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Homework Help: Circuit problem, prob really easy, please check

  1. Jan 13, 2008 #1
    1. Find the missing values for the given circuit.

    2. the given circuit is attached..
    View attachment phys.bmp

    3. soo, so far, i used ohms laws to get the total resistance, Rt = 1.4 ohms. can someone please tell me if this is right?
  2. jcsd
  3. Jan 13, 2008 #2
    are all of them in parallel here ?
  4. Jan 14, 2008 #3
    The resistance of the whole circuit is indeed 1.4 ohm. The resistances are NOT all in parallel.
    In general The resistance of every network of resistances can be computed by using kirchhofss laws and ohms law to get simultaneous equations for all the currents in the circuit.
    You can use kirchhofs law for every 3 or more wire connection
    I_1 = I_3 + i_4 for example
    and ohms law and the fact that the sum of all voltages differences in a closed loop
    is 0
    V = R_1*I_1 + R_4*I_4
    V = R_1*I_1 + R_3*I_3+R_5*I_5
    (use always the same direction for I_3)
    If you use enough of those you can get 5 simultaneous equations for I_1 ... I_5

    There's a faster way, if you've covered Thevenins Theorem.


    draw another 14V voltage source to the right of R2 and R5 and parallel to the first voltage source. This won't change any currents through the resistances.
    Then cut the wire between the tops of R1 and R2. Both sides of the cut will be at the same voltage before and after, so that won't change any currents either.
    Then replace the left voltage source, R1 and R4 with their thevenin equivalent and do the
    same with the voltage source on the right, R2 and R5.
    What remains is a simple series circuit.
  5. Jan 15, 2008 #4
    thank youu.
    we havent covered thevenin yet but wat u said made sense =)
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