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Homework Help: Problem with kirchoffs laws

  1. Jan 6, 2008 #1
    Hello to all,
    I have been having problems to solve the exercise below,I tried to apply Kirchoff's current law at every node but still couldn't find the answer.The exercise is about finding the values of i(a),i(b)and i(c).
    I hope I will have my answers here from you.

    Attached Files:

  2. jcsd
  3. Jan 6, 2008 #2
    Just Kirchhofs laws aren't enough. You'll need Ohms law at some point.

    If you assign 0 volts to the wire at the bottom of the circuit you'll see that there
    is actually only one point in the circuit that is at a different voltage. Call this voltage
    V. If you knew V it would be easy to find all the currents in the circuit. You can
    eliminate V from these equations to get a system of equations for I_a,I_b and I_c
  4. Jan 7, 2008 #3
    thanks for your answer,it is a single pair node circuit so the voltage across all the elements is the same.I know what I did wrong(and kept repeating the same mistake),it was in assigning the current's direction in the 60ohms resistor.
    Is there an easy way to assign the direction of the current in each element without making mistakes?
  5. Jan 7, 2008 #4
    Generally No. However it's not really necessary. If you made the wrong assumption the
    current should turn out negative
  6. Jan 8, 2008 #5
    Thanks again,yes it makes sense now.But in the above circuit,is the voltage across the 60ohms resistor the same as the voltage across the 600ohms?Do all the elements in this circuit have the same voltage?
  7. Jan 8, 2008 #6
    I thought that was what you wrote in your previous post. You can of course easily check for yourself. if one end of circuit element 1 is connected with one end of circuit element 2, and the other end of circuit element 1 is connected to the other end of circuit element 2, they will have the same voltage.

    The way the circuit is drawn makes it not very obvious that this is the case.
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