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. B = μ₀ μ_r n I . Given: n = 1200 m⁻¹ , I = 2.5 A , μ_r = 200 , μ₀ = 4π × 10⁻⁷ . B = 4π × 10⁻⁷ × 200 × 1200 × 2.5 = 0.7536 T ≈ 0.75 T . Substituting values gives 0.75 T, which