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## Homework Statement

The figure (http://img.photobucket.com/albums/v80/northerndancer/thefigure.jpg?t=1173558797 [Broken]) is a cross section through three long wires with linear mass density 50 g/m. They each carry equal currents in the directions shown. The lower two wires are 4.0 cm apart and are attached to a table. What current

*I*will allow the upper wire to "float" so as to form an equilateral triangle with the lower wires?

## Homework Equations

F=I

_{1}LB

_{2}(magnetic force between two parallel wires)

F=mg (force due to an object's weight)

## The Attempt at a Solution

The two bottom wires exert attractive magnetic forces on each other because they are parallel and both of their currents move in the same direction. In order for the third wire to "float", it must be repelled by the two lower wires by a magnetic force equal and opposite to its weight. But:

-The upper wire is not parallel to the lower wires, so I'm not sure how to calculate the force each lower wire exerts on the upper wire.

-How do I calculate the mass of the upper wire if I don't know its length?

Thanks for any hints.

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