Effect of open on parallel circuit

Discussion in 'Homework Help' started by Circuits123, Oct 11, 2014.

  1. Circuits123

    Thread Starter New Member

    Dec 7, 2012
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    I'm trying to understand the answer to 2 questions involving this circuit (attached)
    Q. 1 and the answer say: if R3 opens the voltage across it will decrease.
    Q. 2 and the answer say: if R1 opens the voltage across it will stay the same.

    Why doesn't the voltage decrease for both of them? My understanding is that this is a simple parallel circuit with 3 branches, so shouldn't an open in one resister be the same as any other?
     
  2. shteii01

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    Feb 19, 2010
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    What is it?
     
  3. JoeJester

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    Post a pic of the question...
     
  4. Circuits123

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    I attached a screenshot with my question. Can anyone else open it? The questions are what will happen to the voltage if there is an open in R1 or R3.
     
  5. shteii01

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    Which voltage?
     
  6. JoeJester

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    What does your textbook say about voltages and parallel circuits?

    Your screen shot and the answers are not matching up. That is why i wanted to see the question as posed by your instructor.

    There is a serious disconnect between your words and your screen shot.
     
  7. Circuits123

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    Dec 7, 2012
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    OK. Here's another try. I've uploaded a pdf of my textbook. The questions I'm stuck on are #6 and 7 on the first page of the pdf. The diagram the questions refer to is at the end of the 2nd page.
     
  8. JoeJester

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    Ok....

    Here is your question ... again ...

    What does your textbook say about the answer to question 5.

    In question 6, what do you think the voltage acrossed R3 is when the resistor is OPEN? When you hang a meter acrossed R3, is there any current flowing in the meter?
     
  9. Jony130

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  10. shteii01

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    So. You effed up the original circuit? All I see are 100 Ohm(?) resistors in there. The pic that Joe posted got 680 Ohm and 1000 Ohm resistors. Which pic is right?
     
  11. JoeJester

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    Mine came from the pdf he posted.
     
  12. Circuits123

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    Dec 7, 2012
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    The textbook says the answer to #5 is (c) - the current stays the same. I understand that - the voltage is the same across all parallel branches, so if the resistance in a branch stays the same and the voltage remains the same, the current must stay the same. But I still don't understand the answers to #6 (answer b) and #7 (answer c).

    The resister values shouldn't affect whether the voltage changes right? And isn't R3 just another branch like R1 and R2, regardless of how it's drawn? So I'm clearly missing something.
     
  13. shteii01

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    Apply Ohm's Law.
     
  14. studiot

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    Surely shteii's question "which voltage?" has relevence here?

    Why should it matter to an ideal battery voltage which branch you open?
     
  15. Circuits123

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    Could you be more specific? I know what Ohm's Law is. But...
     
  16. JoeJester

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    You know ohms law. You KNOW the voltage across parallel branches remain the same. You know you can use Kirchhoff's current law to find all the currents in each branch and the total current.

    I don't care what your book has for an answer ... what do YOU think the answers are for those questions. You should have no doubt as to your reasoning for the answers you choose.

    Books have been known to have incorrect answers. The publishers website should have an information concerning all known corrections.

    Attached is a table for you to fill out so we know you know what's happening in that circuit.

    Then consider this question:

    What parameter(s) changes when
    a. R3 Opens
    b. R2 Opens
    c. R1 Opens
     
  17. Circuits123

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    Dec 7, 2012
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    I filled-out the table (attached).
    If R(1, 2, OR 3) opens, R goes to infinity [or at least to the resistance for air]. That makes I for that branch go down to zero. So Ohm's law says E must be zero too (infinity times zero = zero). Total voltage and the voltage in the other branches remains 100 because voltage is the same in every branch for a parallel circuit. Total current goes down, but the current in the other 2 branches remains the same.

    Does that sound right? If so, that would mean the answer to #7 in my book is wrong.
     
  18. JoeJester

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    I tend to think the voltage is the same between ground and the hot side of the voltage source when the resistors OPEN. I see your point about multiplying by zero or even the impossible division by zero, but when you go into a lab exercise to "prove" it, you connect a meter across the open resistor and you read the source voltage, because your DMM has an impedance, typically 10 or so Megaohms, and completes the circuit to allow some current flow.

    Did you notice the math doesn't work out ... for instance, the current in the 680 ohm resistor is not 100 V/680 ohms. 666.67 ohms is about 2 percent low, but that could be a discrepancy found in the lab.

    Zero current flow indicates no resistance ... not zero ohms resistance. It is typical to identify zero current flow condition as an OPEN.

    Your book's conflicting answers between question 6 and 7 certainly did not do you any favors.

    My choices would have been the voltage stayed the same across all open resistors in that circuit. The total current would decrease by the amount normally drawn by the branch that opened. All other branches would have normal current draw.
     
  19. Circuits123

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    Dec 7, 2012
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    Thanks
     
  20. WBahn

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    (infinity)*(zero) is indefinite, you have no idea what it is equal to and must look elsewhere for an answer.

    You are making a number of wrong conclusions. You are saying that the voltage across one branch must be zero while the voltage across the other two branches must remain the same (though I have no idea what the "100" you used means). But then you also say that the voltage is the same for every branch in a parallel circuit, thus directly contradicting what you just said about one branch having one voltage across it and the other two having a different voltage across it.
     
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