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Junction Rule not applying

  1. Jan 26, 2016 #1
    1. The problem statement, all variables and given/known data
    In my lab today, I created a circuit with resistor (R1) in series with a set of resistors in parallel (R2//R3).

    Using an ammeter, I measured:
    I1 = 2.30mA
    I2 = 1.25mA
    I3 = 0.38mA

    2. Relevant equations
    Junction Rule: ΣI = 0

    3. The attempt at a solution
    So applying the Junction Rule, I1 = I2 + I3
    However, with what I've measured, 12+13 does not add up to I1. What could cause this "loss" of charge? To me, this doesn't make sense, as this violates the idea of conservation of charge. How can charge just disappear like such, after the parallel connection converges?
     

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    Last edited: Jan 26, 2016
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  3. Jan 26, 2016 #2

    berkeman

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    Can you show your setup? Remember that a DVM in current measuring mode has an internal shunt resistance that cannot always be ignored...
     
  4. Jan 26, 2016 #3
    I have just added an image of my set-up in the original post.
     
  5. Jan 26, 2016 #4

    berkeman

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    Did you really have 6 DVMs running at the same time doing simultaneous monitoring? Or did you have one DVM in voltage mode and one DVM in current mode that you were moving around? What was the actual test procedure? :smile:
     
  6. Jan 26, 2016 #5

    haruspex

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  7. Jan 26, 2016 #6

    gneill

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    It would be of interest to know the component values (so that we can compare your measured values to theoretical ones). What were the values of the resistors?
     
  8. Jan 26, 2016 #7

    berkeman

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    Also, do you see a difference in how you measured the voltage across R1 versus how you handled R2 and R3? :smile:
     
  9. Jan 26, 2016 #8

    NascentOxygen

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    You used a 2 volt supply....so the resistors were around 50 to 150 ohms.

    One thing that might or might not be relevant here, but cheap power supplies can drift with time. So it's possible that what started off as 2.0 volts may have been a different value by the time you took your final current measurement. It is a wise precaution to constantly monitor voltage sources if you are not assured of their stability; an unnoticed drift in supply voltage can result in hours of wasted time, and needless frustration!
     
  10. Jan 27, 2016 #9
    Exactly to the latter procedure -- I measured each resistor's current and voltage individually. How would that affect the current though?
     
  11. Jan 28, 2016 #10

    cnh1995

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    Did the voltages add up to 2V?
     
  12. Jan 28, 2016 #11

    berkeman

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    Can you respond to the other comments and questions in this thread? You have been given some great hints and advice... :smile:
     
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