# Faraday disk with a mass:

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Hi to everybody! Please: can you help with this problem?
A conductive disk with a radius a = 20 cm, with negligible moment of inertia, rotates around its horizontal axis. The disc region around the radius is inserted in a magnetic field B = 0.75 T perpendicular to the disk itself. A mass is connected to the edge of the disc using a thin wire. The disk is connected to a circuit with an e generator. m. f. = 10 V. The overall resistance of the circuit is R = 0.3 Ohm. Under steady conditions, the disc rotates at an Omega angular velcoity, raising the mass. Calculate: a) the current of the regime that runs through the circuit and b) the angular velocity Omega.
My book calculates the current by the following relation:
mga=iB(a^2)/2 that is: the moment of weight force is equal to the moment of force obtained by the magnetic field. My question is: Why you can't establish the same relation between the forces?
I mean: mg=iBa. It's obviously another result, but I mean: why forces aren't equal, but the moments are???