1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Kirchhoff Circuit: Current, Resistors, Voltage

  1. Apr 4, 2004 #1
    Circuit Current

    I have the circuit below (also attached as an image):
    Note:
    ^^^^ & <<<< = resistors
    [-]=power supply
    ...=just space
    (-) & (+) indicates + & - side of battery
    A,B represent the junctions

    |-----^^^^------A-----^^^^------|
    |........................|........................|
    <.......................<.......................(-)
    <.......................<......................[-]
    <.......................<.......................(+)
    <.......................<.......................|
    |........................|........................|
    |----(+)[-](-)----B-----^^^^------|

    R1=100 ohm (center far left)
    R2=560 ohm (top left)
    R3=330 ohm (center)
    R4=300 ohm (top right)
    R5=100 ohm (bottom right)
    Va=1.5 V (bottom left)
    Vb=1.6 V (center far right)
    (This is also attached as an image. Sorry for the lack of quality, working quickly in paint.)

    Okay, now that we've got the setup, I need help in determining my equations, well for the junction rule at least. My voltage equations are as follows:

    Va-I1R1-I2R2-I3R3 = 0 (left loop)
    Vb-I4R4-I3R3-I5R5 = 0 (right loop)
    Va-I1R1-I2R2-I4R4+Vb-I5R5 = 0 (entire loop)

    Okay, for the junction rules. I need two of them to have five equ for my five unknowns...which are the I.
    Would they be,
    I1+I2+I4=I3 (junction A)
    I3=I5 (junction B)
    Current in = Current out

    Are these equations correct? I'm trying to find the current for each resistor and then go back and find the voltage. The latter is no problem, it's just determining that my equations are correct for current I.

    Thank you for your assistance and have a nice day!
    Christina
     

    Attached Files:

    Last edited: Apr 4, 2004
  2. jcsd
  3. Apr 4, 2004 #2
    Verify Equations

    Please see attached image for circuit.

    I have labeled all my current directions. For each Rn, that is the corresponding In. (ie. I1 goes w/R1)

    I would like for you to verify my equations below for solving this circuit using Kirchhoff's Rules of voltage and current.

    Junction Rules (Current In = Current out):
    I2 = I3 + I4
    I3 + I5 = I1 + I2 (OR) I3 + I5 = I ???

    Voltage Rules ([delta]V=0):
    Va-I1R1-I2R2-I3R3 = 0 (left loop)
    Vb-I4R4-I3R3-I5R5 = 0 (right loop)
    Va-I1R1-I2R2-I4R4+Vb-I5R5 = 0 (entire loop)

    Thank you.
     

    Attached Files:

  4. Apr 4, 2004 #3

    Janitor

    User Avatar
    Science Advisor

    I presume I3 is the current in the middle wire.

    If so, doesn't I3+I5=I1? (Instead of "I3 + I5 = I1 + I2.")
     
  5. Apr 4, 2004 #4

    Janitor

    User Avatar
    Science Advisor

    Your voltage equations look correct to me.

    You can always eliminate some of the current variables by noting that I1=I2, etc.
     
  6. Apr 4, 2004 #5
    Would the equation be I3 + I5 = I (no subscript) or I3 + I5 = I1?
     
  7. Apr 4, 2004 #6

    Janitor

    User Avatar
    Science Advisor

    If I1 is what one calls the current through R1, then I3 + I5 = I1.
     
  8. Apr 4, 2004 #7
    Ohhh, yeah. I knew that. :) Geez I am good at confusing myself.

    Thank you for your help.
     
  9. Apr 5, 2004 #8
    kashmirekat,

    Since I1 = I2 and I4 = I5, you really just have 3 different currents. Call them I12, I45 and I3. Then, the way you've defined directions, I12 = I3 + I45

    Even as I'm writing this it looks like others are giving you the same help. Good luck!
     
  10. Apr 6, 2004 #9
    Your making so many unnecessary unknowns
     
  11. Apr 6, 2004 #10
    I hope this explains your question.


    This part got cut off

    Ex) If you want to know the purple current, the equation is:

    I1 - I2 = I

    And since you already have I1 and I2 from earlier, I can be found easily.

    I hope this helps :)
     

    Attached Files:

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?