A thin ring of 10 cm radius carries a uniformly distributed charge. The ring rotates at a constant angular
speed of 40 rad s^–1
about its axis, perpendicular to its plane. If the magnetic field at its centre is 3.8 × 10^–9
T, then the charge carried by the ring is close to (₀ = 4 × 10^–7
N/A²
).