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NEET 2019 · Electric Flux & Gauss's Law

Electric Flux & Gauss's Law question from NEET 2019

Q101NEET 2019Medium

Electric Flux & Gauss's Law

Two parallel infinite line charges with linear charge densities +λ\lambda C/m and -λ\lambda C/m are placed at a distance of 2R in free space. What is the electric field mid-way between the two line charges?

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Answer: (c)

  1. Field of an infinite line charge: E=λ2πε0rE = \frac{\lambda}{2\pi\varepsilon_0 r}; here r=Rr = R from each line.
  2. At the midpoint the field of +λ+\lambda points away from it and the field of −λ-\lambda points towards it, so both point the same way (towards −λ-\lambda) and add.
  3. E=2×λ2πε0R=λπε0RE = 2 \times \frac{\lambda}{2\pi\varepsilon_0 R} = \frac{\lambda}{\pi\varepsilon_0 R}
  4. So the answer is (c) λπε0R\frac{\lambda}{\pi\varepsilon_0 R} N/C.
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