# Homework Help: E/m ratio

1. Jan 19, 2009

### barcelonaboy

I'm trying to calculate the ratio of e/m, I've plotted the given data points to get a straight line graph with a gradient of 0.848.

I know that the ratio of e/m is given by e/m = (4.49*10^8 * gradient^2)/volts
where volts = 3000.

When I plug the numbers in I get a value of 107625 which is nowhere near the given book value of e/m 1.7*10^11 Ckg^-1

I think that I'm doing something in the wrong units as I need to get the answer in C/kg^-1, but I can't see where I'm going wrong.

Any pointers owuld be much appreciated

2. Jan 19, 2009

### Haths

I'm afraid that I don't know what you might be doing wrong because I don't know what data varibles you have. Are you plotting a current against a radius of a circle, or velocity against current flow, or voltage against current even I think can be one setup for an electron beam experiment.

Have you checked that your units are in the SI format?

Is your set of axis the right way round?

Yeah could do with some more info bud :(
Haths

3. Jan 19, 2009

### barcelonaboy

Hi Haths,

I'm plotting a graph of Voltage (V) against Current (mA) (V/mA) as part of an electron beam experiment, which is giving a gradient of 0.848 with the R^2 coming out as 0.9991 which is a good fit for the data points, so I'm confident in the graph and the data points.

The equation that I need to plug my numbers into is, this includes the 4.49*10^8 as a dimensionless constant

e/m = (4.49*10^8)(gradient^2)(1/ 3000 volts)

I think that I'm going wrong on the units in the gradient as I can't see how to get from an equation that has units in volts and mA to one that involves Coulombs/kg or if there is some conversion factor that I'm not aware of.