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Primitive physics class question


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#1 nullsquared   Members   -  Reputation: 122

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Posted 09 February 2011 - 05:38 PM

We took a quiz in AP Physics B today, and, according to my teacher, I got one of the questions wrong. I don't have the quiz with me at the moment, but here is what I could come up with from memory:
Posted Image
For my solution, I solved for the magnitude of F:
Posted Image
And I solved for the magnitude of the new force, assuming the new charge of both spheres would be -Q/2:
Posted Image
And I came up with the answer F/8.

Am I correct, or am I wrong (and why)?

My teacher said that the magnitude of the new force is equal to just F (no coefficient) because "the total charge is still the same."

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#2 Buckeye   Crossbones+   -  Reputation: 6214

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Posted 09 February 2011 - 06:06 PM

Given what you posted: I would disagree with your teacher. Before the spheres touch, there is an attractive force, due to the separation of unlike charges. After they touch, there is no longer any positive charge (assuming unlike charges combine) and the remaining total charge is -Q. Like charges repel and, as you assumed, should distribute themselves -Q/2 to a sphere. That results in a repulsive force between the spheres.

Just from what you posted (and the assumptions about combination and distribution noted), I would say the answer is: Before +F. After -F/8. Your magnitude appears correct, but not the sign.

"The total charge is the same" is not sufficient. Consider 2 spheres, one with -Q, one with +Q charges before they touch. There will be an attractive force until the spheres touch. Then the force will drop to zero as the charges neutralize. Strictly speaking, the total charge before and after is 0. But the separation of those charges is the principle in Coulomb's Law. There is a separation before the touch; no separation after.

However, N.B.

Rule 1: the teacher is always correct
Rule 2: if the teach is ever incorrect, see Rule 1.

Please don't PM me with questions. Post them in the forums for everyone's benefit, and I can embarrass myself publicly.


#3 nullsquared   Members   -  Reputation: 122

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Posted 09 February 2011 - 06:42 PM

Given what you posted: I would disagree with your teacher. Before the spheres touch, there is an attractive force, due to the separation of unlike charges. After they touch, there is no longer any positive charge (assuming unlike charges combine) and the remaining total charge is -Q. Like charges repel and, as you assumed, should distribute themselves -Q/2 to a sphere. That results in a repulsive force between the spheres.

Just from what you posted (and the assumptions about combination and distribution noted), I would say the answer is: Before +F. After -F/8. Your magnitude appears correct, but not the sign.

Thanks for the clarification. I ignored the signs because the question explicitly asked for only the magnitude; perhaps I could have made that clearer.

Rule 1: the teacher is always correct
Rule 2: if the teach is ever incorrect, see Rule 1.

I'm not sure if you've seen any of my previous posts, but this isn't the first time I've had a dispute with a teacher. Unlike the rest of the crowd, I refuse to blindly believe everything a teacher says. This is one of the core issues in today's education system, but that strays from the topic at hand.

#4 szecs   Members   -  Reputation: 2185

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Posted 10 February 2011 - 12:52 PM

My chemist teacher (or whatever) told to our class once, that Oxygen can have 7-8 protons. I asked two times if he meant neutrons. Nope. He meant protons. Then one of my tests was "accidentally" lost. I only could get a pass mark at the end of the course because all the tests were taken into account. That worth a lot of credits and that f*cked my scholarship pretty well.

#5 grhodes_at_work   Moderators   -  Reputation: 1361

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Posted 10 February 2011 - 01:29 PM

nullsquared, actually I am glad that you refuse to just take what your teacher says to be always true. It isn't. Frankly, any good teacher should welcome a good student's challenge and be open to themselves learning from the students. The classroom should be a collaboration, an interaction. The teacher is the master, but the master must also learn from the student.


Graham Rhodes Moderator, Math & Physics forum @ gamedev.net

#6 Álvaro   Crossbones+   -  Reputation: 13897

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Posted 10 February 2011 - 01:39 PM

I encourage you to keep up the fight. If he is wrong, he is wrong. However, one thing I learned from my father when I was your age is that you should always be careful to not embarrass people in public.

The other thing I wanted to point out is that the answer depends on the relative sizes of the spheres. In your diagram they seem to be the same size, but this was not specified in the text. If they are metallic and don't have the same size, each sphere will keep an amount of charge proportional to its radius (because a sphere of a conducting material has a capacity that is proportional to its radius). The resulting force will then be smaller than F/8.[EDIT: Nevermind. I missed the "identical" part. Attention was never my strength. ;)]

#7 nullsquared   Members   -  Reputation: 122

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Posted 10 February 2011 - 03:16 PM

The classroom should be a collaboration, an interaction.

This is true. Unfortunately, it doesn't apply to most of my classes. My physics class consists of the teacher taking around 10 minutes to present a couple of new formulas, then "plugging in the numbers" to get "the answer." Then we do a good amount of nearly identical problems so that we score well on the AP exam. This irritates me to no end. I don't care about memorizing formulas; I care about actually understanding physics and the world around me. In fact, it irritates me to the point where I loathe going to class for what happens to be one of my favorite topics.

And, ironically, my teacher complains that we don't ask him enough questions. Well, if he was actually capable of answering questions without circumventing them by blabbing the same thing over and over, then we would ask more questions.

I encourage you to keep up the fight. If he is wrong, he is wrong. However, one thing I learned from my father when I was your age is that you should always be careful to not embarrass people in public.

Excellent point, but I gave up. He's been wrong before and it is very clear to me that trying to reason with him is fruitless.

#8 unbird   Crossbones+   -  Reputation: 5998

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Posted 10 February 2011 - 05:05 PM

Sounds like a dead end. And a bad teacher. I once heard the following aphorism:

I had bad teachers. That was a good school.

Learn from this on another level. It won't be last time you have to "shut up" or duck from someone you know is wrong. At this point this isn't about reasoning/science/logic anymore, but about social/political/soft skills. I can imagine your pain, but:

- Don't let it spoil your enthusiasm. Sit it through. There are enough alternatives to learn (e.g. here). If you like the topic you don't need a teacher anyway. If he's telling nonsense discuss with your colleagues - after the class.
- Heed szecs' warning. Don't jeopardize your career just to prove yourself. This isn't worth it. I remember a guy at university who repeatedly challenged the professor, especially in the professor's research topic. Well, he was a jerk, to colleagues, too. But even if he was right, it cost him his master. If you are good at something, someone else will notice.

#9 nullsquared   Members   -  Reputation: 122

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Posted 10 February 2011 - 05:39 PM

Sounds like a dead end. And a bad teacher. I once heard the following aphorism:

I had bad teachers. That was a good school.

Learn from this on another level. It won't be last time you have to "shut up" or duck from someone you know is wrong. At this point this isn't about reasoning/science/logic anymore, but about social/political/soft skills. I can imagine your pain, but:

- Don't let it spoil your enthusiasm. Sit it through. There are enough alternatives to learn (e.g. here). If you like the topic you don't need a teacher anyway. If he's telling nonsense discuss with your colleagues - after the class.
- Heed szecs' warning. Don't jeopardize your career just to prove yourself. This isn't worth it. I remember a guy at university who repeatedly challenged the professor, especially in the professor's research topic. Well, he was a jerk, to colleagues, too. But even if he was right, it cost him his master. If you are good at something, someone else will notice.



Your words are wise. However, I am faced with one quirk: I am forced to sit through that class (and others like it). I would much rather spend that time sitting in the library watching Salman Khan's videos on the same topics (Khan Academy). His lessons are absolutely amazing; he aims to teach understanding, not memorization.





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