A coil in the shape of an equilateral triangle of side 0.02 m is suspended from a vertex such that it is hanging in a vertical plane between the pole pieces of permanent magnet producing a horizontal magnetic field of  5×10−2T. Find the couple acting on the coil when a current 0.1 A is passed through it (the magnetic field is parallel to its plane).

The couple acting on a closed loop is given by  τ=NiABsinθWhere, N=number of loops, A=area of loop, B=magnetic field and   θ=angle between magnetic field and normal to the surface of the loop.Here,  N=1, i=0.1 A, B=5×10−2 T , θ=900(the magnetic field is parallel to its plane)And  A=34a2 = 34(0.02)2 =1.732×10−4 m2τ=NiABsinθ  ∴ τ=1×0.1×(1.732×10−4)×(5×10−2)×1=8.66×10−7Nm .