Homework Help: Electrodynamics - relativistic generalization of a formula

1. Jun 3, 2010

Jerbearrrrrr

The formula holds in the rest frame.
Why is this particular extension 'the' extension?
(Context is Electrodynamics/SR. It's just a piddly 16 lecture course that outlines Electrodynamics and SR and a bit of Quantum phenomena at the end. Ohm's law hasn't been mentioned and I don't think you're expected to know a lot about Ohm's law itself)

Oh.
If you use a "taylor series in U" as in,
$$\sigma F^{ab}U_b = J^a + \lambda U^a$$
and solve for lambda, it does turn out as what it's meant to be.

But why is this the correct thing to substitute in? Or is it a naughty question that demands us to be psychic?

Last edited by a moderator: May 4, 2017
2. Jun 5, 2010

gabbagabbahey

Why not just use the first equation you are given (supposed to show)? In the rest frame of the ohmic medium, you have:

$$\sigma F^{ab}U_{b}=(0,\sigma\textbf{E})=(0,\textbf{j})$$

What do you get when you apply a Lorentz boost to this 4-vector?