Consider a charged wire with constant linear charge density λ. The wire has length 2πa and is attached to the edge of a disc with radius a. In the central region of the disc (a circular region of radius b<a) a constant magnetic field B is applied (perpendicular to the disc). The magnetic field is then suddenly turned off. Obtain the torque due to the emf induced in the wire, in terms of ∂B/∂t. From this result calculate the disc final angular momentum. Now... when we suddently turn the magnetic field off a emf is induced in the wire. This emf is given by ε=-πb^2∂B/∂t. It is the variation of the magnetic flux. Right? I suppose I could obtain the electric field associated to ε by taking its gradient. The B field is, however, say, in the z direction. How come it will generate a central force to rotate de disc? Thank you.