(a) Use an energy method to determine how much current must be passed through the coil if it is to remain unstretched when a mass m is hung from its bottom end. Neglect stray fields at the ends of the coil and purely mechanical forces which might produce a “spring constant”.(b) Give a simple explanation of the forces at work. Why does this problem not contradict the fact that no system can exert a net magnetic force on itself?

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