1. The problem statement, all variables and given/known data You have a spring of lenght l, radius R, with N loops and n loops per unit length. If you consider it a solenoid, what current do you need to apply to withstand a mass m hanging from it, without stretching or shrinking the spring. 2. Relevant equations Magnetic field inside a solenoid: B=μ0nIz Magnetic energy: W=∫∫∫B2/(2μ0)dV 3. The attempt at a solution I calculated the magnetic energy of a solenoid and I got: W=μ0N2I2πR2/(2l) The force applied by the mass is: F=mg So, the magnetic force that I need is: F=∇W(length constant)=μ0N2IπR2/(l) Both forces must be equal. I solve for I and I get: I=mgl/(μ0N2πR2) Do you thing it is right?