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Old Oct 26th 2016, 07:34 AM   #1
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Errors and uncertainties questions

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

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

The answer I got was 1/3.5x10^3 (x100%) = 0.029% but the answer is 2.9%. What am I doing wrong?

2) State the absolute uncertainty in the value of the spacing?

The answer is +- 0.01mm but I don t know how this is got either.

Any help would be great.
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Old Oct 26th 2016, 08:34 AM   #2
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Given 3.5 x 10^3 the possible range of "true" measurement is anywhere from 3.45 x 10^3 to 3.55 x 10^3. That's a range of possible error of 0.1 x 10^3. So as a percentage:

0.1 x 10^3/3.5 x 10^3 = 2.9%

Note that you actually can ignore the powers of ten:

0.1/3.5 = 2.9%
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Old Oct 26th 2016, 08:50 AM   #3
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Thank you so much for explaining this to me
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Old Oct 26th 2016, 11:17 AM   #4
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Originally Posted by Clyner82 View Post
The answer I got was 1/3.5x10^3 (x100%) = 0.029% but the answer is 2.9%.
I'm betting that you are using a calculator with a % symbol. If you hit the % key it automatically divides your answer by 100. You don't need the % key for anything, really.

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