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JEE Main 2016 (10 Apr, Shift 1) · Electric Potential & Equipotentials

Electric Potential & Equipotentials question from JEE Main 2016 (10 Apr, Shift 1)

Q102JEE Main 2016 · 10 APR · SHIFT 1Difficult

Electric Potential & Equipotentials

Within a spherical charge distribution of charge density ρ\rho(r), N equipotential surfaces of potential V0, V0 + Δ\DeltaV, V0 + 2Δ\DeltaV, .......... V0 + NΔ\DeltaV (Δ\Delta V > 0), are drawn and have increasing radii r0, r1, r2,..........rN, respectively. If the difference in the radii of the surfaces is constant for all values of V0 and Δ\DeltaV then:

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Answer: (c)

  1. Equal ΔV\Delta V at equal Δr\Delta r means E=ΔVΔrE = \dfrac{\Delta V}{\Delta r} is constant.
  2. Gauss: E⋅4πr2=q(r)ε0⇒q(r)∝r2E \cdot 4\pi r^2 = \dfrac{q(r)}{\varepsilon_0} \Rightarrow q(r) \propto r^2.
  3. dq=ρ⋅4πr2 dr∝2r dr⇒ρ∝1rdq = \rho \cdot 4\pi r^2\,dr \propto 2r\,dr \Rightarrow \rho \propto \dfrac{1}{r}
  4. So the answer is (c) ρ∝1r\rho \propto \dfrac{1}{r}.
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