Homework Help: Work done by the magnetic field on the resistor

1. Oct 24, 2011

syhpui2

1. The problem statement, all variables and given/known data

A magnetic field, B= 0.8 T, is directed out of the page in a region containing a rectangular up-side-down "U-wire" having width W = 0.5 m, as shown. A resistor of mass m and resistance R = 6 Ω, which is free to slide without friction on the vertical rails, is released from rest and starts falling in the presence of the earth's gravitational field, reaching a terminal speed v = 3.8 m/s.

http://i.imgur.com/ukw8m.png

Which statement(s) below is (are) valid during the time the resistor is falling at terminal velocity?

(a) The work per unit time done by the earth on the resistor is equal to the power dissipated in the resistor.

(b) The work done by the magnetic field on the resistor is equal in magnitude, but opposite in sign, to the work done by the earth on the resistor.

(c) Both of the statements above are valid.

2. Relevant equations

P=UI

3. The attempt at a solution

I tried to analyze with P=UI. However, I get answer B. Should I analyze it with W=Fd?