Homework 3 is available for you to work on at the mastering physics website. I will add commentary here in response to your comments and questions. I look forward to seeing your comments, questions or other thoughts below.
For Monday, please finish reading chapter 23. (I think that section 23.4 is particularly interesting and important as it contains the idea that there is always energy in electric fields.) Then, when you have time, I would suggest starting to read chapters 24 and 25, with your emphasis on chapter 25. Our next homework, homework 4, will involve both chapters 24 and 25; the emphasis will be on problems from chapter 25.
In reference to homework or reading i really do look forward to seeing comments and questions here. What points, either conceptual or computational, are you stuck on or would like to know more about?
Zack
PS. If you would like to see derivation of the formula for the electric potential of the dipole, then I could do a post on that. I heard that some people might be interested.
If there is anything else you would like to see derived or explained, please feel free to ask.
I have a question about part B of 23.20, which asks us how much work is required to transfer a second charge of magnitude "q" across a capacitor of some given area a^2 and a distance d apart.My first instinct was to simply add the work it takes to bring the charge q across the capacitor (1/2CV^2, the answer to part a) and then add the additional electrostatic work of bringing the charges together (kq^2/a). This answer was wrong so I thought maybe the electrostatic work had to be integrated for the movement of the charge (involving ln(r)). This did not work either, where should I go?
ReplyDeleteThe integral that is involved is already in the result you mention,(1/2)CV^2, which worked okay for part a, yes? So there is no ln(r) or anything like that. part B. is just like part A except that you need to subtract off the energy from part in a, because of the way the problem is worded.
ReplyDeleteSo actually, with twice the charge you get four times the energy. Does that make sense?
One thing, conceptually, with this problem is to realize that these are continuous charges made up of lots of little charges, so that changes this problem compared to problems with discrete charges,Like in some of the earlier problems (14, 15 and 16).
with regard to reading, i added above a special mention of section 23.4.
ReplyDeleteI put some comments regarding problem 80 in the top post. Inadvertently in today's class we stumbled on how to do that in the context of thinking about problems 40.
ReplyDeleteThis week's extra credit problems have a wide range of difficulty. Don't worry if you can't do them all. if you have any questions about specific problems, including whether you should really be able to do them or not, please feel free to post that here.