# Homework Help: Finding e/m ratio

1. Jan 5, 2009

### Lorentz09

Hi all, I'm new to this forum so sorry if this post is a bit scrappy!

1. The problem statement, all variables and given/known data

This is part of a lab report I am doing: Plot a graph of V against I^2 and find the e/m ratio from the gradient.

2. Relevant equations

e/m = 2V/B^2 r^2 where B is magnetic field, r is radius of electron beam

B = U (4/5)^(3/2) nI/R where U is magnetic constant, n is no. of turns per coil, R is radius of Helmholtz coils

3. The attempt at a solution

I'm not sure what exactly to do once I get the gradient!

Many thanks if anyone can help and feel free to ask if I havn't provided enough info :)

2. Jan 12, 2009

### chrisk

Note that B is proportional to I and V is proportional to B2 giving V proportioanl to I2. If you plot V vs. I2, the plot is linear and the slope (gradient) of the line is e/m times the other constants. Simply eliminate B using the two given equations and solve for V and the slope term will be evident i.e. V = e/m*K*I2 where K are the combined constants of U, R, n and r.