A loudspeaker uses the motor effect to move its cone. At one instant, the current in the loudspeaker coil is 0.35 A.
For each turn of the coil, only a 32 mm length of conductor is inside a magnetic field of flux density 0.40 T0.40\ \mathrm{T}0.40 T. This length is at right angles to the field. The forces on these lengths act in the same direction and add together.
The coil must provide a total force of at least 0.50 N.
Calculate the minimum number of turns needed in the coil.