1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: I need a 2nd opinion on a circuit analysis answer

  1. Feb 1, 2007 #1
    1. The problem statement, all variables and given/known data
    Use superposition to find [tex]I_L[/tex]


    2. Relevant equations
    -The overall current is the sum of the currents supplied by each source individually

    3. The attempt at a solution

    for source B only (current source is open):

    ---ohm's law: [tex]\frac{100V}{15k + j25k} = 3.43mA( -59.04 degrees)[/tex]

    Here is where I'm having to defend my answer:

    for souce A only (voltage source shorted):

    ---since there is no phase shift labeled, it should be assumed that the current source can be treated as a DC source, treating the inductor as a short, making [tex]I_L[/tex] -20mA

    Is this wrong? Should I treat a current source as AC if it has other AC sources involved in the circuit? If this is so I'd just do a current divider to get [tex]I_L[/tex]
  2. jcsd
  3. Feb 1, 2007 #2
    I believe the question would have to be more explicit in defining whether the 20mA is an ac or dc source.
  4. Feb 1, 2007 #3
    Well no extra information was given than what's shown, yet I got it marked wrong for treating it one way over the other...

    Am I going to have to call out my professor again? I'm losing all my brownie points because of his dud test questions
  5. Feb 1, 2007 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I would imagine the "+" by the current source tells you it's a dc source.
  6. Feb 1, 2007 #5
    Well actually he graded it as it being an AC source.
  7. Feb 1, 2007 #6
    If you are answering the question as part of a test and if the professor is not available to clarify, then I guess you could always offer both solutions, one for each case.

    Failing that, then I suppose at the last resort you should treat it as ac since, as you said, there are other ac sources in the circuit.
  8. Feb 1, 2007 #7
    Hmm... but a "+" sign also appears beside the dependent voltage source and that one is clearly ac.
  9. Feb 1, 2007 #8


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Haha! Yes, it does. What on earth is it for, then?
  10. Feb 1, 2007 #9
    For the dependent voltage source, I would imagine that it is there to indicate where the +terminal is (neccesary in defining voltage). I don't know why it appears beside the current source.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook