Why is my calculated resistance higher than the resistor used

In summary, the speaker is trying to determine why the calculated total resistance of their series circuit is higher than the theoretical total resistance. They used a 100Ω resistor and measured the current at 1.0v intervals from 1.0-5.0v, with the current measured at 8.70mA at 1.0v and 17.60mA at 2.0v. The resistor measured 97Ω using a DMM and a DC circuit was used. The theoretical total resistance of the series circuit is equal to the sum of resistor(s) (97Ω). However, when calculating total resistance at different voltage outputs, the values exceeded 98Ω. Upon further questioning, it was
  • #36
Well, I'm afraid that nothing else is coming to mind that would explain the discrepancies you've encountered. If you can get back to the lab before your report is due, set up the circuit and take voltage readings across the current-measuring DMM. Also take a close-up picture of the DMM if you can.
 
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  • #37
I am going to redo my experiment hopefully it won't affect my grade
 
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  • #38
I know it's not relevant to my earlier question, however how would I calculate resistance of my circuit at each voltage interval in parallel set up like this : https://gyazo.com/25a70bf570d626ac0c1373184075e77d? using voltage and current? R1=V2/A3, R2=V3/A4, R3=V4/A5 than 1/Rt=1/R1 + 1/R2 = 1/R3?
 
  • #39
Yes that works.

However if the meters are "ideal" meters then the equivalent resistance of the lot could be calculated as Vm1/A1.
 
  • #40
CWatters said:
Yes that works.

However if the meters are "ideal" meters then the equivalent resistance of the lot could be calculated as Vm1/A1.
I didn't think about that. thanks saves a lot of button smashing :D
 
  • #41
CWatters said:
Yes that works

Was meant to write R1=V1/A3, R2=V1/A4, R3=V1/A5
Because I am measuring the resistance at 1v-5v at 1v intervals
 
  • #42
Daniel2244 said:
I know it's not relevant to my earlier question, however how would I calculate resistance of my circuit at each voltage interval in parallel set up like this : https://gyazo.com/25a70bf570d626ac0c1373184075e77d? using voltage and current? R1=V2/A3, R2=V3/A4, R3=V4/A5 than 1/Rt=1/R1 + 1/R2 = 1/R3?

Daniel2244 said:
Was meant to write R1=V1/A3, R2=V1/A4, R3=V1/A5
Because I am measuring the resistance at 1v-5v at 1v intervals

If the meters are ideal then all the voltmeters will read the same (VM1 = VM2 = VM3 = VM4 = V1) so both methods will give the same answer.

The resistor values in the circuit are quite low (99 to 327 Ohms) compared to the resistance of most voltmeters (20K to 10M Ohm). So for example 20,000 Ohms in parallel with 327 Ohms = 322.7 Ohms so the volt meters hardly have any effect on the resistance and you can consider them to be ideal.

The internal resistance of an ammeter or DMM on the current range setting can vary. I think most are around 0.02 Ohms (eg they have a 200mV drop at 10A) but some might be more like 1 Ohm ? So for example you have a 1 Ohm meter in series with the 99 Ohm resistor = 100 Ohms. So even a 1 Ohm ammeter should only affect your circuit by 1%. so you can pretty much consider the Ammeters to be ideal as well.
 
  • #43
CWatters said:
If the meters are ideal then all the voltmeters will read the same (VM1 = VM2 = VM3 = VM4 = V1) so both methods will give the same answer.

The resistor values in the circuit are quite low (99 to 327 Ohms) compared to the resistance of most voltmeters (20K to 10M Ohm). So for example 20,000 Ohms in parallel with 327 Ohms = 322.7 Ohms so the volt meters hardly have any effect on the resistance and you can consider them to be ideal.

The internal resistance of an ammeter or DMM on the current range setting can vary. I think most are around 0.02 Ohms (eg they have a 200mV drop at 10A) but some might be more like 1 Ohm ? So for example you have a 1 Ohm meter in series with the 99 Ohm resistor = 100 Ohms. So even a 1 Ohm ammeter should only affect your circuit by 1%. so you can pretty much consider the Ammeters to be ideal as well.

I am still getting really high resistance on each branch higher than the resistor itself and I just don't know why. The circuit was set up correctly and all the
components were fine (to my knowledge). For example, at 1V the resistance of branch 1 (R1) was 103.09Ω, branch 2 = 232.56Ω tand branch 3 = 378.78. after using the Rt equation the resistance of the circuit at 1v was 60.24Ω but I still shouldn't be having the excessive resistance of the branches. Also from 1v-5v the resistance of the circuit stay roughly the same is this corrct?
 
  • #44
Did you measure the voltage across any of the current-reading DMM's? Maybe snap a picture of the DMM?
 
  • #45
gneill said:
Did you measure the voltage across any of the current-reading DMM's? Maybe snap a picture of the DMM?
No but in future I am
 
  • #46
Did you implement the full circuit that you showed in post #38? You had sufficient meters to do so? If so I'm impressed by your lab's resources.
 
  • #47
Does the variable power supply have a meter on it and are you using that meter to set the supply voltage?

Daniel2244 said:
I am still getting really high resistance on each branch higher than the resistor itself and I just don't know why. The circuit was set up correctly and all the components were fine (to my knowledge). For example, at 1V the resistance of branch 1 (R1) was 103.09Ω, branch 2 = 232.56Ω tand branch 3 = 378.78.

