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Homework Help: Current and Power in DC Circuits

  1. Mar 22, 2010 #1
    I have two problems that have been bothering me, which I just cannot seem to figure out. Both have to do with current in a DC circuit with multiple resistors.

    The first one looks like this:
    12 V 4 ohms
    | |
    | 18 V 12 ohms |
    | 8 ohms |

    I am trying to find the current through the 4 ohm resistor, but I am not quite sure how to go about doing so. I tried subtracting the voltages since they are in parallel and finding the equivalent resistance and then using Ohm's Law, but that was unsuccessful. I have a feeling it might intend for me to use Kirchoff's rules, so I then tried doing so from junction a to junction b. I got Vab=16-4I1=18-12I2=8(I1-I2 (I1 is from b to a around the top, where the 16 V and 4 ohm resistor is, I2 is from b to a directly across the middle, and I3 is from b to a along the bottom, where the 8 ohm resistor is), but just didn't know where to go from there. Would the current be I1, or something else?

    The second problem is quite similar. In this one, I am trying to find the power given off my one of the 10 ohm resistors; I think my problem here is again the current, as power=I^2 * R.
    It looks like this:
    12 V
    | |
    | |
    | 2 ohms |--\/\----| <----20 ohms
    |---\/\------|---\/\---| <----10 ohms
    |---\/\---| <----10 ohms
    So basically I am trying to find the current through one of the 10 ohm resistors so I can use it to find power, which is I^2 * R.

    I would really appreciate some help on this, as I am studying for a test, and these two problems have been troubling me for much too long.

  2. jcsd
  3. Mar 22, 2010 #2
    Somehow those diagrams turned out horribly. I've attached some better ones; "Untitled" is corresponds to the first problem and "Untitled 2" corresponds to the second.


    Attached Files:

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