# Can somebody help me with errors and uncertainties for Physics

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#1
I have been writing up my Physics A2 Coursework, and I have come to the point where I have to give error values + or - from my results. I really don't know how to do this very well! I have worked out that there are errors by using half the value for the spacing on a ruler (ie, ruler is in 1mm divisions, I have an error value of + or - 0.5mm) but after that, I am totally stuck. Do i have to put the values into percentage error, and if so how do I do that?

Also, if I have errors for 2 values (in my case, depth of crater[h] and diameter of crater[2r]) and I put these into an equation (which is V = 1/3*πh^2[3r - h]) how do I know what the error value for V is?

That sounded rather confusing didn't it....please somebody who is good at Physics, could you give me some pointers for how to work out error values, epecially the type that is required for A2 Physics.

ThunderCat8.
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16 years ago
#2
If you have an e-mail address I'll send you something from the Cambridge Physics Handbook about errors. What is the address?
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#3
[email protected]

Thanks!
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16 years ago
#4
I tried to explain but did it wrong myself, and now I'm waiting for this answer. lol.

You can work out the upper and lower bounds, work each one through and fine the difference but I know there is an easier way.

edit: can you send it to [email protected]
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16 years ago
#5
(Original post by ThunderCat8)
I have been writing up my Physics A2 Coursework, and I have come to the point where I have to give error values + or - from my results. I really don't know how to do this very well! I have worked out that there are errors by using half the value for the spacing on a ruler (ie, ruler is in 1mm divisions, I have an error value of + or - 0.5mm) but after that, I am totally stuck. Do i have to put the values into percentage error, and if so how do I do that?

Also, if I have errors for 2 values (in my case, depth of crater[h] and diameter of crater[2r]) and I put these into an equation (which is V = 1/3*πh^2[3r - h]) how do I know what the error value for V is?

That sounded rather confusing didn't it....please somebody who is good at Physics, could you give me some pointers for how to work out error values, epecially the type that is required for A2 Physics.

ThunderCat8.
Ok, Ill give you a general overview, firstly, when using rulers with 1mm spacings, the error is +/- 1mm, not 0.5 mm.
Now, when working out the percentage error, it is the error divided by the value, x 100. For example: 0.2m +/- 0.001m the percentage error will be:
0.001/0.2 x 100 = 0.5% error.

Now, if you want to get an overall percentage error(for example with V) you need to work out the individual percentage errors, and add them together. So say you have errors of 5% 3% and 0.5%, the overall percentage error will be 8.5%.

When you work out errors, and the term is squared(as with your value of h I think?) then you times the error by two. So, if the error for h = 20%, because the term is squared, its actual error is 40%.

This is just a general error overview, hope it helps!
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#6
Thanks mate! That really helps! One thing, when you are doing the percentage error, is it possible to have one percentage error for all my results, ie, having a percentage error of 5% for ALL the values for depth, rather than having a percentage error for each individual value, which is what it sounds like when you said it:

(Original post by imasillynarb)
Now, when working out the percentage error, it is the error divided by the value, x 100. For example: 0.2m +/- 0.001m the percentage error will be:
0.001/0.2 x 100 = 0.5% error.
If it is impossible to have a overall percentage error for each thing (depth, diameter etc) then how do I put the individual percentages onto error bars in Excel? I only know how to put one percentage error in, not all different ones.

Thanks for the help though, that really should get me going! Just need to get that one thing rectified.

Oh and chrisbphd, if you could still send me that info, i'd greatly appreciate it.
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16 years ago
#7
I am sorry for the delay...I am scanning from a handout and it's a bit slow!

Do you have MS Word...I'll put the pictures in that as pages.
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#8
Yes, I have MS Word. Thanks for doing this for me. I honestly appreciate it. Anything you want from me, just ask!!
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16 years ago
#9
Some rep would be nice! Nearly finished now.
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#10
Rep? This is new to me! Only just seen that for the first time today! How exciting!! Hope I did it right.

Thank you soooo much.
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16 years ago
#11
(Original post by ThunderCat8)
Thanks mate! That really helps! One thing, when you are doing the percentage error, is it possible to have one percentage error for all my results, ie, having a percentage error of 5% for ALL the values for depth, rather than having a percentage error for each individual value, which is what it sounds like when you said it:

If it is impossible to have a overall percentage error for each thing (depth, diameter etc) then how do I put the individual percentages onto error bars in Excel? I only know how to put one percentage error in, not all different ones.

