A conducting circular loop is placed in a uniform magnetic field of $0.4 \mathrm{~T}$ with its plane perpendicular to the field. Somehow, the radius of the loop starts expanding at a constant rate of $1 \mathrm{~mm} / \mathrm{s}$. The magnitude of induced emf in the loop at an instant when the radius of the loop is $2 \mathrm{~cm}$ will be ___________ $\mu \mathrm{V}$.