HELP: physics 2 homework-capacitor question

[pinkfloyd4ever]

ok, I know this isn't headphone related, but I knew people here could help. I'm lost on number 4

Ya I know I shoulda asked sooner than the night b4 it's due, but w/e, thats how I always do stuff.

feel free to help on other questions too, but I REALLY need your help most with #4 pleeeaassseeeee. Thanks

 

[rb67]

Hope this is correct

Then use
Q = C*V
E = 1/2 C*V^2
to answer the rest of the question

 

[pinkfloyd4ever]

Quote:

Originally Posted by rb67 /img/forum/go_quote.gif Then use Q = C*V E = 1/2 C*V^2 to answer the rest of the question

wow awesome, THANKS a ton

if this forum had Karma/reputation, you'd def be getting as much as possible from me right now

 

[tomb]

Yes - the back of a napkin is quicker than CAD when drawing on a one-to-one basis.

However, I found the same thing, more or less. I'm usually deciphering fluid schematic or P&ID's but re-drawing the problem is often helpful. The diamond config is very confusing, but if you follow the paths, it's really the same as this:

Consequently, it's easy to recognize the middle two caps are in series. So:
1/Ctot = 1/C1 + 1/C2, or 1/Ctot = 1/5 + 1/7 = 7/35 + 5/35 = 12/35,
Ctot = 35/12.

That becomes another capacitor in parallel with the other two, making three caps in parallel. So,
Ctot = 35/12 + 4 + 6 = 155/12, or 12.91uf

EDIT: no stars for being too slow, I guess.

 

[pinkfloyd4ever]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif Yes - the back of a napkin is quicker than CAD when drawing on a one-to-one basis. However, I found the same thing, more or less. I'm usually deciphering fluid schematic or P&ID's but re-drawing the problem is often helpful. The diamond config is very confusing, but if you follow the paths, it's really the same as this: Consequently, it's easy to recognize the middle two caps are in series. So: 1/Ctot = 1/C1 + 1/C2, or 1/Ctot = 1/5 + 1/7 = 7/35 + 5/35 = 12/35, Ctot = 35/12. That becomes another capacitor in parallel with the other two, making three caps in parallel. So, Ctot = 35/12 + 4 + 6 = 155/12, or 12.91uf

AHHH, yes that new schematic helps, I knew the diamond was messing w/me but I wasn't sure how to redraw it correctly. Man, this stuff is why I'm an ME major rather than EE. Gauss's law was worse, I still don't get it at all. Fortunately my DIY experience has helped a little with caps. Oh and if anyone feels like it, I can't figure out how to get started with # 2 either. It seems like, with the formulas I have (our text is Sears & Zemansky's University Physics, 11th ed, this stuff is in Ch 24), I don't have enough info to solve the problem. I could do it if it was a parallel plate cap, but I can't find & dont remember the teacher mentioning any formulas on calculating C of a cap with a coaxial geometry. And he will help us a little if we ask, but it's also kinda like asking Simon from AI if I'm a good singer...he likes to grill you for not understanding everything just from his lectures. Don't spend alot of time on this anybody, cause our teacher is rather lenient about turning stuff in; as long as it's in by the end of the day. Thank god, cause our class is @ 10 and I'd be screwed if it was due then-there's 2 more questions on the back. God I need to get off here and actually do it. Thanks again, tomb & rb67

 

[pinkfloyd4ever]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif

wait, so you're telling me that that's electrically the same as this:

or did I draw that wrong?

man, I'll be lucky if I pass my circuits class next sem

EDIT: OHHHHHH OK now I see it. You basically just turned the pair of caps in the middle perpendicular to what I drew, which makes more sense and is less confusing. I guess seeing it both ways helped. Thanks again tomb!

 

[DaKi][er]

Look at what is connected to node A and what is connected to node B and you'll see it

 

[monolith]

Yeah, I'm not sure you have enough to do Q2. Then again, I'm a theoretical guy. Not big on calculating things or engineering things. I can give you capacitance in terms of potential difference, but it doesn't give you that until the second part of the question, so I don't know.



By the way, I'm a physicist, so I change subscripts and add/drop vector signs at will. Hope it's clear.

 

[rb67]

For #2 you might find this link useful:
http://hyperphysics.phy-astr.gsu.edu...capcyl.html#c2

It's pretty much along the same lines as monolith's excellent example. You should have enough info for it.

 

[jcx]

strictly speaking only a) is answerable

the problem needs more initial conditions for the latter 2 parts, the node between the 5 and 7 uF caps could be any V, those caps could have any total of charge and stored energy

of course calling your instructor on it could have consequences...

 

[ezkcdude]

Quote:

Originally Posted by jcx /img/forum/go_quote.gif of course calling your instructor on it could have consequences...

What would be really funny is if his prof was a head-fier!

 

[monolith]

Actually, nevermind. My answer seems to be correct. I don't know what I was thinking before.

Isolate Q/L from my answer to get part (b) and you're done.