Chapter 13, Problem 83CP

A square bar of mass m and resistance R is sliding without friction down very long, parallel conducting rails of negligible resistance I see below) The two rails are a distance ' apart and are connected to each other at the bottom of the incline by a zero-resistance wire. The rails are inclined at an angle $\theta$, and there is a uniform vertical magnetic field $\stackrel{\to }{B}$ throughout the region, la) Show that the bar acquires a terminal velocity given by

$v\frac{mgR\sin \theta }{B^2{I}^2{\cos }^2\theta }$ Calculate the work per unit time done by the force of gravity. (c) Compare this with the power dissipated in the Joule heating of the bar (d) What would happen if $\stackrel{\to }{B}$ were reversed?