A square loop $ABCD$ of thin wire has sides $20\ \text{cm}$ long and carries a current of $3.0\ \text{A}$. It is folded along its diagonal $AC$ until the two halves are at right angles. The triangle $ABC$ lies flat in a horizontal plane, and the triangle $ACD$ stands in a vertical plane, as in the figure. A uniform horizontal magnetic field of $0.40\ \text{T}$ acts at right angles to $AC$, so it lies in the plane of $ABC$. The current still flows round the loop in the order $A$, $B$, $C$, $D$. What is the size of the torque of the field on the folded loop?

Fig. 1: A square loop ABCD of side 20 cm, folded along its diagonal AC so that its two halves are at right angles. The triangle ABC lies flat in a horizontal plane and the triangle ACD stands up in a vertical plane above AC. Arrows show the current of 3.0 A going round A, B, C, D. Horizontal arrows across AC show a uniform magnetic field of 0.40 T, in the plane of ABC.