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Homework Help: Parallel Current Sources

  1. Jan 28, 2014 #1
    1. The problem statement, all variables and given/known data
    Determine the voltage v in the circuit after first replacing the three sources with a single equivalent source.


    2. Relevant equations

    3. The attempt at a solution
    I've arrived at the correct answer. I just want to make sure I did it correctly and didn't just get lucky.
    I know that the equivalent current source will be +10A upward. So I redraw it with the source in the middle and the resistors on either side. The current splits evenly, and half flows to the left resistor, while half flows to the right resistor. This gives v=iR=(5)(10)=50V.

    My question: Does the current split equally in that situation regardless of the elements and their values on either side?

    For example, if there were a 10Ω resistor on the left and a 3V battery on the right, would half the middle current flow through the resistor and half flow through the battery?
  2. jcsd
  3. Jan 28, 2014 #2


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    you solved it correctly with good reasoning.

    to answer your question. You cannot put to ideal sources in parallel with each other. an ideal voltage source (a battery) has 0 resistance. that means 100% of the current would flow throw the battery. If you tried to implement that in the real world, you would risk blowing the battery.
  4. Jan 28, 2014 #3
    Ah, I see. Thanks for the confirmation. 'Preciate it.
  5. Jan 28, 2014 #4


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    No, that is not correct. An ideal current source and an ideal voltage source CAN be put in parallel, provided that there is something (like a resistor) that allows the total circuit to not require either of those sources to have a value other than what is specified.

    For example, a circuit consisting of a 10volt source, a 10amp source, and a 10ohm resistor, all in parallel, would be no problem, since it would just require that that 10volt source have 9 amps flowing through it the "wrong" way. Ideal sources don't care. A real-world source would of course be a different story.
  6. Jan 28, 2014 #5


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    the fact that there is something like a resistor in between the 2 sources would mean the sources are not in parallel.....

    It is NEVER good practice to drive a load with multiple sources unless proper control circuitry is in place.

    that being said, yes in an ideal case 9 amps would go through the voltage supply and 1 amp would go through the resistor. my mistake... note please never try to do that in lab
    Last edited: Jan 28, 2014
  7. Jan 28, 2014 #6


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    No, it does not. I don't see how you get that. Think of exactly the configuration shown in the OP. There are 3 current sources in parallel. How can you say they are not? Just remove one of them and replace one of the remaining ones with a voltage source and you have exactly the configuration I am talking about.

    Agreed, but that is NOT a limitation that is place on ideal circuits in introductory courses.
  8. Jan 28, 2014 #7


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    the definition of parallel is that 2 components share the same nodes. if you put a limiting resistor in series with one or both of the power sources then that implies that the power sources are not in parallel.

    note op has all required information. i am no longer replying to this thread
  9. Jan 28, 2014 #8


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    Yes, if you were to put something in series with any element, it wouldn't be parallel, but that's not the case I'm describing. You clearly don't understand what you are saying.
  10. Jan 28, 2014 #9


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    Hi kandrew! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]
    The current leaving a node like you have divides up according to the resistance ratio. More correctly, it splits according to the inverse of the resistance ratio.

    So if the two resistors are of equal value, the current divides equally. If one resistor is x3 that of the other, then that larger resistor carries precisely one-third of what the other carries. You can see this from Ohm's Law, once you recognize that the voltage across the parallel components is common, i.e., it's the same for each.

    In the example you have conjured up, the current source puts out its constant 10A of current, the 3v ideal voltage source mantains its terminal voltage at 3v come hell or highwater, and the 10Ω resistor carries 0.300A because it has 3 volts across it. The remainder of the current source's output goes through the voltage source without ill effect. Ideal voltage sources can give and take any amount of current you wish, except infinite current.

    I think that answers your question?
    Last edited by a moderator: May 6, 2017
  11. Jan 28, 2014 #10
    That clears it all up. Thanks a bunch, Nascent. :)
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