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Unformatted text preview: 20. From symmetry, we see that the net ﬁeld at P is twice the ﬁeld caused by the upper semicircular charge
+q = λ · πR (and that it points downward). Adapting the steps leading to Eq. 23-21, we ﬁnd
Enet = 2 −ˆ
4πε0 R 90◦
−90◦ =− q
ε0 π 2 R 2 ...
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