The magnetic field on the axis of a circular current loop of radius a is given by the formula \mathbf{B}=\frac{\mu_0 I a^2}{2(a^2+z^2)^{\frac{3}{2}}} \mathbf{\hat{z}}, where I is the current and z is the distance from the center of the loop. For an insulating disc with uniform surface charge density \sigma rotating at angular velocity \omega, the surface current density is defined as \mathbf{K(r)}=\sigma \mathbf{\omega} \wedge \mathbf{r}. The contribution to the magnetic field from a ring of radius r is expressed as B_z = \frac{\mu_0}{2} \int_{0}^{a} \frac{\sigma \omega r^3 dr}{(r^2+z^2)^{\frac{3}{2}}}. As z approaches infinity, the magnetic field behaves as \mathbf{B}(z) \sim \frac{1}{8} \mu_0 \sigma \omega \frac{a^4}{z^3} \mathbf{\hat{z}}.
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