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![[image loading]](http://img6.imageshack.us/img6/7291/madzt.jpg)
I need your help, physicists of TL. I'm currently in electricity and magnetism and we had our first midterm (which was really easy, I got an 80 which was lower than I would like but this is why I'm here). This question, I feel, is wrong. I got this question wrong because I answered D, the correct answer being C.
A. Q inside, 5Q outside
B. Q inside, -Q outside
C. -Q inside, 7Q outside
D. 0Q inside, 6Q outside
E. There is not enough information to tell.
My thought process was this: C is not correct because that is with regards to the picture of the whole shell, which it not asking about. So I say D is the correct answer, because all 6Q will distribute itself on the outside of the shell. The second sentence reads: "If an excess charge of 6Q is placed on the conducting shell, how does this charge distribute itself on the inner and outer surfaces of the shell?".
Am I missing something? I was like duh, it distributes itself over the outside of the shell, none inside. Of course the shell has a -Q induced by the +Q charged sphere on the inside. Since the shell is initially neutral the outside experiences a net +Q due to -Q on the inside surface of the shell. So if we introduce 6Q the whole picture looks like -Q on the inside and 7Q on the outside. But, if that is the case, it proves that all 6Q does in fact distribute itself over the outside of the shell considering the initial state without the introduction of the charge.
I talked to my lecturer and he waved his hands and disagreed with me. What is the case, TL?
[I filled in the charges in the picture just to show work. Nothing was labeled initially.]
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uhhhh, what are the choices, because if you are just forcing me to pick a letter randomly, that is not associated with anything Im gonna choose B
EDIT: Ahh there we go choices
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I realized and edited as soon as I could. Sorry for the inconvenience. I am quite...not-elated...about this so I just tried to get it out as quick as I could.
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You're not really asking a physics question. You're arguing about how the question is phrased. I don't think anybody here could help you with that. That's entirely up to the professor what he meant by the question.
It seems like you know the physics from your argument (which I think is correct). So I think you should just move on.
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dosnt the center positive charge attract a negative charge off the shell?
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It looks like bad wording in the question to me. If they asked about the total charge on the shell then C would be correct, but as the question is stated I'd say D is the correct answer.
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Calgary25955 Posts
This isn't necessarily for you, it's for other people (I think this thread is fine and interesting):
In the future, instead of making a homework blog, use the Team Liquid Manpower thread. In the past, I've contacted engineers and VB programmers through that thread to help me with my work. You will get a better response and you won't take up space on the forums.
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On October 06 2010 01:10 garbanzo wrote:
You're not really asking a physics question. You're arguing about how the question is phrased. I don't think anybody here could help you with that. That's entirely up to the professor what he meant by the question.
It seems like you know the physics from your argument (which I think is correct). So I think you should just move on.
This really does seem like the case. You are right, the charge is distributed +6Q to the outside of the, but the state in the end is going to be +7Q out, -1Q in. The question is phrased somewhat ambiguously if you take "this charge" to mean the charge that was added then yes C, if you assume "this charge" is the net charge on the sphere then the answer is D, but I would have answered D
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On October 06 2010 01:10 garbanzo wrote:
You're not really asking a physics question. You're arguing about how the question is phrased. I don't think anybody here could help you with that. That's entirely up to the professor what he meant by the question.
It seems like you know the physics from your argument (which I think is correct). So I think you should just move on.
Sure it's more semantics than physics. But if you don't know the physics, then what use is a contribution? Anyone can argue semantics.
On October 06 2010 01:10 Rhaegar99 wrote:
dosnt the center positive charge attract a negative charge off the shell?
It does. In the initial case without the introduction of a +6Q charge the shell has the +Q sphere inside it, -Q on the inside, and +Q on the outside as a result of the shell being neutral initially.
On October 06 2010 01:12 glassmazarin wrote:
It looks like bad wording in the question to me. If they asked about the total charge on the shell then C would be correct, but as the question is stated I'd say D is the correct answer.
This is what I think as well. I just wanted opinions before I emailed the department head with my concern, because I want them to either be more specific in the future or correct it. That's if it is how i perceive it, which is why I made this post - to see what the case is before I send an email. Best case I expect them to be more careful later on. I think they're too lazy to adjust scores for 948 exams, but it can't hurt to try.
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On October 06 2010 01:14 Chill wrote:This isn't necessarily for you, it's for other people (I think this thread is fine and interesting): In the future, instead of making a homework blog, use the Team Liquid Manpower thread. In the past, I've contacted engineers and VB programmers through that thread to help me with my work. You will get a better response and you won't take up space on the forums.
