1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

General DC Circuit: Voltage drop different from supply source

  1. Feb 1, 2015 #1
    1. The problem statement, all variables and given/known data
    I performed a lab with a simple DC circuit. The circuit has 4 resistors (i.e. R1, R2, R3 and R4), which is the same as shown in the attached image. When the voltages across all resistors were added, they were higher than the source. How is this possible? The KVL rule states that the voltage across a loop in a circuit should equal zero but no in this case.

    2. Relevant equations
    V=IR

    3. The attempt at a solution
    The lab started off where we had to adjust the power supply until the voltage across R4 was 6V. After measuring the currents and voltages across each resistor, I found that the voltage drop a loop did not add up to what we measured across the source. The voltage drops were greater than the source. For example, the voltage across the source was 19.91 and the total voltage drops across the first loop (E->R1->R2). was 20.072V. My question is how is this possible?
     

    Attached Files:

  2. jcsd
  3. Feb 1, 2015 #2
    Does the total voltage around a loop without the battery give you 0? For example R3>R4>R2 and back to R3
     
  4. Feb 1, 2015 #3
    I do not have access to the circuit anymore as its in a classroom but without the battery, I would assume the there wouldn't be any voltage drops across the resistors since there isn't any source right?
     
  5. Feb 1, 2015 #4
    No the battery is still there and supplying voltage, but you are just going around a loop without the battery and checking if the total voltage drop around that loop is zero.
     
  6. Feb 1, 2015 #5
    Oh, okay. I understand what you mean now. I am not able to physically verify this but I am going to make an assumption that the total voltage drop around that loop is zero without the battery since there would not be any source connected to the circuit without the battery.

    I still don't understand how the sum of the voltage drops across the resistors in a loop is greater than the voltage source (e.g. battery).?
     
    Last edited: Feb 1, 2015
  7. Feb 1, 2015 #6

    phinds

    User Avatar
    Gold Member
    2016 Award

    It isn't. There was undoubtedly something wrong with your measurement technique. You have not found a violation of the laws of physics, you've just made a mistake. Come at it from that point of view and you might figure out where you made the mistake.
     
  8. Feb 1, 2015 #7
    Is this because the voltmeter itself has a voltage drop, which would account for the additional total voltage drop across the circuit? In that case, we would have a internal resistance in the voltmeter and hence the total equivalent resistance would be less which would mean the voltage across it would be even less and not more. Not sure how this works and I have been spending the last 2 hours thinking about it. Please advice. Thanks for all the help!
     
    Last edited: Feb 1, 2015
  9. Feb 1, 2015 #8
    Yes, they can have an internal resistance and wires, connectors etc. can also have a bit of resistance so the perfect model is not the same as what you are doing practically. That is why I said check another loop.
     
  10. Feb 2, 2015 #9

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    I don't think it's due to the impedance of the voltmeter. I would expect that to have the opposite effect.

    Did you measure the supply voltage with the volt meter or just write down the value displayed on bench power supply?
     
  11. Feb 2, 2015 #10

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    I've just noticed you wrote...

    If the circuit was powered by a battery how was the source voltage adjusted so that R4 was 6V?
     
  12. Feb 2, 2015 #11

    NascentOxygen

    User Avatar

    Staff: Mentor

    I doubt that OP will understand this, without you explaining. :eek:
     
    Last edited: Feb 2, 2015
  13. Feb 3, 2015 #12
    We had the circuit set up as shown with the power supply connected. Then, we connected a meter across R4 and adjusted the power supply until we read 6V across R4. This would then result in a power supply which would give 6V across the R4 resistor. I am still unsure how the experiment resulted in me getting a sum of higher voltage drops across circuit than the power supply. Was this a measurement mistake or is there something logical to why I got such results?
     
  14. Feb 3, 2015 #13
    Can you write all the voltage drops you measured over the resistances
     
  15. Feb 3, 2015 #14
    I very much agree with you. If it was due to the impedance of the voltmeter, I would get an even higher voltage drop across the circuit. The supply voltage was displayed on the bench power supply. Any idea where I made the mistake? Would there be a logical explanation to why the voltage drops don't add up correctly?
     
  16. Feb 3, 2015 #15
    Hi,
    I have attached a copy of the voltage drops I measured. I am not sure if this note helps but the power supply only showed up to a certain amount of significant figures. For example, it showed 3.00 whereas the current through R1 was showed to be 3.708 mA.
     

    Attached Files:

  17. Feb 3, 2015 #16

    NascentOxygen

    User Avatar

    Staff: Mentor

    So your power supply delivers 19.9 volts. Looking at your table of measurements, are the various voltages you recorded consistent with this 19.9v?
     
  18. Feb 3, 2015 #17
    When I summed the voltages across the loops, I got 20.072 (9.982+10.09) for loop 1 with R1 and R2, and 20.069 (9.982+4.087+6) for loop 2 with R1, R3 and R4.
     
  19. Feb 3, 2015 #18

    NascentOxygen

    User Avatar

    Staff: Mentor

    Would you say those results agree with theory within expected experimental error?

    By what percentage do they differ?
     
  20. Feb 4, 2015 #19

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    Always check that the display on the bench supply is accurate by confirming it with the voltmeter.

    The two numbers you quoted (19.91V and 20.072V) differ by about 0.8%. It could be something as simple as an in accurate display on the bench power supply.
     
  21. Feb 5, 2015 #20
    Thank you all for your replies. It has been very informative and helpful. Based on all the posts, I am going to assume that there isn't a theoretical mistake I made? I raise this question because one of the conclusions of the experience was "Comment on the possible causes for any deviations from what you would expect theoretically.". My answer to this so far is 1- an inaccurate display on the bench power supply. It is most likely that the power supply wasn't showing accurate results OR the readings from the multimeter was a little off? Please advice.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: General DC Circuit: Voltage drop different from supply source
Loading...