Physics Help Forum Errors and uncertainties questions

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 Oct 26th 2016, 08:34 AM #1 Member   Join Date: Mar 2016 Location: Manchester Posts: 37 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.
 Oct 26th 2016, 09:34 AM #2 Physics Team     Join Date: Jun 2010 Location: Naperville, IL USA Posts: 2,271 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%
 Oct 26th 2016, 09:50 AM #3 Member   Join Date: Mar 2016 Location: Manchester Posts: 37 Thank you so much for explaining this to me
Oct 26th 2016, 12:17 PM   #4

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 Originally Posted by Clyner82 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.

-Dan
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### Percentage error of diffraction grating

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