Practice question

**Field outside long solenoid** considered negligible because magnetic lines are concentrated inside and return path spreads over large area outside, making B_out ≈ 0. This justifies using solenoid for uniform field experiments, with n = 950 m⁻¹, I = 1.4 A giving B = 4π×10⁻⁷×950×1.4 = 1.67×10⁻³ T. r = (mv/qB) . r = (1.67 × 10⁻²⁷ × 4 × 10⁷/1.6 × 10⁻¹⁹ × 0.05) = (6.68 × 10⁻²⁰/8 × 10⁻²¹) = 8.35 m . Using F = q v B sinθ, F = I l B sinθ, B = μ₀ I/(2π r), B = μ₀ N I/(2R) and τ = N I A B