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Homework Help: Two EE questions - nodal analysis and circuit modeling

  1. Apr 11, 2014 #1
    Two EE questions -- nodal analysis and circuit modeling

    1. The problem statement, all variables and given/known data
    Photo 11-4-14 1 16 49 pm.jpg
    Photo 11-4-14 1 17 20 pm.jpg
    Photo 11-4-14 1 17 49 pm.jpg

    2. Relevant equations
    I was given R1, R2, and Iα(t), the time for the switch to closed (50μs<=t<=100μs)
    The question is asking about the energy associated with 2 sources and dissipated in R1, R2.

    3. The attempt at a solution
    For the 1st question, part should be ok for me. But i have no idea on how to start the part b).

    take integration for Iα(t) (since Iα(t)=da/dt) , then obtain a i=(-1/5)e^-500t
    but what value for the integrals? integrated from t = 50μs to 100μs? or from t = 0μs to 200μs ? then "I" can be found?
    E=Pt, = (IV)t. What value of t will be used here for each source and resistor to find the energy?

    For Q4, totally no idea on it. It is about the topic of diode?
  2. jcsd
  3. Apr 11, 2014 #2


    User Avatar

    Staff: Mentor

    In Q3, once you know the voltage & current associated with each element, the product of the two will be its power. Integration of power leads to energy.

    Q4 does call for a diode arrangement.
  4. Apr 11, 2014 #3

    Thanks so much for your reply.
    After calculation I find :
    I1: 2/7(ia)
    I2: 5/7(ia)
    I3= -5/7 ia
    V1= 20/7 (ia)
    V2= 100/7 ia

    Then , power of R1 = v1 i1
    I get 4000/49 e^-5000t
    Then I need to integral it to find energy but what should I set for the upper and Lower intergal? Since the switch is closed for 50u<T< 100u, but the question asks for the energy for 0<T<200u, so which range should I use ?
    And why?
  5. Apr 11, 2014 #4


    User Avatar

    Staff: Mentor

    You will have to evaluate the integral in pieces, until you have covered the full interval. The circuit changes when R2 is shorted out.
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