Unformatted text preview: Physics 21 Fall, 2004 Solution to HW11 27 P 9 A straight 2.0 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 . × 10 5 T. What current does the wire carry? x y z B mg F I The diagram shows a section of wire (of some length L ) carrying current I in the ˆ z direction. The force F on that piece of wire that balances gravity is given by F = IL ˆ k × B = ILB ˆ j We need the mass of the section of wire to get the gravita tional force acting on it. If the radius of the wire is r , the mass m is m = πr 2 Lρ Cu , where ρ Cu is the mass density of copper (8 . 9 × 10 3 kg/m 3 , from Ex. 2512 in the text or other tables). Balancing the gravitational and magnetic forces, we have ILB = mg = πr 2 Lρ Cu g. Solving for I , we see that L drops out and I = ρ Cu πr 2 g B Using r = 1 . × 10 3 and the other numbers given, the numerical result is I =...
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This note was uploaded on 08/06/2008 for the course PHYS 21 taught by Professor Hickman during the Fall '07 term at Lehigh University .
 Fall '07
 Hickman
 Physics, Force

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