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Homework Help: Open circuit voltage

  1. Mar 29, 2010 #1
    VS = 10 volts

    R1 = 165 ohms
    R2 = 146 ohms
    R3 = 294 ohms

    http://users.on.net/~rohanlal/elec.jpg [Broken]

    what is the value of the open circuit voltage, VOC, in volts ?

    my attempt:
    I'm not sure where to begin as I don't know whether r2 and r3 are in series or parallel.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Mar 29, 2010 #2
    Well, to determine whether or not resistors are in parallel with each other is to ask yourself, "Do the two resistors share the same node pair?" If they do, they are parallel. Does this get you anywhere?
  4. Mar 29, 2010 #3
    i dont think it does. do those circles on the right side of the diagram represent ground? so is current passing through R2 and R3 even when no open circuit is connected?
  5. Mar 29, 2010 #4
    The circles on the right side do not symbolize ground. In circuit theory, I suppose you can place the ground where ever it is easier to solve the given problem. In this case, you can easily determine the open circuit voltage by implementing either mesh method or nodal method. I assume you have learned these techniques by now if you are receiving a problem such as this...
  6. Mar 29, 2010 #5
    Let me know what you get when you determine the open circuit, and I will compare it with what I received.
  7. Mar 29, 2010 #6


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    Resistors with no current flowing in them have the same voltage on each side of them because there cannot be a voltage drop across them.

    So, what is the voltage on the left side of the two open circuit resistors? This is also the voltage on the right side of these resistors.
  8. Mar 29, 2010 #7
    i never knew that you can have a potential difference between two points without there being a current flowing through those points.
  9. Mar 29, 2010 #8


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    The voltage across R1 is the same as Vs and this is the same as Voc because there cannot be a voltage across R2 or R3.
  10. Mar 31, 2010 #9
    i guess you could just think of the two wires that have r2 and r3 as an extension of the cables of a multimeter couldn't you?

    if i was to take two extremely large resistors, and tape one end of one of the resistors to the negative cable of a multimeter and do likewise with the other resistor and the positive cable of a multimeter then the voltage that would show on the multimeter would be no different to what it was before i taped those resistors to the multimeter's cables would it?

    Edit: I just realized that couldn't be the case because that would be no different from measuring the voltage drop across R1.
    Last edited: Mar 31, 2010
  11. Mar 31, 2010 #10
    if you analyze the loop where the open circuit voltage exists, you get

    Kirchoff Voltage Law
    (1) -VS + VR2 + VR1 + VOC = 0


    What is the voltage across R2 (VR2) and R1 (VR1). Since no current passes through R1 and R2, as said before, there is no voltage drop across them.

    Therefore solve equation 1, and you will get your simple answer.
  12. Apr 1, 2010 #11


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    Yes, as long as the multimeter did not draw any current, the actual values of the resistors in this circuit have no effect

    Kirchoff is a bit of an overkill in this case.
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