1. The problem statement, all variables and given/known data A long wire carries a current i uniformly distributed over a cross section of the wire. Show that the magnetic energy of a length l equals μ_0 i2 l / 16π. Describe why this doesn't depend on diameter. Show that the inductance for a length of the wire l associated with the flux inside the wire is μ_0 l / 8π 2. Relevant equations u_B = B2/2μ_0 U = L i2 /2 Biot-Savart law for Field in a long wire: B = μ_0 i / 2 π r 3. The attempt at a solution Solve for u_B as u_B = B2/2μ_0 = (μ_0 i / 2 π r ) 2 / 2μ_0 Multiply by volume of wire to find U: U = ( (μ_0 i / 2 π r ) 2 / 2μ_0 ) * π r^2 l = μ_0 i2 l / 8π I'm off by a factor 1/2. Any suggestions?