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

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 question: comparing horizontal velocity components

Physics 205A Quiz 3, fall semester 2019
Cuesta College, San Luis Obispo, CA

Two velocity vectors shown at right have different speeds and directions. All angles are measured counterclockwise from the +x axis. Velocity vector __________ has the larger horizontal component magnitude.
(A) v_A.
(B) v_B.
(C) (There is a tie.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (B)

Since these θ angles are measured counterclockwise from the +x axis, the horizontal components of these velocity vectors are given by:

v_A,x = v_A·cosθ_A,
v_B,x = v_B·cosθ_B.

Then the horizontal components are these velocity vectors can be calculated and compared:

v_A,x = v_A·cosθ_A = (10 m/s)·cos(80°) = 1.736481776669303... m/s,

or to two significant figures, the horizontal component of v_A has a magnitude of 1.7 m/s, while:

v_B,x = v_B·cosθ_B = (4.0 m/s)·cos(60°) = 2.0 m/s.

Thus the horizontal component of v_B is greater than the horizontal component of the horizontal component of v_A.

Sections 70854, 70855
Exam code: quiz03Ch3V
(A) : 4 students
(B) : 49 students
(C) : 1 student
(D) : 0 students

Success level: 91%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.19

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: soccer ball horizontal velocity component

Physics 205A Quiz 3, fall semester 2018
Cuesta College, San Luis Obispo, CA

A Physics 205A student kicks a soccer ball such 
that it has an initial speed of 21 m/s, with an angle of 155°, as measured counterclockwise from the +x direction. Neglect air resistance. The horizontal component of the soccer ball's initial velocity vector is:
(A) –19 m/s.
(B) –8.8 m/s.
(C) +8.8 m/s.
(D) +19 m/s.

Correct answer (highlight to unhide): (A)

The initial velocity vector of the soccer ball is at an angle of 155° as measured counterclockwise from the +x axis (i.e., the "unit circle angle"). Using this θ = 155° angle allows the ± signs in the initial velocity vector components to naturally result from the calculations:

v_0x = v₀·cosθ,
v_0y = v₀·sinθ.

Then the horizontal component of the initial velocity vector is then:

v_0x = v₀·cosθ,

v_0x = (21 m/s)·cos(155°) = –19.032463527769649 m/s,

or to two significant figures, v_0y = –19 m/s.

(Response (D) is the initial horizontal speed, but directed to the right; response (C) is the vertical component of the initial velocity vector (21 m/s)·sin(155°); response (D) is the initial vertical speed, but directed downwards.)

Sections 70854, 70855
Exam code: quiz03pRH5
(A) : 42 students
(B) : 5 students
(C) : 3 students
(D) : 6 students

Success level: 75%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.60

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 question: direction of total electric field

Physics 205B Quiz 3, spring semester 2014
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 16.33

Two point charges are held at fixed locations. The total electric field at x = +0.5 cm is directed to the:
(A) left.
(B) right.
(C) (There is no direction, as the electric field is zero.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (A)

The electric field vectors due to the –2.0 nC source charge are shown at right, where they all point in towards the negative source charge. (The magnitudes of these electric field vectors are not quite to scale, but do get smaller with increasing distance from the –2.0 nC source charge).

The electric field vectors due to the +8.0 nC source charge are shown at right, where they all point out away from the positive source charge. (The magnitudes of these electric field vectors are not quite to scale, but do get smaller with increasing distance from the +8.0 nC source charge, and are proportionally larger than the electric field vectors of the less –2.0 nC source charge).

The total electric field at x = +0.5 cm due to both source charges would be the vector addition of the individual electric field vectors at that location, as they point in the same direction (to the left).

Sections 30882, 30883
Exam code: quiz03cDvD
(No results compiled)

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!"