圆弧磁场

$$\begin{array}{rl}F& =\int dF\times sin\theta \\ & =\int B_{0}Isin\theta dl\\ & =\int B_{0}I(Rd\theta )sin\theta \\ & =B_{0}IR(-cos\pi )-BIR(-cos0)\\ & =-2B_{0}IR\end{array}$$