# Homework Help: Finding error in Calculation

1. Dec 11, 2011

### Phystudent91

1. The problem statement, all variables and given/known data
I'm in the middle of a project finding the e/m ratio of the electron by the use of the formula below. I know all required values and can find a result, I'm unsure how to calculate an error for the calculation.
In the formula below, D, n, A and f are all given quantities (therefore no error/the error can be taken as 0, right?)

2. Relevant equations

$\frac{e}{m}$ = (32/D2)*(nA2∏)2*(Va/Is2)*(Is/Vs)2

3. The attempt at a solution

The last 2 parts of the equation are where the trouble arises. I have had to measure and record results in two different ways to find values and to plot the 2 graphs - Va/Is2 and Is/Vs - and the software used gives a percentage error in the line.
Do I need to find the error in each measurement for Va and Is etc?

The current way I know to calculate error would mean using the smallest value measured and half of the smallest possible measurement (half the equipment increment). This would result in a stupidly high error, eg:
smallest V measurement was 5V, this is also the smallest increment on the equipment, therefore error is ±2.5V meaning an error in that set of measurements of 50%??
I'm then taught to add the percentage errors... this leads to 350% error.
However, because I know the gradients of the graph, is it acceptable to only use these errors? This would give a total are of ≈10%. Much better!

Which is the correct way of going about this? And if the answer is neither of these, what is the correct way of going about this!?