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# Help with percentage uncertainty question? watch

1. It's only one mark I cannot answer it .
Someone please explain how you get then answer.

Is the percentage uncertainty of a value= Precision/ value x100.
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2. (Original post by Questioness)
It's only one mark I cannot answer it .
Someone please explain how you get then answer.

Is the percentage uncertainty of a value= Precision/ value x100.
It's Uncertainty / Value, but in this case you could claim that precision would also mean uncertainty (not always though!)

So, if I tell you that I've measured something as 3.5cm, what is the implied uncertainty in my measurement?

Spoiler:
Show
Why didn't I say 3.54cm for instance?
3. (Original post by lerjj)
It's Uncertainty / Value, but in this case you could claim that precision would also mean uncertainty (not always though!)

So, if I tell you that I've measured something as 3.5cm, what is the implied uncertainty in my measurement?
Spoiler:
Show
Why didn't I say 3.54cm for instance?
Your using a mm scale? So it's +\- 0.1 cm ?
4. (Original post by Questioness)
Your using a mm scale? So it's +\- 0.1 cm ?
Yep. So if the diffraction grating claims to have 3.5x10^3 lines/mm, then what is the implied uncertainty in that? For % uncertainty you just divide by the value (and multiply by 100%)
5. (Original post by lerjj)
Yep. So if the diffraction grating claims to have 3.5x10^3 lines/mm, then what is the implied uncertainty in that? For % uncertainty you just divide by the value (and multiply by 100%)
Do you mean per meter?
6. (Original post by Questioness)
Do you mean per meter?
Apparently, yes. 3.5x10^3 lines/m is not very many, but I guess it'll still work.

Also, it's a % uncertainty, so the units really don't matter here. Neither does the x10^3 bit either actually. Any measurement given to you as 3.5 x10^p mkgsA... etc will have the same % uncertainty in it.
7. (Original post by lerjj)
Apparently, yes. 3.5x10^3 lines/m is not very many, but I guess it'll still work.

Also, it's a % uncertainty, so the units really don't matter here. Neither does the x10^3 bit either actually. Any measurement given to you as 3.5 x10^p mkgsA... etc will have the same % uncertainty in it.
So
My answer is wrong?
8. (Original post by Questioness)
So
My answer is wrong?
I think I may have confused you

My point with the 3.5cm reading is that if someone gives you a number to 1d.p. then that implies an uncertainty.

In this case, they've said the number of lines is 3.5 x10^3/m

The implied uncertainty in that is 0.1 x10^3/m. As before, the end bit doesn't really matter, your percentage uncertainty =0.1/3.5 = 3%
9. (Original post by lerjj)
I think I may have confused you

My point with the 3.5cm reading is that if someone gives you a number to 1d.p. then that implies an uncertainty.

In this case, they've said the number of lines is 3.5 x10^3/m

The implied uncertainty in that is 0.1 x10^3/m. As before, the end bit doesn't really matter, your percentage uncertainty =0.1/3.5 = 3%
Ahah right! Thank you. Makes much more sense now.
Might be a one marker but I've had some trouble with this for some time now
10. I don't follow completely, can someone please re-explain this? :/ Also I need to know how to generalise this to any question involving pretty much anything.

(Original post by Questioness)
Ahah right! Thank you. Makes much more sense now.
Might be a one marker but I've had some trouble with this for some time now
(Original post by lerjj)
I think I may have confused you

My point with the 3.5cm reading is that if someone gives you a number to 1d.p. then that implies an uncertainty.

In this case, they've said the number of lines is 3.5 x10^3/m

The implied uncertainty in that is 0.1 x10^3/m. As before, the end bit doesn't really matter, your percentage uncertainty =0.1/3.5 = 3%
11. (Original post by Someboady)
I don't follow completely, can someone please re-explain this? :/ Also I need to know how to generalise this to any question involving pretty much anything.
Let's say I measure something as 3.5cm with a ruler. Why didn't I measure it as 3cm or as 3.54cm? Because the the former isn't as precise as I can be and because the latter is beyond the capabilities of my instrument to measure.

Therefore, if I tell you I measured something as 3.5cm, you should assume that I have some level of competence and that the uncertainty in that is 1mm.

Likewise, if somebody tells you e.g. the implied uncertainty is again 0.1m/s^2.

Tl;dr The uncertainty in a measurement is reflected in the number of significant figures you give it to. The percentage uncertainty can therefore be calculated.
12. AQA actually specifically talk about this question in their practical handbook, and I thought it might help:

http://filestore.aqa.org.uk/resources/physics/AQA-7407-7408-PHBK.PDF - page 55

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