Linear Mass Density of current and wires

[splac6996]

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Homework Statement

The figure is a cross section through three long wires with linear mass density 43.0. They each carry equal currents in the directions shown. The lower two wires are 4.0 cm apart and are attached to a table.

Homework Equations

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

The Attempt at a Solution

I am not sure where to start could someone give me a hint at what to do?

 

[salmon]

Hint: A Repulsive force exists between straight current-carrying conductors carrying currents in opposite directions. By Ampere's law, each conductor generates a magnetic field around it and the forces acting on them are basically the forces of interaction between the magnetic fields. Any standard textbook should explain this phenomenon. Look up the Bio-Savart Law or try this link:

hyper-text-transfer-protocol://world-wide-web.physics.upenn.edu/~uglabs/lab_manual/force_between_conductors.pdf

(I'm not allowed to post URLs yet, so just copy-paste the above link in the address bar and rewrite the beginning of the link)

I'm thinking that the only reason why the upper conductor won't float is because of gravity. This means that you probably have to assume that the lower conductors are fixed, otherwise they'd fall too.