In a typical speaker, a stiff cone made of plastic or paper pushes air back-and-forth creating pressure waves that we perceive as sound. The force to drive the cone comes from a coil of wire attached to the base of the cone, which, when current passes through it, experiences a magnetic force. The coil of wire is situated between a concentric magnet as shown below, whose field points N to S radially outward everywhere with a field strength is 0.2 T.  The coil of wire has a diameter of 5 cm and is composed of 40 turns of wire with a total resistance of 4 Ω.  What is the magnitude of the instantaneous force on the cone in N if a 6 V potential is applied across the ends of the coil of wire?  Give your answer to 2 significant digits.

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Ferromagnetism

Ferromagnetism is a mechanism by which some materials show magnetic properties. Iron and magnetite, an oxide of iron, are two naturally occurring materials that show the property of ferromagnetism. The word ferromagnetism is derived from the Latin name of iron, "Ferrum" as it was the first observed material to show such property. The property of ferromagnetism is shown by only a few substances such as iron, nickel, cobalt, and their alloys. Some transition metals and alloys of rare-earth metals also show ferromagnetism.