(b) Charge on helium nucleus, q=+2e=2×1.6×10−19C=3.2×10−19C Radius of helium nucleus, r=0.8m Time period, T=2s ∴ Current associated with helium nucleus, I=‌
q
T
=‌
3.2×10−19
2
=1.6×10−19A ∴ Magnetic field at the centre of the loop, B=‌