1. The problem statement, all variables and given/known data An electromagnetic "eddy current" brake consists of a disc of conductivity [tex]\sigma[/tex] and thickness d rotating about an axis passing through its center and normal to the surface of the disc. A uniform B is applied perpendicular to the plane of the disc over a small area a^2 located a distance P from the axis. Show that the torque tending to slow down the disc at the instant its angular speed is W is given approximately by [tex]\sigma[/tex]*W*d*[B*P*a]^2 3. The attempt at a solution I assume that you need to calculate the force on the disc at the small section a^2 and then from this a torque can be easily found. Can you say: [tex]F=q\cdot(E+v \times B)[/tex] And since the electric field is motional make this [tex]F=q\cdot(E+v \times B)=q\cdot(v \times B+v \times B)=q\cdot(2v \times B)[/tex] [tex]v \times B = BPW[/tex] [tex]F=2qBPW[/tex] If so, how do you find the charge enclosed inside of the little region of volume d*a^2? I don't quite see how the conductivity plays into all of this, or where the second factor of B comes from in the solution. Any help would be appreciated.