# Need help in determining a source of error in an experiment

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1. May 10, 2015

### Philip.plsHelp

1. The problem statement, all variables and given/known data
It is known that the theoretical resistivity of copper is approximately: 2*10^-8 Ωm. But after I conducted an experiment to find the resistivity of copper, the results turned out to be: 1.68*10^-8Ωm.

This is a very small error that can be easily dismissed due to "human error" but i am not allowed to mention "human error".

What other factors might have caused this small percentage of error between the theoretical and obtained results??.... hint: i have already researched into the corrosion and crystal structure of copper and it doesn't derive into any possible conclusions.

2. Relevant equations

3. The attempt at a solution

Last edited: May 10, 2015
2. May 10, 2015

### Staff: Mentor

You were there conducting the experiment, so you should be able to point to sources where inaccuracy of some description may have occurred. Suggest some.

3. May 10, 2015

### Philip.plsHelp

1. The problem statement, all variables and given/known data
I have obtained a relative error from the absolute error using the formula and received an answer of 5% while my total experimental error ((result-theory)/theory) was nearly 20%.

I have a rough idea of what they both represent but can someone please explain these two to me and how they can be explained in an assignment? Basically the similarities and differences between the two...

thank you

2. Relevant equations

3. The attempt at a solution

4. May 10, 2015

### Staff: Mentor

Which absolute error do you mean? The experimental uncertainty? And what is "the formula"?
If you mean the experimental uncertainty, then I guess you underestimated the uncertainty, or something else went wrong, or the theory prediction was wrong, or you were very unlucky.

Edit: I merged the two threads as they appear to have the same origin.

5. May 10, 2015

### Philip.plsHelp

The formula = max-min/total... I just want to know the difference between relative and total experimental error.

6. May 10, 2015

### Staff: Mentor

They are different concepts.
"x is 10% larger than y" and "x is 7 meters larger than y" are different things.

7. May 11, 2015

### Staff: Mentor

That is not the formula. This is: relative error = (max-min)/total
As you know, those brackets make a world of difference, and change a wrong formula into a right one.

Suppose you were to measure a distance, and estimated the error in your measurement to be 15km. That might seem a large experimental error. But once it was made known that the distance being measured was the distance from the Earth to the moon, then you could say it is good accuracy, because the relative error is a tiny 15/384403 = 0.000039
and multiplying by 100 we can convert this to a percentage,
so the percentage error is just 15*100/384403% = 0.0039%

Percentage errors are often preferred, because most people do understand what a percent is.

8. May 11, 2015

### Merlin3189

Firstly, listen to O
If the discrepancy is due to experimental error, then you need to examine your experimental method and your calculations. You haven't told us anything about that, so it would be hard to comment on it. All experiments have experimental errors: you need to identify and quantify them.

Another possibility lies in your theoretical calculation of resistivity
Do you know how theoretical resistivity is calculated? What assumptions are made which may not be accurate in your experiment?
The first two sources I checked also measure resistivity of Cu as significantly different from your theoretical calculation. You don't say what sort of copper you are talking about, so perhaps you calculated the theoretical resistance for pure copper, but tested an alloy or a specimen with different physical properties (eg. work hardened - this would not give all the error you found, but may account for some of your difference.)

9. May 11, 2015

### Staff: Mentor

It is worth pointing out that you are not embarking on a witch-hunt for the one in your group who blundered. This error you are chasing is not a blunder, not a human's mistake. It is a measure of how accurate the equipment, procedure, and calculations were. Nothing in science is 100% accurate. When you measure a length using a ruler, your result is accurate to only about 0.5mm, so you would record your result as, for example, 57.0 mm ± 0.5mm. That accounts for a possible error of 0.5mm

Some of the topic threads at the foot of this page may hold something of interest to you.