Sunday, September 5, 2010
Chapter 16, question 12
This question, SV8 problem 16.52, has some problems in its wording. First, the capacitor is made from two strips of aluminum foil, separated by paraffin-coated paper. The question wants you to think about the capacitor unrolled, so that there are two flat plates, separated by one layer of the dielectric. What are the dimensions of that configuration in order to have the given capacitance? (Then, as an editorial note, the statement of the problem goes on to say that if you put in another piece of paper and roll the thing up as illustrated, it will actually have twice the capacitance because now charge accumulates on both sides of each plate, essentially doubling the active area.)
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5 comments:
Hello all,
I'm having trouble visualizing everything here in WA#12. I can't seem to figure out what to put for d, separation.
Any clarification other than what has originally been posted here, would be great! Thanks!
The plates are the two strips of aluminum foil, which are separated by a strip of paraffin-coated paper. So the separation is the thickness of the paper, and you need to find the dielectric constant of the paper. You are given one dimension of the plates, the width, and you are asked to find the length.
Hi everyone,
I don't know why I'm not getting the correct answer to this question - I first did the "Practice another version" to make sure I was doing the right thing, and I got the answer correct. When I applied the same formula with different number to my actual problem, it says i'm off by more than 10%...
The formula I'm using is C = 2*Eo*Er*A/D
With my numbers it becomes 8.5e-8 = 2*8.85e-12*3.7*A/2.5e-5. I then find A = .0344m^2. I then set that = to .04*L to find the length and I get a length of .8112.
What am I missing? Am I doing something wrong or am I just calculating incorrectly?
Never mind... I didn't multiply by 2 and I got the correct answer, however, I thought I was supposed to do so because in the parentheses of the problem, it said capacitance doubles because it's in a roll.
Can anyone clear this up for me?
Re: Anonymous
Yes-- the problem states that by adding a second strip of paper and rolling up the capacitor, its capacitance will double. However, the question is asking for the desired length BEFORE the capacitance is rolled up.
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