When you set V1 to 1V what values did you measure for VM2, VM3 and VM4 ?

What were the corresponding currents A3, A7, A8 ?

after using the Rt equation the resistance of the circuit at 1v was 60.24Ω but I still shouldn't be having the excessive resistance of the branches. Also from 1v-5v the resistance of the circuit stay roughly the same is this corrct?

If the meters are ideal the overall resistance calculated should stay constant but not 60 Ohms. I calculate it should be about 56 Ohms.
 
  • #48
gneill said:
Did you implement the full circuit that you showed in post #38? You had sufficient meters to do so? If so I'm impressed by your lab's resources.
I took out the DMM (amp setting) and (volt setting) and placed it where I needed to measure. For example when measuring the voltage at v1 and current at A1 I moved it to V2 and A2. I wish they had more resources would be a lot easier
 
  • #49
CWatters said:
Does the variable power supply have a meter on it and are you using that meter to set the supply voltage?
When you set V1 to 1V what values did you measure for VM2, VM3 and VM4 ?

What were the corresponding currents A3, A7, A8 ?
Yes, the supply pack has a meter on it, additionally I measured the output from the power pack using a voltmeter as well.
VM2 = 0.94v, VM3=0.73, VM4=0.97
A3=09.4mA, A7=4.10mA, A8=2.59
If the meters are ideal the overall resistance calculated should stay constant but not 60 Ohms. I calculate it should be about 56 Ohms.
R1=1/0.0094=106.38 R2=1/0.0041=243.90 R3=1/0.00259=386.1
1/Rt=1/106.38+1/243.90+1/386.1=0.0160
Rt=1/0.0160=62.5Ω
 
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  • #50
Daniel2244 said:
I took out the DMM (amp setting) and (volt setting) and placed it where I needed to measure. For example when measuring the voltage at v1 and current at A1 I moved it to V2 and A2. I wish they had more resources would be a lot easier
I'm still not clear on what the precise circuit was for each reading taken. How many meters were in play at any given time? Can you provide a figure showing the setup for taking readings?
 
  • #52
Your ammeters don't appear to be placed in series with the resistors that you're measuring the voltage across :confused:
 
  • #53
gneill said:
Your ammeters don't appear to be placed in series with the resistors that you're measuring the voltage across :confused:
Does that really matter? because I am using voltage from power supply over the voltage on the branch
 
  • #54
Daniel2244 said:
Does that really matter? because I am using voltage from power supply over the voltage on the branch
It should be okay if the meters are essentially ideal. But we are trying to find the source of your discrepancies, so it would be nice to see the branches handled separately. However, let's see what we can make of the measurements that you took.

What were the V and I readings for these two setups:
upload_2018-2-28_15-29-43.png
upload_2018-2-28_15-29-53.png

Were the current values different? Did you have to adjust the source voltage after moving the ammeter and voltmeter?
 

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  • #55
What were the V and I readings for these two setups
First: VM1=1v, A1 =3.70 second: VM1=0.94v A1=13.22
 
  • #56
Daniel2244 said:
First: VM1=1v, A1 =3.70 second: VM1=0.94v A1=13.22
Don't you find that odd? If the meters were ideal, shouldn't the readings be the same for both setups?
 
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  • #57
gneill said:
Don't you find that odd? If the meters were ideal, shouldn't the readings be the same for both setups?
of course I do, but I don't know why my results have turned out how they have as my circuit was accurately built. Unless they were faulty connection with the wires (which are terrible).
 
  • #58
Even relatively shabby cables should behave okay for the low currents and potentials involved. Did readings vary wildly with mild jostling of the wires?

The type of things that go wrong in these sorts of labs, even when all the circuit connections are correct and to spec, are related to the measuring equipment and its use. Things such as:

- Inappropriate range selection (affects reading accuracy)
- Choosing and AC range rather than a DC range (or vice versa)
- Incorrect meter connections (set of jacks for the various reading types)
- Meter's internal batteries are failing
- Meter is actually out of calibration or broken due to a previous misuse by another user (it happens!)

I think I would really very much like to see photos of the lab setup for at least two of the arrangements where you record the readings. A closeup of the meters while connected would be fantastic.
 
  • #59
I think I would really very much like to see photos of the lab setup for at least two of the arrangements where you record the readings. A closeup of the meters while connected would be fantastic
I will provide photographs when I redo this experiment.
I am not sure what went wrong as I set my DMM to DC and power pack DC. However, the other points are valid but I'm not quite sure what you mean by "set of jacks for the various reading types"
 
  • #60
Daniel2244 said:
I will provide photographs when I redo this experiment.
I am not sure what went wrong as I set my DMM to DC and power pack DC. However, the other points are valid but I'm not quite sure what you mean by "set of jacks for the various reading types"
Remember when I asked whether or not your DMM had different places to plug the leads into it for different types of readings or ranges? Sometimes it's easy to forget to change or select the right plug positions.
 
  • #61
gneill said:
Remember when I asked whether or not your DMM had different places to plug the leads into it for different types of readings or ranges? Sometimes it's easy to forget to change or select the right plug positions.
Yeah, now I understand, the wires I used were placed in the right sockets I'm pretty sure.
 

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