Thanks. I wasn't exactly sure where the appropriate place to put this was so I just blogged it rather than putting it in general or something.
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On October 06 2010 01:15 NukeTheBunnys wrote:Show nested quote +On October 06 2010 01:10 garbanzo wrote:
You're not really asking a physics question. You're arguing about how the question is phrased. I don't think anybody here could help you with that. That's entirely up to the professor what he meant by the question.
It seems like you know the physics from your argument (which I think is correct). So I think you should just move on. This really does seem like the case. You are right, the charge is distributed +6Q to the outside of the, but the state in the end is going to be +7Q out, -1Q in. The question is phrased somewhat ambiguously if you take "this charge" to mean the charge that was added then yes C, if you assume "this charge" is the net charge on the sphere then the answer is D, but I would have answered D
I forgot to add that I probably would have answered D also. But that's generally because I have this bad habit of ignoring the actual question and just answering what the problem is asking. So for this problem I would have interpreted it as: "Take the picture drawn (I assume that initial picture is there) and add +6Q to the conducting shell. What's the final picture?"
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On October 06 2010 01:15 NukeTheBunnys wrote:Show nested quote +On October 06 2010 01:10 garbanzo wrote:
You're not really asking a physics question. You're arguing about how the question is phrased. I don't think anybody here could help you with that. That's entirely up to the professor what he meant by the question.
It seems like you know the physics from your argument (which I think is correct). So I think you should just move on. This really does seem like the case. You are right, the charge is distributed +6Q to the outside of the, but the state in the end is going to be +7Q out, -1Q in. The question is phrased somewhat ambiguously if you take "this charge" to mean the charge that was added then yes C, if you assume "this charge" is the net charge on the sphere then the answer is D, but I would have answered D
How I read it was basically: what does 6Q do with itself?, to which I responded 6Q places all of itself on the outside of the shell, none on the inside (D). Nothing to do with net charge was taken into consideration considering the question.
On October 06 2010 01:19 garbanzo wrote:Show nested quote +On October 06 2010 01:15 NukeTheBunnys wrote:On October 06 2010 01:10 garbanzo wrote:+ Show Spoiler +
You're not really asking a physics question. You're arguing about how the question is phrased. I don't think anybody here could help you with that. That's entirely up to the professor what he meant by the question.
It seems like you know the physics from your argument (which I think is correct). So I think you should just move on.
This really does seem like the case. You are right, the charge is distributed +6Q to the outside of the, but the state in the end is going to be +7Q out, -1Q in. The question is phrased somewhat ambiguously if you take "this charge" to mean the charge that was added then yes C, if you assume "this charge" is the net charge on the sphere then the answer is D, but I would have answered D I forgot to add that I probably would have answered D also. But that's generally because I have this bad habit of ignoring the actual question and just answering what the problem is asking. So for this problem I would have interpreted it as: "Take the picture drawn (I assume that initial picture is there) and add +6Q to the conducting shell. What's the final picture?"
Here is the unaltered original picture of the problem, the one I gave in the original post was just showing work of how 6Q distributed itself. They don't ask about the final charge distribution of the whole shell, just how the 6Q introduced distributes itself. That is where my contention is and how C and D are different.
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On October 06 2010 01:17 Hizzo wrote:
This is what I think as well. I just wanted opinions before I emailed the department head with my concern, because I want them to either be more specific in the future or correct it. That's if it is how i perceive it, which is why I made this post - to see what the case is before I send an email. Best case I expect them to be more careful later on. I think they're too lazy to adjust scores for 948 exams, but it can't hurt to try.
Please don't bother the department head over such a trivial matter. They are very busy people. Just speak to the professor. If you're truly concerned about how the question is phrased (and not the points) then just voice that concern. If you talk to him like you're just trying to get a point or two for your assignment then they will likely ignore you.
This is speaking as a former physics TA.
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seems to me the problem with your answer is you are ignoring the side of the conducting sphere which is closer to the charge
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On October 06 2010 01:35 garbanzo wrote:Show nested quote +On October 06 2010 01:17 Hizzo wrote:
This is what I think as well. I just wanted opinions before I emailed the department head with my concern, because I want them to either be more specific in the future or correct it. That's if it is how i perceive it, which is why I made this post - to see what the case is before I send an email. Best case I expect them to be more careful later on. I think they're too lazy to adjust scores for 948 exams, but it can't hurt to try.
Please don't bother the department head over such a trivial matter. They are very busy people. Just speak to the professor. If you're truly concerned about how the question is phrased (and not the points) then just voice that concern. If you talk to him like you're just trying to get a point or two for your assignment then they will likely ignore you. This is speaking as a former physics TA.
