Many earphones are designed in the way pictured below. A coil of wire carrying an electric current is wrapped around a tiny speaker cone. The cone and coil are placed near a small magnet. Sound is generated by the speaker cone vibrating in different ways. The cone vibrates as different magnetic forces act on the current carrying coil. Why do the magnetic forces acting on the coil change as the current running through the coil changes?

Along steel shaft of 10 cm diameter is austenised at 870°c and then quenched in a liquid at 30oc. the average surface heat transfer coefficient is 2000 w/m2. °c. the relevant thermophysical properties of the shaft are: p=7800 kg/m", k=20 w/mk, cp=0.46 kj/kg.k calculate: a)-the time required for the center temperature to drop to 200°c. b)-the temperature at half-radius at that moment. c)-the amount of heat that had been transferred to the liquid by that time per meter length of the shaft