't understand this."

"I think both of these are the same?"

Explain the conceptual difference between the electric potential V, and electric potential energy, EPE.

"A source creates a potential V, everywhere around it and the EPE is stored with the test charge. The source charge creates an electric potential everywhere around it on its own. EPE is stored when the source charge and test charge are brought closer together or farther apart. EPE is only made from both charges."

"EPE is a measure of energy. Electric potential (V) is a measure of energy per charge."

"Still a little foggy on this. It usually takes me seeing examples in class to fully understand the differences between these things and why they are different."

"I don't know"

Briefly summarize the difference (if any) between "voltage" and electric potential.

"There is none."

"I believe voltage is the same as electric potential."

"Potential and voltage are the same thing."

"Really not making sense."

Identify the changes in electric potential energy EPE (if any) for the following test charges (±q):
(Only correct responses shown.)

Positive test charge +q brought closer to a positive source charge +Q: increase [67%]
Negative test charge –q brought closer to a positive source charge +Q: decrease [61%]
Positive test charge +q brought closer to a negative source charge –Q: decrease [64%]
Negative test charge –q brought closer to a negative source charge –Q: increase [64%]

Identify the changes in electric potential V (if any) for the following test charges (±q):
(Only correct responses shown.)

Positive test charge +q brought closer to a positive source charge +Q: increase [44%]
Negative test charge –q brought closer to a positive source charge +Q: increase [36%]
Positive test charge +q brought closer to a negative source charge –Q: decrease [58%]
Negative test charge –q brought closer to a negative source charge –Q: decrease [31%]

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

"I am still not comfortable about the difference between electric potential and electric potential energy. I don't understand why work increases electric potential energy when two like charges are moved closer together or when two different charges are moved away from one another." (Two like charges repel each other, so it will take "effort" in trying to push them together. This "effort" is the work you are doing in increasing their electric potential energy (like compressing a spring raises its elastic potential energy). Likewise two opposite charges attract each other, so it will take "effort in trying to pull them apart. This "effort" is the work you are doing in increasing their electric potential energy (like stretching a spring raises its elastic potential energy.)

Can V be likened to a field if EPE can be likened to a force?" (Yes, as potential is measure of the energy that a charge could have if placed at a location near a source charge, while electric potential energy is energy two charges have with each other. Likewise electric field isa measure of the force that a charge could have if placed at location near a source charge, while electric is the force two charges exert on each other.)

"The book is difficult to read and the blogs are a bit hard to understand as well. It would be nice if actual problems where posted on the blogs with the answers and the step-by-step how to solve that way we would try and understand how to do these problems before coming to class. The blog is a bit vague and shows the formulas but posting actual examples like how we work examples in class would be very helpful." (Point taken; keep letting me know which problems from the homework reports you would like me to work out in class, while I will still go over other examples proactively in class (as time allows) before you do the subsequent homework report. Also the electric forces/fields/energy/potentials concepts and problems are very abstract, so keep at it, and we'll develop these ideas further as well get into magnetic forces and fields later.)

"Can you go over the snowmobile model in class?"

Online reading assignment: charges, electric forces and fields

Physics 205B, spring semester 2019
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.

"Electrons are free to move in conductors, whereas in an insulator electrons are more or less fixed to their location. For insulators, when a charged object is introduced, particles of the opposite charge are drawn to the object. For conductors, particles will actually separate creating a positive half and negative half that is dependent on the orientation of the object introduced."

"The positive charge of a proton equals the negative charge of an electron. The law of conservation of electric charge says the net electric charge of an isolated system remains conserved. Coulomb's law is the force that charges exert on each other."

"When positive or negatively charged objects get close to an insulator or conductor, the electrons will move accordingly in order to create a net attraction to the object."

"Neutral objects will have an electrostatic attraction to objects that are charged. The attraction will be the same for positively or negatively charged objects."

"The greater the charges and the closer together they are, the greater the force. Unlike signs are attracted, like signs are repelled."

"Electrons are able to transfer between objects through the air. This is done by an electric shock or arc, where several electrons all jump at the same time along the path of least resistance."

"The electric force between charges is directly proportional to the product of the absolute value of the charges and a constant (k) and inversely proportional to the square of the distance between the charges. As the distance between the charges increases, the force decreases. As the distance between the charges decreases, the force between the charges increases."

"The difference between electric force and electric fields, and how to solve for them. A simple equation for force, and a two-step process for electric fields."

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.

"A lot of chemistry concepts overlapped--I would like clarification on the gasoline and electric spark question."

"I don't quite understand how something that is a neutral insulator can be attracted to a charge I would assume it would want to keep its neutral balanced state."

"What does Q mean?"

"The concept of a test charge, q was confusing. I do not understand how the electric field is a vector."

"I am pretty confused about source charges and the field model. I understand that one source will exert a force on another but I got lost when we started talking about source charge Q creating an electric field."

"Coulomb's law. Also the field model: electric fields, electric forces, and source and test charges."

"The difference between electric force and electric field. I feel like I understand the definition of each, but I am unsure of how they relate to one another."