Thanks for the help though, that really should get me going! Just need to get that one thing rectified.

Oh and chrisbphd, if you could still send me that info, i'd greatly appreciate it.
I didnt quite understand what you were asking there? In regards to the first part, which I understood , Yes!

What you need to do is find the range(biggest value - smallest value) and then divide it by the number of results. This gives you the absolute uncertainty, then to get the percentage uncertainty, you simply divide the absolute uncertainty by the value, and x 100.
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#12
I understand what you say, but I can't seem to get any numbers that fit. I keep getting something stupid like 1600% which isn't very good!!

Here are my results for Diameter in cm:

2.00
2.10
2.38
2.48
2.66
2.70
2.80
2.99
2.92
3.18

I have got an error value of +/- 0.2cm from the ruler etc. How do I now get the percentage error for Diameter? (Diameter as a group, not individual errors for each value)

You said take range, which is 1.18cm. (3.18 - 2.00)

Divide by no. of readings (10) = 0.118

Divide by value...which one? And what do I do about my +/-0.2cm which i worked out before?

Sorry for being really dense, but for some reason nothing is clicking today! I want to get a high a mark as possible, and this is the stuff that I need!! Thanks for helping so much, i just need a teeny bit more help.
0
16 years ago
#13
(Original post by ThunderCat8)
I understand what you say, but I can't seem to get any numbers that fit. I keep getting something stupid like 1600% which isn't very good!!

Here are my results for Diameter in cm:

2.00
2.10
2.38
2.48
2.66
2.70
2.80
2.99
2.92
3.18

I have got an error value of +/- 0.2cm from the ruler etc. How do I now get the percentage error for Diameter? (Diameter as a group, not individual errors for each value)

You said take range, which is 1.18cm. (3.18 - 2.00)

Divide by no. of readings (10) = 0.118

Divide by value...which one? And what do I do about my +/-0.2cm which i worked out before?

Sorry for being really dense, but for some reason nothing is clicking today! I want to get a high a mark as possible, and this is the stuff that I need!! Thanks for helping so much, i just need a teeny bit more help.
Right:

Error value divided by the average reading. then times by 100. to get % error.
0
16 years ago
#14
(Original post by ThunderCat8)
I understand what you say, but I can't seem to get any numbers that fit. I keep getting something stupid like 1600% which isn't very good!!

Here are my results for Diameter in cm:

2.00
2.10
2.38
2.48
2.66
2.70
2.80
2.99
2.92
3.18

I have got an error value of +/- 0.2cm from the ruler etc. How do I now get the percentage error for Diameter? (Diameter as a group, not individual errors for each value)

You said take range, which is 1.18cm. (3.18 - 2.00)

Divide by no. of readings (10) = 0.118

Divide by value...which one? And what do I do about my +/-0.2cm which i worked out before?

Sorry for being really dense, but for some reason nothing is clicking today! I want to get a high a mark as possible, and this is the stuff that I need!! Thanks for helping so much, i just need a teeny bit more help.
Ahar ok, get the mean value of your readings (2.621cm) and the absolute uncertainty is 0.118cm as you calculated above. Therefore, the percentage error is 0.118/2.621 x 100 = 4.5% error
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#15
Ok, I think at last I understand perfectly! Thank you all so much for your help! If anybody else would like to add anything, then feel free to. I'm sure there is something that I will come across later on in my investigation that I need help with, so every little helps!!

THANK YOU!
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16 years ago
#16
E-mails sent and on their way. Sorry for the delays.
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16 years ago
#17
(Original post by chrisbphd)
E-mails sent and on their way. Sorry for the delays.
Chris, can you send one to [email protected] as well. Thanks.
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16 years ago
#18
hey chris, any chance of that error publication for me too please?

[email protected]

P.S. Does it cover error bars on graphs? If not NP
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16 years ago
#19
Sorry for the delay in all of this. You can now find it at chrisbphd.150m.com . Hope it helps.
0
16 years ago
#20
(Original post by chrisbphd)
Sorry for the delay in all of this. You can now find it at chrisbphd.150m.com . Hope it helps.
Quality, thanks!
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