A small ball of charge q and mass m is “falling” vertically at a constant speed u through a viscous fluid. At some point, a uniform constant horizontal magnetic field is turned on, and, after a while, the ball begins to move at a different constant speed so that the thermal energy dissipated in the fluid per second is reduced by the factor of k, compared with the ball’s motion in the absence of the magnetic field. Find the maximum possible magnitude of the magnetic field. The acceleration due to gravity is g.

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