"The field model looks pretty confusing based on all the variables but I think will be easier to understand in lecture with examples."

"The test charge--I do not understand how it is a test charge and what exactly is being tested."

"Why does +q always has a force exerted on it along the direction of a field line and –q always has a force exerted on it against the direction of a field line?"

"I remember learning about this in a previous physics class so I understood most of it."

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

"When she sat in the car, the seat rubbed against her creating a charge, and her rubber shoes improperly grounded her, so when she touched the nozzle, static electricity was produced sparking a fire."

"She became electrically charged when she rubbed her hands on her sweater causing friction and transfer of charges."

"Nothing like fire to get the tough stains out, am I right?"

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

a positive.	*** [3]
a negative.	**** [4]
either a positive or a negative.	********************* [21]
zero (neutral).	** [2]
(Unsure/lost/guessing/help!)	* [1]

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

a positive.	***** [5]
a negative.	***** [5]
either a positive or a negative.	******************* [19]
zero (neutral).	* [1]
(Unsure/lost/guessing/help!)	* [1]

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

a positive.	*** [3]
a negative.	********* [9]
either a positive or a negative.	******************* [19]
zero (neutral).	* [1]
(Unsure/lost/guessing/help!)	* [1]

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

"Force F has units of N or newtons and electric field E has units of N/C or newtons per coulomb."

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

"Electrical field is proportional to the electrical force divided by the test charge. The direction of the electric field is the same direction of the force, they both are vectors."

"Electric force is dependent on the force a source charge exerts on a test charge. Electric field is dependent on source charge creating an electric field around it."

"The electric field E is created by the source Q and then the electric force F is exerted by the electric field onto the test charge q."

"Electric force is the attraction or repulsive interaction between two objects. The electric field is the region around a charged object with which a force would be exerted on other charged particles."

"Not quite sure."

Explain the conceptual difference between a source charge (±Q), and a test charge (±q).

"Q exerts a field out around it and q obeys the force exhibited by the electrical field."

"A source charge Q is the one applying force onto source charge q. I am still a little confused with this stuff."

"Well the field is coming from the source charge, and the test charge is just something we use to see how it will react to the field."

"A source charge creates an electric field and the test charge is placed within the field. This is confusing to me, I think I would benefit from more visuals on this."

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 [80%]

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:

along the E field direction.	******************* [19]
opposite the E field direction.	********** [10]
(Unsure/guessing/lost/help!)	** [2]

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:

along the E field direction.	****** [6]
opposite the E field direction.	*********************** [23]
(Unsure/guessing/lost/help!)	** [2]

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

"We HAVE to go over electric forces and fields. The whole presentation makes no sense to me."

"More explanation would be nice."

"Can we do some examples in class?"

"I'm sure we will go over the equations in class."

"Not exactly sure what to ask but I am a little confused. Hopefully more explanation in lecture will clear some things up."

"What will our next quiz be on?" (Interference and electrostatics. The study guide for next week's quiz is already posted.)

Physics quiz question: change(?) in magnitude of total electric field

Physics 205B Quiz 3, spring semester 2018
Cuesta College, San Luis Obispo, CA

Originally a –9.0 nC charge is held at the origin.

Subsequently, if a +0.1 nC point charge were to be held in place at x = +1.0 cm, the magnitude of the electric field at x = +0.5 cm would:
(A) decrease.
(B) be unchanged.
(C) increase.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (C)

The electric field vector at x = 0.5 cm due to the –9.0 nC source charge points to the left, in towards this negative source charge.

The electric field vector at x = 0.5 cm due to the +0.1 nC source charge also points to the left, out away from this positive source charge.

The total electric field at x = 0.5 cm due to both source charges is the vector addition of the individual electric field vectors at that location. Since the electric fields due to the –9.0 nC source charge and the +0.1 nC source charge point in the same directions at x = 0.5 cm, the direction of the total electric field there would point to the left, and its magnitude would be greater than just the electric field of the –9.0 nC source charge alone).

Sections 30882, 30883
Exam code: quiz03Ch4R
(A) : 10 students
(B) : 8 students
(C) : 14 students
(D) : 0 students

Success level: 44% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.64

Physics quiz archive: interference, electrostatics

Physics 205B Quiz 3, spring semester 2018
Cuesta College, San Luis Obispo, CA
Sections 30882, 30883, version 1
Exam code: quiz03Ch4R



Sections 30882, 30883 results

0- 6 :
7-12 :	** [low = 9]
13-18 :	****
19-24 :	************ [mean = 23.3 +/- 4.9]
25-30 :	************** [high = 30]

Online reading assignment: electric potential energy

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 electric potential energy.


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 EPE can increase when two like charges (positive-positive, negative-negative) are pushed together or opposite charges (positive-negative, negative-positive) are pulled apart. Also, EPE decreases when two like charges (positive-positive, negative-negative) are left to repel each other or when opposite charges (positive-negative, negative-positive) are left to attract one another."

"Objects such as batteries can store electrical potential energy or 'voltage.' Changes in EPE are caused by either moving like charges towards each other or opposite charges away from each other since this is doing work. There is still a one-step and a separate two-step approach."

