Practice question

**Motion of charged particle perpendicular to uniform B** is circular because force F ⊥ v, no work done, speed constant, radius r = m v/(q B), period T = 2π m/(q B) independent of v. If v has component parallel to B, helical path results, pitch = v_parallel·T. Force F = q v B sin θ , θ = 90° , so sin θ = 1 . F = 1.6 × 10⁻¹⁹ × 7.5 × 10⁷ × 0.12 = 1.44 × 10⁻¹² N . Using F = q v B sinθ, F = I l B sinθ, B = μ₀ I/(2π r), B = μ₀