Calculate the current density needed to `levitate' a copper wire in the earth's magnetic field (that is, the current required for the lorentz force on the wire to be equal to its weight). assume that the wire is at the magnetic equator where the field is horizontal with magnitude 10-4 tesla. take the density of copper to be 8.9 g cm3 .
(b) calculate the power dissipated in the wire per unit volume. will the wire become hot? the resistivity of copper is 1.7 x 108 ω.m and the heat capacity of copper is 385 j kg-1 k -1 .
i have part (a) i just need part (b)
answer to part (a) is j=8.73x10^8 a/m^2.

The ultracentrifuge is an important tool for separating and analyzing proteins. because of the enormous centripetal accelerations, the centrifuge must be carefully balanced, with each sample matched by a sample of identical mass on the opposite side. any difference in the masses of opposing samples creates a net force on the shaft of the rotor, potentially leading to a catastrophic failure of the apparatus. suppose a scientist makes a slight error in sample preparation and one sample has a mass 10 mg larger than the opposing sample. if the samples are 12 cm from the axis of the rotor and the ultracentrifuge spins at 70,000 rpm, what is the magnitude of the net force on the rotor due to the unbalanced samples? ( be thorough on your answer)