Homework Help: Derivation of Larmor formula

1. Nov 7, 2014

albega

1. The problem statement, all variables and given/known data
I need to derive this formula, and I'm basing it upon the following method (this has the diagram which I will be talking about below)
http://isites.harvard.edu/fs/docs/icb.topic1214926.files/Purcell-appendix-B.pdf

3. The attempt at a solution
I'm fine with everything apart from the reasoning that the parallel field lines (from where q is, and where it would be) have to join up in the middle region. The reasoning for this is given as

'A field line such as AB lies on a cone around the x axis which includes a certain amount of flux from the charge q. If CD makes the same angle θ with the axis, the cone on which it lies includes the same amount of flux. Hence AB and CD must be parts of the same field line, connected by a segment BC.'

I don't see how the cones having equal flux implies the two field lines must be the same and so joined. Could anybody explain this? Thanks!

2. Nov 8, 2014

3. Nov 8, 2014

haruspex

I've tried a couple of times to view the PDF, but it comes up blank on my system. Maybe others have the same problem.