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30. (a) It is clear from symmetry (also from Eq. 2216) that the field vanishes at the
center.
(b) The result (
E
= 0) for points infinitely far away can be reasoned directly from Eq. 22
16 (it goes as 1/
z
² as
z
→
∞
) or by recalling the starting point of its derivation (Eq. 2211,
which makes it clearer that the field strength decreases as 1/
r
² at distant points).
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 Spring '10
 BB
 Physics

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