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LuxAurum
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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 I0 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 - s2}[/itex]
contributes (outside this range tr is negative..."
Homework Equations
tr = 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(tr) = 0 when tr 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.