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!

Finding the Potential (Kirchoff's Law)

  1. Feb 24, 2012 #1
    1. The problem statement, all variables and given/known data
    http://puu.sh/i8Bg


    2. Relevant equations
    V=IR
    ƩV = 0

    3. The attempt at a solution
    Starting behind the left EMF source I sum the potential changes using Kirchoff's Voltage Law
    150V - R2I - 50V - R1I = 0
    Solving for I, I get 20Amps

    However I know this can't be right because the potential at point Q is 97V
    and after traversing the circuit, the potential according ot the calculated I is 110 Volts..

    Can someone perhaps show me step-by-step to do problems like these?
     
  2. jcsd
  3. Feb 24, 2012 #2

    PeterO

    User Avatar
    Homework Helper

    Note that there is no Earth connected to this circuit, so no point on the circuit is necessarily at 0 V.

    If the current flows anti-clockwise, the potential goes up 150 as you pass through the left battery, drop 40 or 60 through the bottom resistor [having trouble which resistor is which - one you start to reply the picture of the circuit is gone], drop another 50 going "backwards" through the ideal battery on the right, then another 60 or 40 through the top resistor.
    So I think P should be at -13 or 7 depending which resistor was the 2Ω and which was the 3Ω
     
  4. Feb 24, 2012 #3
    Okay I understand everything but the very end. What calculations did you do to get those two numbers?
     
  5. Feb 24, 2012 #4
    Using loop laws you can get the above answer ! join PQ by arbitrary line and can calculate the voltage difference between points P and Q using Loop law.
     
  6. Feb 24, 2012 #5

    PeterO

    User Avatar
    Homework Helper

    As I said, as soon as I start answering the post, I no longer have access to the circuit diagram. I remembered that the 150V battery was on the left and the 50V battery was on the right. I didn't remember which resistor was on the top and which was on the bottom. I do know there is a 60v drop across the 3Ω resistor and 40V drop across the 2Ω. Depending which one was top/bottom determines whether Q is at -13 [110 below], or 7 [90 below] point Q.
    I figured you would be able to work out which one - as you ahve the original circuit, and a greater need to solve the problem.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Finding the Potential (Kirchoff's Law)
  1. Kirchoffs Laws (Replies: 6)

Loading...