Practice question

**Solenoid with magnetic core** produces field B = μ₀ μ_r n I inside, μ₀ = 4π×10⁻⁷ T·m/A, μ_r relative permeability, n = N/L turns per meter, I current. Core enhances field μ_r times, so given B, μ_r, n, current I = B/(μ₀ μ_r n) can be found, illustrating core effect on field strength. tau = m B sinθ . Given: m = 0.4 A m² , B = 0.8 T , θ = 60° , sin 60° = (√(3)/2) ≈ 0.866 . tau = 0.4 × 0.8 × 0.866 ≈ 0.277 N m ≈ 0.28 N m . Substituting values gives 0.28 N m, which