[例]求同轴传输线之磁能 H= 2死r B=HI 2死r dV=元rldr w.-lw.dv-uHdv =(r广rdr int 4饥 i dr π = 2 r I H B = μ 2π r I dV = 2π r l dr = μ 4π I 2 ln( ) R1 l R2 [例] 求同轴传输线之磁能 = ( ) R 1 R 2 2 1 μ 2π r I 2  2π r l dr Wm =V wmdV 2 = 1 μ H 2 V  dV r dr l 1 R2 R I I