You are Here: Home >< Physics

# Help with percentage uncertainty question?

Announcements Posted on
Would YOU be put off a uni with a high crime rate? First 50 to have their say get a £5 Amazon voucher! 27-10-2016
1. It's only one mark I cannot answer it .

Is the percentage uncertainty of a value= Precision/ value x100.
Attached Images

2. (Original post by Questioness)
It's only one mark I cannot answer it .

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
8. (Original post by Questioness)
So
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%
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.

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: May 7, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### University open days

Is it worth going? Find out here