A thin ring of 10 cm radius carries a uniformly distributed charge. The ring rotates at a constant angular
speed of 40 \pi 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 (\mu
0
= 4\pi × 10
–7
N/A
2
).