Jump to content
Posted 09 February 2011 - 05:38 PM
Posted 09 February 2011 - 06:06 PM
Please don't PM me with questions. Post them in the forums for everyone's benefit, and I can embarrass myself publicly.
You don't forget how to play when you grow old; you grow old when you forget how to play.
Posted 09 February 2011 - 06:42 PM
Thanks for the clarification. I ignored the signs because the question explicitly asked for only the magnitude; perhaps I could have made that clearer.
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.
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.
Rule 1: the teacher is always correct
Rule 2: if the teach is ever incorrect, see Rule 1.
Posted 10 February 2011 - 12:52 PM
Posted 10 February 2011 - 01:29 PM
Posted 10 February 2011 - 01:39 PM
Posted 10 February 2011 - 03:16 PM
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.
The classroom should be a collaboration, an interaction.
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.
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.
Posted 10 February 2011 - 05:05 PM
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.