Dazed and confused..

Discussion in 'Homework Help' started by TwoPlusTwo, Feb 16, 2011.

  1. TwoPlusTwo

    Thread Starter Member

    Oct 14, 2010
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    Hi, I'm having trouble understanding this problem.

    The voltage Va for the network below is -1 V. If it suddenly jumped to 20 V, what could have happened to the circuit structure?

    Since the chapter introduces short and open circuits, I'm assuming it has something to do with that but I don't really understand the drawing. Is this supposed to be a branch of some larger network with some unknowns that I have to determine first?
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    No, you do not need to determine any unknown.
    You need to think what could have happened to 4K and 3K resistors to get Va equal 20V
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    Label the resistors Ra, Rb, And Rc. What happens to the voltage at point a when any one of them changes? Do you know how a voltage divider works? That the voltage sources will cause currents in the resistors, and that those currents can be calculated to find the voltage dropped over each resistor?
     
  4. TwoPlusTwo

    Thread Starter Member

    Oct 14, 2010
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    Ok, how is this?

    Total voltage=20 V - 4 V=16 V

    Current=16 V/(4+3+1)kΩ=2mA

    U1=2mA*4kΩ=8 V

    U2=2mA*3kΩ=6 V

    U3=2mA*1kΩ=2 V
     
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Wrong
    Total voltage is equal
    20V - (-4V) = 24V
     
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  6. TwoPlusTwo

    Thread Starter Member

    Oct 14, 2010
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    Ok. I fixed it. So is the rest of it ok?
     
  7. TwoPlusTwo

    Thread Starter Member

    Oct 14, 2010
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    And why exactly is Va = - 1 V?
     
  8. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Yes, looks ok
     
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  9. TwoPlusTwo

    Thread Starter Member

    Oct 14, 2010
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    I still don't understand why Va has to be -1 V in this configuration. But if I were to guess at a solution to the original problem it would be that Va has jumped from -1 V to 20 V because the 4k and 3k resistors have been shorted out. Does that make sense?
     
  10. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Becaues Va voltage in not a voltage drop across 1K resistor.
    Va is a voltage between Va and GND.

    [​IMG]


    Yes, you right
     
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  11. TwoPlusTwo

    Thread Starter Member

    Oct 14, 2010
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    Great! I think I understand. Va is given by

    - 4 V + U3 = - 4 + 3 V = - 1 V
     
  12. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Great job

    [​IMG]
     
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  13. BillB3857

    Senior Member

    Feb 28, 2009
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    Overly thought out problem. What caused the voltage drops? What if the current somehow became zero (series circuit)
     
  14. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You also need to consider a fault in the 1k resistor.
     
  15. addousas

    New Member

    Jul 15, 2010
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    Twoplustwo dont let the change of schematics throw you off. Think of this as a branch of huge circuit that you are inspecting in a lab. You have a branch that has these resistor in series and the voltages you have (20v,-1v,-4v) you have attained using a voltmeter.
    the voltage drop over (4k+3k) is simply 20-(-1) =21v =>21v/(7k)=3mA
    the voltage drop over (4k+3k+1k) is simply 20-(-4)=24v=>24v/8k=3mA
     
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