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Homework Help: How to go about evaluating these simple circuits? (w/ capacitors)

  1. Nov 8, 2011 #1
    I have a midterm in a few days, and I'm having some problems understanding the process behind evaluating circuits. I was wondering if you guys could simply run me through, step by step, how you evaluate a circuit, so that I can get a better understanding? I'm using the Khan Academy site as well for this, since they have some good videos, but I'd love for someone to help me with a few specific examples.

    I know this sounds a bit sketchy, but none of this is for homework or anything. My teacher simply enjoys showing us equations and proofs, but doesn't really show us any examples. My book has this flaw as well.

    List of Relevant Equations

    I'm going to list a few equations I know will come up. This might not be all that are used, but hopefully they will be.

    V = IR

    P = (I^2)V

    P = E/J

    Q = CV

    KE = (1/2)mv^2

    PE = mgy

    W = Fdcosθ

    F = qE

    V(series) = IR(series)

    R(series) = Ra + Rb + ...

    1/(Rparallel) = (1/Ra) + (1/Rb) + ...

    Rparallel = (Ra*Rb*...)/(Ra+Rb+...)

    i = (ε/R)e^(-t/(RC))

    Vr = εe^(-t/(RC))

    Vc = ε(1-e^(-t/(RC)))

    (In series) 1/Ceq = Ʃ from i = 1 to n of 1/Ci

    (In parallel) Ceq = Ʃ from i = 1 to n of Ci

    If you're helping me, hopefully you know what the symbols mean, haha. If you need any clarification, just ask.

    A few problems

    Here are three problems taken from a practice sheet for the test. Note that I have the current directions marked, but if you feel the need to change them, go ahead and do it.

    1. http://i.imgur.com/Jdh36l.jpg"

    For this problem I know that in the answer εa ends up supplying no power to the circuit. I know that I'm supposed to use Kirchoff's Loop Rule and do at three loops, and then use the junction rule in order to relate the different currents to one another. It's frustrating, because I know WHAT to do, but when carrying it out, I get stuck.

    2. http://i.imgur.com/2KVVQl.jpg"

    Most of the blurriness in this picture is actually a result of the paper. The image they used was pretty crappy. Sorry! I know you're supposed to use the Loop Rule again, but I can't seem to figure out what to do. I know this is the easiest problem, but I still can't seem to do it.

    3. http://i.imgur.com/Niarkl.jpg"

    This problem is just a genuinely difficult problem. I know that the moment the switch is opened, there is a current flowing through the capacitor, and as the capacitor charges, the current dwindles. I know that the use of the charging/discharging of a capacitor means that we use these equations somehow:

    i = (ε/R)e^(-t/(RC))

    Vr = εe^(-t/(RC))

    Vc = ε(1-(e^(-t/(RC))))

    However, we did not use this in any examples in class, so I'm pretty stuck.

    4. http://i.imgur.com/XPbm2.png"

    This is an online homework problem that was due about a week and a half ago. As you can see, I got stuck, and couldn't really figure it out. This seems like it would go a long way to help me figure this business out.


    I feel like I'm asking a lot of you guys at this point, so this'll be the last one I put up for now. Hopefully this will be enough to supplement my study :) It is apparently pretty easy, so that's good. I figured out the first part, but I couldn't figure out the second for the life of me.

    Thank you in advance!

    You guys have seriously never let me down. I know this will probably be easy for a lot of you, but this is my first time experiencing this stuff, and I'm trying to wrap my head around it. Thank you so much :) If you need clarification on ANYTHING, just ask!

    Sorry if this is a bit much.
    Last edited by a moderator: Apr 26, 2017
  2. jcsd
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