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Homework Help: Help understanding current source

  1. Mar 24, 2014 #1
    1. The problem statement, all variables and given/known data

    This is a really basic problem, just a closed circuit with a current source and a resistor. We are meant to give the voltage and the current across the resistor in terms of the current from the source and the resistance of the resistor. (From the diagram, express Vr and Ir in terms of I1 and R). I have never encountered a current source before and am having trouble learning to deal with it.

    2. Relevant equations

    Ohm's law: V = IR
    Kirchoff's Laws: sum of voltages = 0, and current into a node = current out

    3. The attempt at a solution

    My thought was to say that I1 = Vs/Rs, so Vs = I1/Rs = Vr = Ir/R.
    But then I don't know how to solve these for Ir and Vr in terms of I1 and R.

    Any help understanding the concept would be greatly appreciated! Thanks :)

    Attached Files:

  2. jcsd
  3. Mar 24, 2014 #2


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    Staff: Mentor

    Welcome to the PF.

    There is nothing tricky here. The current source sets the series current that flows around the circuit. The resistor value is what determines what voltage is dropped across it, just based on Ohm's Law V=IR.
  4. Mar 24, 2014 #3


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    Science Advisor
    Homework Helper

    You are probably familiar with voltage sources, even if you didn't use the name. You can think of a voltage source as being like a battery. It always has the same voltage across its terminals, but the current depends on the circuit that is connected to it.

    A current source is the opposite way round. It always produces the same current, and the voltage across it depends on the rest of the circuit.

    If you increase the resistance of a circuit connected to a voltage source, voltage stays the same but the current decreases. If you increase the resistance connected to a current source, the current stays he same but the voltage increases.

    Unlike a battery, it doesn't make any sense to have a current source "not connected to anything", because its output current has to go somewhere.

    In real life you have to make a current source from a circuit containing transistors or op-amps, so the idea is a bit more abstract than a battery. But current and voltage sources are both very useful ideas in circuit analysis.

    Note, all the above is a bit over-simplified. For example both current and voltage sources can produce a variable output that is related to something else in the circuit. But the important thing is to understand the basic idea before making it more complicated.
    Last edited: Mar 24, 2014
  5. Mar 24, 2014 #4
    Thanks! So in this case Ir = I1?
  6. Mar 24, 2014 #5


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    Staff: Mentor

    Yep! :smile:
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