General DC Circuit: Voltage drop different from supply source

  • #1
sugz
110
0

Homework Statement


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.

Homework Equations


V=IR

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?
 

Attachments

  • dc circuit.JPG
    dc circuit.JPG
    39.5 KB · Views: 528

Answers and Replies

  • #2
barefeet
59
2
Does the total voltage around a loop without the battery give you 0? For example R3>R4>R2 and back to R3
 
  • #3
sugz
110
0
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?
 
  • #4
barefeet
59
2
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.
 
  • #5
sugz
110
0
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:
  • #6
phinds
Science Advisor
Insights Author
Gold Member
2021 Award
17,809
9,873
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).?
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.
 
  • Like
Likes NascentOxygen
  • #7
sugz
110
0
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:
  • #8
barefeet
59
2
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.
 
  • #9
CWatters
Science Advisor
Homework Helper
Gold Member
10,541
2,315
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?
 
  • #10
CWatters
Science Advisor
Homework Helper
Gold Member
10,541
2,315
I've just noticed you wrote...

I do not have access to the circuit anymore as its in a classroom but without the battery....

If the circuit was powered by a battery how was the source voltage adjusted so that R4 was 6V?
 
  • #11
NascentOxygen
Staff Emeritus
Science Advisor
9,242
1,073
Does the total voltage around a loop without the battery give you 0? For example R3>R4>R2 and back to R3
I doubt that OP will understand this, without you explaining. :eek:
 
Last edited:
  • #12
sugz
110
0
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?
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?
 
  • #13
barefeet
59
2
Can you write all the voltage drops you measured over the resistances
 
  • #14
sugz
110
0
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?
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?
 
  • #15
sugz
110
0
Can you write all the voltage drops you measured over the resistances
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.
 

Attachments

  • results.JPG
    results.JPG
    22.1 KB · Views: 504
  • #16
NascentOxygen
Staff Emeritus
Science Advisor
9,242
1,073
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?
 
  • #17
sugz
110
0
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?
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.
 
  • #18
NascentOxygen
Staff Emeritus
Science Advisor
9,242
1,073
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.
Would you say those results agree with theory within expected experimental error?

By what percentage do they differ?
 
  • #19
CWatters
Science Advisor
Homework Helper
Gold Member
10,541
2,315
The supply voltage was displayed on the bench power supply.

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.
 
  • #20
sugz
110
0
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.
 
  • #21
CWatters
Science Advisor
Homework Helper
Gold Member
10,541
2,315
I can think of other reasons why the measured voltages might not have been as calculated - to do with the resistor values.
 
  • #22
sugz
110
0
I can think of other reasons why the measured voltages might not have been as calculated - to do with the resistor values.
Right, that makes sense because the resistors have tolerances and so that may affect the practical results versus theoretical results. When calculation is performed, we typically ignore the tolerances for the resistors. Is this correct?
 
  • #23
CWatters
Science Advisor
Homework Helper
Gold Member
10,541
2,315
Yes.

Edit: Well OK we don't always ignore the tolerance of the resistors. Ideally we design circuits so that they aren't "sensitive" to the exact value. However sometimes it can be important (an example would be the design of complex analogue filters). Some circuit simulation programs can do a sensitivity analysis to help work out which ones are critical and need to have a higher tolerance.
 
  • #24
sugz
110
0
Yes.

Edit: Well OK we don't always ignore the tolerance of the resistors. Ideally we design circuits so that they aren't "sensitive" to the exact value. However sometimes it can be important (an example would be the design of complex analogue filters). Some circuit simulation programs can do a sensitivity analysis to help work out which ones are critical and need to have a higher tolerance.
Thank you so much for your reply as well as the others! My answer to the question "comment on the possible causes for any deviations from what you would expect theoretically" is that the results were within 1% of the theoretical results. In addition, the results had indicated that the voltage drops across the resistors in a loop were higher than the supply voltage by 1%, which is possible due to incorrect display of the power supply and the tolerances in the resistors. The tolerances in the resistors are the result of many factors such as environment temperature, humidity, abuse and aging.

I would like to know what can cause the reading from a supply to be incorrect? From my understanding, these are industrial power supplies and should be very correct. Thanks again!
 
  • #25
CWatters
Science Advisor
Homework Helper
Gold Member
10,541
2,315
Sorry to be picky but...

The resistor tolerance doesn't explain an apparent breech of KVL. It only makes the calculated voltages different from the actual voltages.
 
  • #26
sugz
110
0
Sorry to be picky but...

The resistor tolerance doesn't explain an apparent breech of KVL. It only makes the calculated voltages different from the actual voltages.
Hi, I am aware of this but to my knowledge, I do not know what breeched the KVL rule. Any ideas of what I could have done incorrectly?
 
  • #27
NascentOxygen
Staff Emeritus
Science Advisor
9,242
1,073
Hi, I am aware of this but to my knowledge, I do not know what breeched the KVL rule. Any ideas of what I could have done incorrectly?
You've done nothing incorrectly. If the resistor values are not exactly equal to their nominal value, the sum of the resistor voltages must still exactly equal the power supply voltage. It can't be less, and it can't be greater. The individual resistor voltages may differ owing to value tolerances, but their sum remains the same.
 
  • #28
sugz
110
0
In that case, is it safe to say that the experiment was done correctly even though the power supply voltage was different than the sum of the resistor voltage by about 0.8% percent. In addition, is this expected when performing a lab as such? This is my first circuits lab and so I was confused. If you can clear up this for me, I would really appreciate it!
 
  • #29
CWatters
Science Advisor
Homework Helper
Gold Member
10,541
2,315
Yes experiment seems fine. The only thing I would have done differently is to check the bench supply voltage using the same meter you used to measure the voltages on the resistors.
 
  • #30
NascentOxygen
Staff Emeritus
Science Advisor
9,242
1,073
In that case, is it safe to say that the experiment was done correctly even though the power supply voltage was different than the sum of the resistor voltage by about 0.8% percent. In addition, is this expected when performing a lab as such? This is my first circuits lab and so I was confused. If you can clear up this for me, I would really appreciate it!
To get measurements to agree within 0.8% is outstanding!! Congratulations! :)

Experiment measurements in later lab classes will probably never be in such close agreement. You'll just have to get used to it! :w
 

Suggested for: General DC Circuit: Voltage drop different from supply source

Replies
4
Views
12K
Replies
4
Views
6K
Replies
5
Views
11K
Replies
9
Views
986
Replies
2
Views
3K
  • Last Post
Replies
1
Views
4K
Replies
20
Views
2K
Replies
4
Views
7K
Top