The Student Room Group
Original post by user1-4
i've forgotton how to do percentage uncertainity. Can anybody explain it to me :smile:
Thanks


The % uncertainty in a reading needs an absolute uncertainty (the ± part)
call it ± x, and a reading with this uncertainty, call it X ± x

The % uncertainty is 100(xX)100(\frac{x}{X})
(edited 10 years ago)
Reply 2
Hi thanks, but for physics A level isas i remember that we had to know a formula and when to use them.

it was something like +/-range over 2 or range over mean. I can't remember which.

And then there was another method, which I think was +/-precision over two?
So what's your question? You asked about % uncertainties.
Reply 4
If you're doing an experiment to calculate the volume of something you would work out half of the range of the values you took for the width, length and height.
Then for each of the errors which is the half range you would calculate a percentage error for the width, length and height by doing the (half range/ the width )*100. Finally add all the percentage errors together since your multiplying the width, length and height to work out the volume.

Hope that made some sense
Reply 5
Original post by MP2205
If you're doing an experiment to calculate the volume of something you would work out half of the range of the values you took for the width, length and height.
Then for each of the errors which is the half range you would calculate a percentage error for the width, length and height by doing the (half range/ the width )*100. Finally add all the percentage errors together since your multiplying the width, length and height to work out the volume.

Hope that made some sense


thanks. This makes sense.

How do you know when to use the precision as the uncertainty and not the range?
And is the precision method:

Uncertainty = precision/2

or is it uncertainty=precision?
(edited 10 years ago)
Reply 6
The uncertainty is the precision when the range is zero (all your readings are the same).

The only dodgy part is what to do when the uncertainty is smaller than the precision. For example: if your precision was 1mm and you got two results which were 50mm and 51mm, half the range and thus the uncertainty would be 0.5mm. However, this is smaller than the precision of the instrument you used to measure with. My tutor says in this case you should put the precision (the larger number) as the uncertainty and write why you're putting the precision instead of the range. The fact remains though that the mark scheme only ever mentions doing half the range, and the precision if the range is 0.
Reply 7
thank you
Original post by Stonebridge
The % uncertainty in a reading needs an absolute uncertainty (the ± part)
call it ± x, and a reading with this uncertainty, call it X ± x

The % uncertainty is 100(xX)100(\frac{x}{X})


How many s.f. should the percentage uncertainty have?
Original post by asadmoosvi
How many s.f. should the percentage uncertainty have?



One.


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