"The direct approach to analyzing electric potential energy is that a source charge q₁ stores potential energy EPE on a separate test charge q₂. The change in EPE is caused by moving the source charge and the test charge close together or far apart. In the two-step approach, the field model explains that a source charge Q creates an electric field everywhere around itself and thus it exerts a force F on test charge q."

"That there are two different approaches to electric potential energy and both give you the same answer. One involves knowing the source while the other just deals with the effects of that source."

"The formula to calculate electric potential energy is very similar to Coulomb's law except there is no r-squared in the denominator portion of the formula when calculating electric potential energy."

"I learned that source Q creates electric potential V and can be stored with a test q."

"I'm having a hard time understanding this presentation, I feel once I see a few examples in class it will click for me."

"I'm so confused..."

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 am still a bit confused with the differences between electric potential, and electric potential energy. I could use some more practice with different scenarios fro using each."

"I don't really understand why electric potential energy goes up when two like charges are moved closer together or when two different charges are moved away from one another. Shouldn't it be the opposite since like charges repel one another and opposite charges attract each other which would increase and decrease the distances of the charges from one another respectively. Really just a quick explanation should sort me out right."

"I am struggling with the difference between electric potential and electric potential energy."

"I didn't really understand the 'peak' and 'well' analogies."

"This whole section is pretty confusing. The two-step approach, potential energy. I'm having trouble conceptualizing how it all works rather than just the equations. I need to come to your office hours."

"I've never had a good intuitive understanding of potential energy as it relates to kinetic energy, so I'm having a bit of a difficult time understanding how exactly it applies in different scenarios."

"I understand some of the material. It's just a lot of information and I don't think I have it all down yet."

"Still very confused."

"So far nothing is too confusing."

"I didn't find anything confusing but will know for sure if I have any questions in lecture."

Explain the difference between the units of electric potential V, and electric potential energy, EPE.

"Electric potential uses joules per coulomb (J/C) while electric potential energy uses joules (J)."

"Electric potential is in units of volts (V). Electric potential energy is in units of joules (J)."

"V is joules per coulombs (J/C) and EPE is newtons times meters (N·m)."

Explain the conceptual difference between the electric potential V, and electric potential energy, EPE.

"V describes the potential to have EPE, whereas EPE is the actual potential to do work?"

"Two charges can contain electric potential energy. Only one charge can be a voltage source."

"A source charge Q can create an electric potential V around itself and is used to store EPE with a separate test charge?"

"EPE is used in the one step model and V is used in the middle of the two-step model."

"I'm not sure how to explain this conceptually."

Briefly summarize the difference (if any) between "voltage" and electric potential.

"I am not certain about the differences between voltage and electric potential. Voltage is the amount of potential at a location. The electric potential is also the amount of potential at a location."

"Potential and voltage really mean the same thing."

"These terms are interchangeable."

"There is no difference between the two. 'Voltage' is just another name for electric potential."

Identify the changes in electric potential energy EPE (if any) for the following test charges (±q):
(Only correct responses shown.)

Positive test charge +q brought closer to a positive source charge +Q: increase [71%]
Negative test charge –q brought closer to a positive source charge +Q: decrease [68%]
Positive test charge +q brought closer to a negative source charge –Q: decrease [55%]
Negative test charge –q brought closer to a negative source charge –Q: increase [55%]

Identify the changes in electric potential V (if any) for the following test charges (±q):
(Only correct responses shown.)

Positive test charge +q brought closer to a positive source charge +Q: increase [55%]
Negative test charge –q brought closer to a positive source charge +Q: increase [45%]
Positive test charge +q brought closer to a negative source charge –Q: decrease [23%]
Negative test charge –q brought closer to a negative source charge –Q: decrease [45%]

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

"I need a review of the difference between EPE and V."

"Please go over some more examples in class."

"So batteries store energy by having a lot of like charges close together?" (That's what capacitors do. Batteries store energy by having dissimilar materials close together, that want to give up or take in electrons from each other via chemical reactions.)

"Do you have the solutions posted anywhere for the previous quizzes?" (Not for all the questions, but there are solutions for some of them; just click on the blog link next to the PDF link for each quiz.)

"Do our pre-lab question responses effect our pre-lab grade?" (As long as you try your best to answer them, you still get full credit. But do try your best; your responses help you figure out how little (or how much) background you need for the briefing at the start of each lab.)

"So the wave strips in lab work because they're sinusoidal waves that are in phase. If they were cosine waves, would they be out-of-phase from the sine waves?" (If both wave strips were cosines, then they would still be in phase sources. If one wave strip was a sine and the other strip was a cosine, then they would be out-of-phase sources, and then to find the maxima angles you would need to use d·sinθ = (m + (1/2))·λ instead of d·sinθ = m·λ.)

"Hey, P-dog how long have you been teaching physics?" (After getting my doctorate, about forty semesters so far, I have to teach sixty more semesters until I get to retire.)

"Hey I was looking at your webpage and do you teach swing classes?" (I don't teach swing dance, but Mrs. P-dog and I sometimes DJ for swing dancing at the Madonna Inn on Mondays.)