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Question:

The energy stored in magnetic field produced by a metal sphere is 4.5 J. If the sphere contains 4µC charge, its radius will be : [Take : 1/4πε₀ = 9 × 10⁹ N-m²/C²]

16 mm

32 mm

20 mm

28 mm

Solution:

Energy stored by charged metal sphere → 3/5 Q²/4πε₀R = 4.5 → 9 × 10⁹ × 3/5 × 16 × 10⁻¹² /4.5 = R → R = 19.2mm ≈ 20mm