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The electric field along the axis of a uniformly charged disk of radius R and total charge Q was calculated in Example 23.3. Show that the electric field at distances x that are large compared with R approaches that of a particle with charge Q = σπR2. Suggestion: First show that x/(x2 + R2)1/2 = (1 + R2/x2)−1/2 and use the binomial expansion (1 + δ)n = 1 + nδ, when δ << 1.
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