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Homework Help: Output Voltage

  1. Mar 31, 2010 #1
    1. The problem statement, all variables and given/known data
    The circuit below combines variable resistors R1 and R2 with fixed resistors to make a complex voltage divider.

    If the switch is closed and R1=8000ohms and R2 is 5000ohms, what is the output voltage.


    2. Relevant equations
    V=IR


    3. The attempt at a solution
    the equivalent resistor of 1000 and 8000 is 3077 and the current through that is 0.03A. Im not sure what to do after that.
     

    Attached Files:

  2. jcsd
  3. Mar 31, 2010 #2

    Integral

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    You do not need to know the resistor values. Look at the diagram and think about it.
     
  4. Mar 31, 2010 #3
    um..a current goes through the first parallel part and then splits up when entering the next parallel part...
     
  5. Mar 31, 2010 #4
    not sure whrre to look
     
  6. Apr 1, 2010 #5
    ...???
     
  7. Apr 1, 2010 #6
    is it 0 because the current will go through the part of the circuit where there's the least resistance?
     
  8. Apr 1, 2010 #7
    Yes. All the current will go through the closed switch and none of it through the two resistances that are parallel with it.
    The output voltage is I_switch * R_switch and R_switch = 0.

    Although it's not relevant for the answer, the equivalent resistance of 1000 ohm and
    8000 ohm resistances in parallel is not 3077 ohm. The equivalent resistance of 2 parallel resistances should be smaller then the smallest of the resistances.
     
  9. Apr 1, 2010 #8
    okay thnx, my mistake the equivalent resistor should be 889ohms. Question, why does the current only go through the switch? Is it because its the easiest path?
     
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