Past exam of the physics B course of the University of Cambridge / 2013 / Paper 2 / Section A / A1

For the force on a current loop in a bilinear magnetic field, the arrow gives current in the positive $y$ direction on the edge $x=a$, and in the negative $x$ direction on the edge $y=a$. Use $d\mathbf{F}=Id\mathbf{\ell }\times \mathbf{B}$. The edges on the coordinate axes contribute zero because the applied field vanishes there. The other two giveThus the force on the square itself, excluding the supply leads, isIt lies in the plane along the diagonal towards increasing $x$ and $y$, at $45^{\circ }$ to either positive axis.