安培定则

INSIDE

0

OUTSIDE

$$\begin{array}{rl}\oint \mathbf{B}\cdot d\mathbf{l}& ={\mu }_0{I}_{\text{enc}}\\ \mathbf{B}\cdot \mathbf{l}& =\end{array}$$ $$B={\mu }_0\frac{N}{l}I={\mu }_{0}nI$$