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Two point charges +4q and +q are placed at a distance L apart. At what distance from +4q (on the line joining them) is the net electric field zero?
AL/3 from +4q
B2L/3 from +4q (and L/3 from +q)
CL/2 (midpoint)
DThere is no such point
Answer & Solution
Correct answer: B. 2L/3 from +4q (and L/3 from +q)
Let x = distance from +4q where E = 0. From +4q: E₁ = 4kq/x². From +q at distance (L−x): E₂ = kq/(L−x)². Set equal in magnitude (both point away from same direction, must cancel between them): 4kq/x² = kq/(L−x)² → 4(L−x)² = x² → 2(L−x) = x → 3x = 2L → **x = 2L/3 from +4q**.
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