Find e/m Ratio: Lab Report Graph Analysis

[Lorentz09]

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

Homework Statement

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.

Homework 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

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 :)

 

[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.