JEE Main 2017 · 8 Apr · Shift 1 · Physics Q15

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The energy stored in the electric field produced by a metal sphere is 4.5 J. If the sphere contains 4 μC charge, its radius will be : $\left[\text{Take : }\dfrac1{4\pi\epsilon_o}=9\times10^9\text{ N-m}^2/\text{C}^2\right]$

20 mm

32 mm

28 mm

16 mm

Answer and worked solution

Official answer: D

$U=\dfrac{q^2}{8\pi\epsilon_0R}$ , so
$R=\frac{9\times10^9\times(4\times10^{-6})^2}{2\times4.5}=0.016\text{ m}=16\text{ mm}.$

Topics:Energy stored in a capacitor

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JEE Main 2017 · 8 Apr · Shift 1, Physics question 15