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Help with percentage uncertainty question for AS physics!

A student has a diffraction grating that is marked 3.5x10^3 per m.

Calculate the percentage uncertainty in the number of lines per metre suggested by this marking. (1 mark)


I have seen the mark scheme says it is 2.9%, however i don't know how to get that answer... I got the answer of 0.000029%....
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Kyx
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Original post by Smile Generator
A student has a diffraction grating that is marked 3.5x10^3 per m.

Calculate the percentage uncertainty in the number of lines per metre suggested by this marking. (1 mark)


I have seen the mark scheme says it is 2.9%, however i don't know how to get that answer... I got the answer of 0.000029%....


Can you post the exact question, and your working out pls?
Just from what you have given, i assume that you have
1) not taken the factor of ×103\times 10^3 into consideration
2) Not multiplied by 100 to get percentage instead of decimal as
2.9×10−5=0.0000292.9 \times 10^{-5}=0.000029.

Besides, what the mark scheme wants is 13.5\frac{1}{3.5} it seems.
Original post by Kyx
Can you post the exact question, and your working out pls?


That was the exact question.

I did (1/3.5x10^3)x100 :smile:
I'm having trouble finding the absolute uncertainty in the number of lines per metre, for 3.5 x 10^3 lines per metre.
To get to the answer, you need to do (100 / 3.5 x 10^3) x 100, which gives you an answer of 2.857142%, which rounds to 2.9%
Would that not give you per cm though?
This same Q is confusing me now as well... surely the mark scheme isnt correct? (1/3.5x10^3)x100 gives 0.029%
I think the absolute uncertainty implied is 0.1 x 10^3 (ie the value lies between 3.4 and 3.6 x 10^3)
So the % uncertainty would be (0.1 x 10^3) / (3.5 x 10^3) x 100
Original post by old_teach
I think the absolute uncertainty implied is 0.1 x 10^3 (ie the value lies between 3.4 and 3.6 x 10^3)
So the % uncertainty would be (0.1 x 10^3) / (3.5 x 10^3) x 100

But how would you figure out the absolute uncertainty implied from just looking at the question without any extra info? I don't think I'd get that in the exam
I agree that, to my mind, there is uncertainty as to what to take as the uncertainty!
When values are given to 0.5 (eg a measurement 6.5 cm) I'd assume it's to the nearest 0.5
Here, as it is given to 1 dp (ignoring POT), I'd assume it's to nearest 0.1, but I think it can be taken to plus/minus 0.05.
I expect your teacher can tell you the exam board's rules which you'll need to apply.
A thing that really bugs me is when uncertainty is quoted to lots of sf!
Original post by old_teach
I agree that, to my mind, there is uncertainty as to what to take as the uncertainty!
When values are given to 0.5 (eg a measurement 6.5 cm) I'd assume it's to the nearest 0.5
Here, as it is given to 1 dp (ignoring POT), I'd assume it's to nearest 0.1, but I think it can be taken to plus/minus 0.05.
I expect your teacher can tell you the exam board's rules which you'll need to apply.
A thing that really bugs me is when uncertainty is quoted to lots of sf!

Okay that makes sense actually, thanks for explaining to me and I will take a look at the AQA rules for percentage uncertainties and other related stuff

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