- #1

LuxAurum

- 8

- 0

## Homework Statement

I'm working through the 3rd edition of Griffiths' Electrodynamics book and have gotten stuck on some details in example 10.2, which describes an infinite straight wire carrying a current I

_{0}for t>0.

The figure included with the example illustrates a wire in the vertical z direction with element dz. A point P is located at distance s from the wire. The distance from dz to P is given by script_r, and forms a right triangle with base of z.

The part that is unclear to me is the following. Griffiths states:

"For t < s/c, the 'news' has not yet reached P and the potential is zero. For t > s/c, only the segment |z| [itex]\leq \sqrt{(ct)

^{2}- s

^{2}}[/itex]

contributes (outside this range t

_{r}is negative..."

## Homework Equations

t

_{r}= t - (script_r/c)

## The Attempt at a Solution

I understand that the expression t = s/c represents the time it takes for the wave to traverse the distance s to get to P. What I'm unclear about is how to properly setup and analyze this problem if it wasn't an example. Is the distance s used to establish a lower bound for the time? Griffiths then uses (ct)

^{2}in the expression for |z|.

Also, why does I(t

_{r}) = 0 when t

_{r}is negative?

Hopefully this doesn't come across as scattered. Sometimes I get stuck on minor details that should be obvious but aren't at the time.