Practice question

**SI unit of magnetic field** is tesla (T), defined as force 1 N on 1 A·m wire perpendicular to field. Moving coil galvanometer uses torque τ = N I A B balanced by spring torque k φ, so deflection φ ∝ I, enabling current measurement, with radial field ensuring τ = N I A B always maximum. Radius r = (mv/qB) . r = (9.1 × 10⁻³¹ × 2 × 10⁶/1.6 × 10⁻¹⁹ × 0.7) = (1.82 × 10⁻²⁴/1.12 × 10⁻¹⁹) = 1.625 × 10⁻⁵ m = 1.625 × 10⁻³ cm . Using F = q v B sinθ, F = I l B sinθ,