Introduction to Electrodynamics Week 2

[unfocusedg...@gmail.com]

Welcome to the second week of Introduction to Electrodynamics by Griffiths for everyone that's following. This week we'll start reading about the subject proper, starting with Electrostatics. For now let's aim to read from page 58 to 77.

[unfocusedg...@gmail.com]

In Exercise 2.2 it is shown that provided we are outside a charged sphere, the force it enacts can be modelled by shrinking it to a point particle at the centre with the same change. Note that due to the pretty much identical forms of electrostatic attraction (ie. in the case where one particle is fixed), the same can be said for modelling gravitation between massive bodies.

Incidentally, if we are inside a charged shell, it can be shown that the surface forces all cancel out to give zero, so there is no net acceleration of a body inside. This result can be used to show that if a particle lies inside a charged (or massive) sphere at a distance r from the centre, the force it experiences is just the same as if we ignored all charged/gravitational mass beyond a radius r and just considered it sitting on the surface of the subball (which in turn can be modelled as a point charge at the centre, though with a reduced charged compared to the total body). 

[unfocusedg...@gmail.com]

Hmm, thinking a bit ahead, am I right in thinking that an accelerating charge generates a magnetic field, but one with constant velocity doesn't? I guess that explains why if we fix the charged body in a (Newtonian) frame the electrodynamic theory simplifies to electrostatics. The free test particle will generate its own magnetic field as it accelerates, but that magnetic field won't impart any force on the test charge (ie. no self-interaction), only on the fixed body, right?

[Thomas Lane]

This last week has been super busy for me. So far I've only had time to look at the first and second section (so far) and do problems from those. When I took the freshmen level E&M course, I remember struggling to set up these integrals (it's much easier now, anyways). But I'm extremely confused about the author's method of solving problems. He defines angles and doesn't use them. You can get the right answers just from simple vector addition. Maybe, this is why I thought basic E&M was confusing last time around.

[unfocusedg...@gmail.com]

>He defines angles and doesn't use them. You can get the right answers just from simple vector addition. 

I'm not sure what you're referring to exactly, could you perhaps give an example from the book?

I've been busy too (playing FFXI), so I've been slacking a little on the problems from this week, but I think I solved most of them in the past so I think I'm okay.