# AS Physics Practical help!!!!

Hi, I was wondering if anyone could give me some help on how to work out percentage errors, how to work out how many sif. figs to put, uncertanties etc. Thanks. BTW I have my exam in 2 days times so as much help as possible would be apprechiated!!!
When physical quantities are added or substracted, errors of observation are always added.
Eg:
L = 10.64 +/- 0.01 cm
b = 4.46 +/- 0.01 cm
t = 1.98 +/- 0.01 cm
L + b + t = (10.64 + 4.46 + 1.98) cm +/- 0.03 cm = 17.08 +/-0.03 cm
L - b + t = (10.64 - 4.46 + 1.98) cm +/- 0.03 cm = 8.16 +/-0.03 cm

When physical quantities are multiplyed or devided, percentage errors of measured are added.
Eg:
L = 10.64 +/- 0.01 cm
b = 4.46 +/- 0.01 cm
t = 1.98 +/- 0.01 cm
Percentage uncertainty of L = (0.01/10.64) x 100 = 0.09%
Percentage uncertainty of b = (0.01/4.46) x 100 = 0.22%
Percentage uncertainty of L = (0.01/1.98) x 100 = 0.50%
L x b x t = (10.64 x 4.46 x 1.98) cm3 +/-(0.09 + 0.22 + 0.50)% = 93.96 cm3 +/-0.81% = 93.96 cm3 +/-0.76cm3
L / (b x t) = 10.64 /(4.46 x 1.98) cm-1 +/-(0.09 + 0.22 + 0.50)% = 1.20 cm-1 +/-0.81% = 1.20 cm-1 +/-0.0097cm-1

When physical quantities are squared, cubed, ..... etc, percentage errors of measured quantities are doubled, trebled, ..... etc.
Eg:
I'm sorry I can't give an example because I can't type powers. If you need more clarification, PM me.
You can type powers by 3^2 meaning 3 squared.
yea do it dat way..bt wteva uve already done is greatly appreciated. thnx man!!!
Goddess
When physical quantities are added or substracted, errors of observation are always added.
Eg:
L = 10.64 +/- 0.01 cm
b = 4.46 +/- 0.01 cm
t = 1.98 +/- 0.01 cm
L + b + t = (10.64 + 4.46 + 1.98) cm +/- 0.03 cm = 17.08 +/-0.03 cm
L - b + t = (10.64 - 4.46 + 1.98) cm +/- 0.03 cm = 8.16 +/-0.03 cm

When physical quantities are multiplyed or devided, percentage errors of measured are added.
Eg:
L = 10.64 +/- 0.01 cm
b = 4.46 +/- 0.01 cm
t = 1.98 +/- 0.01 cm
Percentage uncertainty of L = (0.01/10.64) x 100 = 0.09%
Percentage uncertainty of b = (0.01/4.46) x 100 = 0.22%
Percentage uncertainty of L = (0.01/1.98) x 100 = 0.50%
L x b x t = (10.64 x 4.46 x 1.98) cm3 +/-(0.09 + 0.22 + 0.50)% = 93.96 cm3 +/-0.81% = 93.96 cm3 +/-0.76cm3
L / (b x t) = 10.64 /(4.46 x 1.98) cm-1 +/-(0.09 + 0.22 + 0.50)% = 1.20 cm-1 +/-0.81% = 1.20 cm-1 +/-0.0097cm-1

When physical quantities are squared, cubed, ..... etc, percentage errors of measured quantities are doubled, trebled, ..... etc.
Eg:
I'm sorry I can't give an example because I can't type powers. If you need more clarification, PM me.

Wow that's a lot of work! Must have taken you a lot of time to type.
This is what the NAS AS and A2 Revision Guide (for Edexcel) says about percentage uncertainties:

* Only work out uncertainties if and where the question tells you to do so, therefore for most of your measurements you won't have to bother with uncertainties.

* For a single measurement take the smallest division of the instrument as its uncertainty e.g. a length might be 19 + 1 mm using a meter rule and 18.8 + 0.1 mmusing vernier callipers.

* Practice finding percentage uncertainties (equation given in test); e.g. 5.3% and 0.53% for above lengths.

* For a set of repeated readings take half the spread of the readings as the uncertainty of the average value e.g. 1.25, 1.29, 1.28, and 1.26 mm giving a spread of 0.04 mm so average value = 1.27 + 0.02 mm with a percentage uncertainty of 1.6%.

* For measurements, such as starting and stopping a timer, where your own error adds significantly to the uncertainty, always take several measurements and use their range to get the uncertainty.

* Add the uncertainties of any measurements that are either added or substracted.

* Add the percentage uncertainties of any measurements that are multiplyed or devided.

* Multiply the percentage uncertainty by any powers to which the measurement is raised e.g. percentage error in r^3 is 3 x percentage uncertainty in r.

* Use 100 x difference/average value to calculate the percentage difference between two values.
Sfteeeeeee
For those taking the A2 edexcel practical tomorrow this is from the specification:

'Candidates should take repeat readings whenever it is reasonable to do so, in which case the
percentage uncertainty should be based on the spread of the readings. The estimated uncertainty