Energy density of magnetic field For ideal solenoid: B L=un'y I= un 1B2 24 V2 u 2 Energy density of magnetic field For ideal solenoid: L n V 2   n B I   V B n B Wm LI n V    2 2 2 2 2 1 2 1 2 1            2 2 2 1 2 1 H B V W w m m     