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Homework Help: Power dissipated and supplied (Circuit Diagram)

  1. Aug 30, 2008 #1
    1. The problem statement, all variables and given/known data

    (See attachment for diagram)

    Known Values:
    R1 = 5
    R2 = 15
    R3 = 25
    R4 = 5
    V1 = 40
    V2 = 20
    I1 = 71/7
    I2 = 25/7
    I3 = 4

    1) Find the power dissipated in each resistor

    2) Find the power associated with each voltage source including the independent
    source and specify whether it is delivered or dissipated.

    3) Verify that the total delivered power equal the total dissipated power

    2. Relevant equations

    3. The attempt at a solution

    Power Dissipated:

    (In the resistors)
    R1*(I1- I3)2= 5(71/7-4)2 = 188.7
    R2*(I1- I2)2=15(71/7-25/7)2 = 647.8
    R3*(I3- I2)2=25(4-25/7)2 = 4.59
    R4*(I3)2=5(4)2= 80

    (By the voltage source in the middle of the branch)

    Total Power Dissipated:1239.9

    Power Supplied:
    V1= 40(I1) = 405.7
    V2= 20(I2) = 71.4
    25I2* I1=905.6

    Total Power Supplied: 1382.7

    What am I doing wrong here in calculating the power supplied/absorbed?
  2. jcsd
  3. Aug 30, 2008 #2

  4. Aug 31, 2008 #3


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    HINT: What's the current through R2 / the dependent power supply?
  5. Aug 31, 2008 #4

    The Electrician

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    Gold Member

    You haven't solved the network correctly. The currents are:

    I1 = 193/7
    I2 = 79/7
    I3 = 12

    You probably made an error in setting up the equations to solve the network, but since you didn't show them, I can't tell you exactly where your error is.
  6. Sep 5, 2008 #5
    Thanks for the above responses - It was a tricky one for me...

    Hope nobody minds, but I've got another question, but I don't want to congest the forum with my homework threads, so I'll just post it in here.

    1. The problem statement, all variables and given/known data
    Find the voltage across Vab (This is for a thevenins circuit).

    Can I solve this using source conversion? (Its the technique that i'm most unfamiliar with, so I decided to try and solve this problem using it...I could have easily used nodal or mesh, I think)


    2. Relevant equations

    3. The attempt at a solution
    My solution:


    Answer in the book is 120V... I also want to check if I can actually do the first step of my solution..combing the resistors and the currents?

    Attached Files:

  7. Sep 5, 2008 #6

    The Electrician

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    Gold Member

    I think the book is wrong. I get 150 volts also, by a different method.
  8. Sep 5, 2008 #7


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    Homework Helper

    I got 150V as well. I'm assuming the nodal voltage at Va is the same as that above the 60k resistor since no current passes through the 15k resistor.
  9. Sep 6, 2008 #8
    Awesome guys..thanks for the responses...I was looking at this problem in complete bewilderment after the book gave me 120V.

    I'm actually really curious as whether I can do the first step of the solution (ie, the add resistors and current sources that are in parallel like that..)?
  10. Sep 6, 2008 #9


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    Homework Helper

    Well you managed to get the same answer, so it is valid. This is known as source transformation.

    I didn't use any source transformation in my working, by the way.
  11. Sep 6, 2008 #10
    I was wondering, why don't we have to define a ground? I am using a program Pspice to check my answers to similar problems and it always makes me define a ground. Yet, circuits like a flashlight obviously don't need a ground node. Perhaps I'm doing something wrong on the program.
  12. Sep 6, 2008 #11


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    Homework Helper

    A ground node is just a reference node from which all other node voltages are evaluated. In circuits, only potential differences and not absolute potentials matter.
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