Ideal sources problem

Discussion in 'Homework Help' started by n_adam, Mar 2, 2014.

  1. n_adam

    Thread Starter New Member

    Mar 1, 2014
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    Hi guys,

    I need some help with these two problems. I think i got them right, but I would need someone to confirm that I am correct.

    Thx a lot!

    2.1 - I think the circuit is valid. When I go around the circuit in the direction of the current there is a 40 volt voltage rise, then a 10 volt drop and a 50 volt drop. So across the current source there is a 20 volt voltage rise. The power delivered is 300 watts, and the power absorbed is also 300 watts.

    2.3 - I think this interconnection is also valid. Going around the circuit in the direction of the current there is a 20 volt voltage drop, then a 15 volt voltage rise, so across the current source there has to be a 5 volt rise.

    b. 20 volt source absorbs power
    15 volt source develops power
    5 amp source develops power
    c. Power absorbed is 100 watts.
    Power developed is 100 watts
    d. The circuit is invalid because there is only voltage rise in the circuit.
     
  2. WBahn

    Moderator

    Mar 31, 2012
    17,743
    4,790
    Your answer and reasoning to 2.3d is incorrect. In order for KVL to be satisfied, what does the voltage across the 5A source need to be?

    For some reason I can't open your attachment for 2.1, so I'm not sure about that. But your description sounds right, at least if the current source is 5A.
     
  3. n_adam

    Thread Starter New Member

    Mar 1, 2014
    19
    0
    2.3d - So there has to be a 35 volt voltage drop across the current source so that the circuit satisfies KVL. In order for that to happen the top side of the current source has to be positive and the bottom negative. But in that case wouldn't the current flow in the opposite direction?
     
  4. WBahn

    Moderator

    Mar 31, 2012
    17,743
    4,790
    You didn't seem to have a problem with current flowing the "wrong" direction into a voltage source -- you recognized that this just meant that the voltage source was absorbing power instead of delivering it. What's different about a current source in which the voltage is the "wrong" polarity?
     
  5. n_adam

    Thread Starter New Member

    Mar 1, 2014
    19
    0
    What confuses me here is that the current direction is given and I thought that means that the top part of the current source has to be negative and the button part positive.
     
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