1. The problem statement, all variables and given/known data A straight 2.47 -mm-diameter copper wire can just 'float' horizontally in air because of the force of the Earth's magnetic field B, which is horizontal, perpendicular to the wire, and of magnitude 5 x 10-5 T. What current I does the wire carry? (The density of copper is 8.96 g/cm3). HELP: The wire feels a downward gravitational force of magnitude mg, where m is mass and g = 9.80 m/s2 is the gravitational field strength near the Earth's surface. 2. Relevant equations F=lIB Gravitational force=mg 3. The attempt at a solution Okay, I know /how/ to do this except for one step. I know that I need to find the force using the equation above. Simple enough, except that I don't know how to find the mass. I know that the mass is the density multipled by the volume. However, I don't know how to find the volume with the information I'm given. If someone could just help me out with that little bit, that would be fantastic!