Chapter 22 - Magnetism - Problems and Conceptual Exercises - Page 799: 110

$C. 7.3\times 10^{10}$

We know that $I=\frac{2\pi rB}{\mu_{\circ}}$ We plug in the known values to obtain: $I=\frac{2\pi(0.13)(1.8\times 10^{-14})}{4\pi\times 10^{-7}}$ $I=1.17\times 10^{-8}A$ Now $n=\frac{I\Delta t}{e}$ We plug in the known values to obtain: $n=\frac{1.17\times 10^{-8}(1.0)}{1.6\times 10^{-19}}$ $n=7.3\times 10^{10}$