If a current carrying coil is placed in a magnetic field a pair of forces act on the sides of the coil. The force on each side is in the opposite direction and forms a couple. As a result, the coil rotates.If the coil has N turns, then each of the two vertical sides experiences a horizontal force of NBIl at right angles to the field lines. The Torque of the couple is Fd where d is the width of the coil. If the plane of the coil is at an angle Θ to the field lines then the torque = FdcosΘ