I wouldn't call it trivial when it could potentially affect what letter grade you end up receiving at the end of a course. 20 questions all equally weighted at 5 points each. I feel like if my lecturer (professor) had to hand wave and couldn't give me a straight answer about the problem then how is it a fair question to give if he cannot even explain why it is the way it is?
On October 06 2010 00:47 Hizzo wrote:
I talked to my lecturer and he waved his hands and disagreed with me. What is the case, TL?
On October 06 2010 01:37 twalf wrote:
seems to me the problem with your answer is you are ignoring the side of the conducting sphere which is closer to the charge
It doesn't ask about anything other than how 6Q distributes itself, which has nothing to do with the inside of the shell other than what the total charge of the outside of the shell is, which they don't ask. Sure, the resulting charge on the outside of the shell is 7Q because of the +Q charged sphere on the inside pulling -Q worth of electrons from the shell to the inside causing a +Q on the outside before the 6Q is brought in. Because they say it is neutral initially, this is the case.
This whole system is really frustrating. I'm not trying to come off badly, I'm just upset about the ambiguity of the question which I believe I interpreted and answered the correct way (in the context of strictly what they are asking), which was (in my mind) unjustifiably wrong.
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When did you talk to the prof, was it during office hours or was it after class when he had a meeting to go to in 10 minutes. Going to the dept head over 5 points will annoy the dept head, who will in turn annoy your professor, who will not give you the points because you caused too much aggravation, and he will also be less likely to help you out at a later date. professors are humans too and they do have favorites, and people they don't like, and that does effect the grades.
If you really feel strongly about it I would go to the professor during office hours, when he has time for it, and very calmly and clearly layout what you have said here, make sure to point out the confusion of what "this charge" means. If he relents and gives you the points party, if he doesn't drop the grades really aren't worth that much trouble over, study for the next test and make it up.
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On October 06 2010 01:51 NukeTheBunnys wrote:
When did you talk to the prof, was it during office hours or was it after class when he had a meeting to go to in 10 minutes. Going to the dept head over 5 points will annoy the dept head, who will in turn annoy your professor, who will not give you the points because you caused too much aggravation, and he will also be less likely to help you out at a later date. professors are humans too and they do have favorites, and people they don't like, and that does effect the grades.
If you really feel strongly about it I would go to the professor during office hours, when he has time for it, and very calmly and clearly layout what you have said here, make sure to point out the confusion of what "this charge" means. If he relents and gives you the points party, if he doesn't drop the grades really aren't worth that much trouble over, study for the next test and make it up.
I talked to him after lecture. He does 2 back to back with 15 min or so in-between (same building, same room). The thing is, you cannot study for something like this (ambiguity with syntax and semantics, lack of question clarity), which is what gets me.
On October 06 2010 01:35 garbanzo wrote:
Please don't bother the department head over such a trivial matter. They are very busy people. Just speak to the professor. If you're truly concerned about how the question is phrased (and not the points) then just voice that concern. If you talk to him like you're just trying to get a point or two for your assignment then they will likely ignore you.
This is speaking as a former physics TA.
And on this, one of the two of my lab TA's who agreed with me (the other one was unsure) suggested that's where I take my concern. So...that's where I got my direction.
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On October 06 2010 01:15 NukeTheBunnys wrote:Show nested quote +On October 06 2010 01:10 garbanzo wrote:
You're not really asking a physics question. You're arguing about how the question is phrased. I don't think anybody here could help you with that. That's entirely up to the professor what he meant by the question.
It seems like you know the physics from your argument (which I think is correct). So I think you should just move on. This really does seem like the case. You are right, the charge is distributed +6Q to the outside of the, but the state in the end is going to be +7Q out, -1Q in. The question is phrased somewhat ambiguously if you take "this charge" to mean the charge that was added then yes C, if you assume "this charge" is the net charge on the sphere then the answer is D, but I would have answered D
I find this strange. It's fairly clear to me that C is the "expected" answer. The right way to go about this is to ask for clarification during the test, if it's possible. To be honest, I don't even think D is correct: if you introduce 6Q charge there's no way to tell how it gets distributed. If there's some way to track the charge carriers you might find that most of them end up on the inside and the positive charges that go to the outside were mostly in there before the extra charge was introduced.
My point is that given the level detail in the question the only sensible question to ask is what the charge distribution looks like in the end.
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I can accept that. Thanks. That's probably why D is incorrect. I just wish they would have asked what the final charge distribution looked like in a clear manner; what poor wording.
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Canada7170 Posts
Yeah with any wording problems just ask during the test, most likely a bunch of other students will get confused too.
I agree that the wording is pretty bad though.
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